Air
Test: A general overview to testing and design
1.0 Key Principles
1.
The air test procedure is set out in CIBSE TM 23 and the ATTMA TS1
2.
An air test involves sealing all ‘normal’ gaps such as vents and pressurising
or depressurising the building. The level of fanpower required to maintain the
pressure differential indicates the ‘leakiness’ or ‘permeability’ of the
building.
3.An Air test are
typically followed by an air test audit (using smoke pencils, for example) to
expose and make visible the various air test leakage routes.
4.
Where projects comprise large quantities of a single component, component
2.0 Climatic conditions
As
mentioned in Chapter 1, the raised pressure differential of 50 Pascals created
during an air test is quite small. Whilst this is adequate to overcome most of
the common pressure differential anomalies, such a small differential is
vulnerable to larger pressure differences created by climatic conditions.
Air
tests uk require calm days – i.e. a reading on the Beaufort Scale of 3 or less
(3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds
to a gentle breeze with leaves and small twigs in constant motion. In winter
conditions, and on exposed sites therefore an air test may not be possible,
although it is often possible to make allowances, so long as these are
carefully recorded during the air tests uk .
3.0 The Test itself
Guidance
on testing buildings for an air test is contained in CIBSE Technical Memorandum
TM23 Testing Buildings for Airtightness and in BS EN 13829: 2001. All
accredited air tightness testers test to the guidelines contained in the BS EN
and within the ATTMA TS1
Essentially
the air test process is one of pressurising or (less commonly) de-pressurising
the inside of the whole building, and of measuring the rate at which air needs
to be blown or sucked to maintain that pressure differential; a building
suffering from high amounts of air leakage uk during the air test will equalise
readily and require a greater measurable effort to maintain the 50 Pascal
differential, while a air tight building will easily contain the enforced
differential and require little additional input during the air test .
The
pressure difference is induced by one or more calibrated fans that are normally
mounted within a suitable doorway. An adjustable door panel system, sealed
around the edges is used which can also be connected to large external fans via
collapsible ductwork if required. The rate of the fan or the volume flow of air
through the fan during the air test can be understood as the rate of air
entering / escaping throughout the remainder of the building envelope.
Buildings are air tested/air leakage tested in such a way as to recreate
‘normal’ conditions. Doors and windows are closed; trickle ventilators closed,
extract fans and such like are closed but not sealed. Internal doors are wedged
open. All of this must be actioned prior to the air test /air tightness test
If
the building is under construction the air test /air leakage testing uk can
be undertaken during working hours, but sometimes this is not
practical so some scheduling of work needs to be thought through in advance.
With all external doors and windows sealed shut, some work becomes impossible
(such as work with solvents requiring ventilation) and internal trades are
normally ‘sealed in’ for a short time, where they can carry on undisturbed
throughout the air test /air tightness test uk
In
existing buildings the air tests
4.0 Air Leakage Audits
The
air test /air tightness test quantifies the rate of air leakage through the envelope as a whole, but
it cannot locate the air leakage paths. Where remedial work is required
therefore, air test /air tightness tests are followed by a range of auditing
techniques designed to identify the specific places where air leakage is apparent through the
building envelope.
In
many cases a simple visual inspection may be sufficient – especially if
undertaken by someone with experience of the likely locations of air leakage.
However,
most air leakage routes are difficult or impossible to spot without visual aids
during the air test /air tightness test. One common technique is to use smoke
tracers – smoke pencils or smoke machines. These render the air paths visible
in certain situations during the
Another
technique, which has certain advantages and disadvantages compared to smoke
tracing along with an air test /air tightness test, is the use of an infrared
camera during a Thermographic test. Used either externally or internally, these
thermographic cameras register the radiant heat levels of surfaces and so are
able to ‘see’ for example, where cold air is cooling the fabric around a gap
internally, or conversely where warm air is escaping and heating the colder
materials on the external face.
To
work effectively, there needs to be a recognisable difference between the
internal and external ambient temperature, so before any heating has been
installed and on a warm summer’s day thermography testing may not be effective.
Similarly on warm and sunny days, sunshine on external surfaces can distort the
true situation so it is better on such days to wait until early evening.
Conversely, rain on external surfaces can be equally distorting of the true
thermal situation. However the Thermographic cameras are useful in
identifying problems at high level or difficult to reach areas, and are also
very helpful in identifying other construction defects such as poorly installed
(or non-existent!) insulation within the fabric.
On
larger commercial buildings, the air test /air leakage testing may be
undertaken at the same time as ‘standard’ ventilation system commissioning and
associated studies, but these are not discussed as part of this guide.
5.0 Component Testing
A
distinct aspect of overall air test /air tightness testing is the individual
component test. This may be undertaken quite separately, in the laboratory or
by the manufacturer of a particular component. Such air testing /air
tightness sampling tests may be deemed necessary on a large project where large
areas of one particular type of component, for example curtain walling, are to
be specified, Insitu element air testing involves isolating the area within a
temporary sealed compartment, which is then pressurized during the air test ,
and the air leakage related to the area of interest assessed. In this way
sample areas of a building may be air test /air tightness tested using
smaller fans as required.
5.1 Concrete frame and panel
1
Introduction
As thermal insulation levels have risen in the last
few years the proportion of energy lost to draughts has increased to the extent
that now in some cases around half of all heat losses are due to air leakage
across the building fabric. Given that approximately half of all energy used in
the
The situation is such that further increasing thermal
insulation levels would be largely unproductive unless air tightness is conscientiously
addressed. Air leakage has been shown to reduce the effectiveness of thermal
insulation by up to 70% and so it is clear that if energy efficiency is to be
improved in buildings, the next efforts will have to focus on air test /air
leakage testing.
Many people make the mistake of thinking that an
airtight building is necessarily a ‘stuffy’ building. This is not the case. All
buildings have to be ventilated for health and comfort and airtight buildings
are no different. An adequate ventilation system (which may well include open
able windows as well as fans etc.) has to be planned for every building. The
difference will be that a great deal of unplanned air leakage needs to be
stemmed, this can be ascertained during the
The additional costs of creating an adequately
airtight building can be negligible, but even where costs are increased e.g.
for the air test , these can be more than offset by a reduction in the
capital cost of heating and ventilation equipment, not to mention the long term
savings in energy.
Given that the vast majority of building stock is
existing, a great deal of attention will need to be given, in the foreseeable
future, to remedial works to existing buildings, all existing air leakage paths
can quickly be found during a air test . This guide specifically includes
examples of good and best practice remedial work in terms of air tightness and
shows that such works can offer substantial benefits without undue disruption
or cost, an air test will have a low impact on site works
Air Pressure Testing hopes to provide practical
guidance on how to save energy and costs and protect building fabric. On the
basis that prevention is cheaper and easier than cure,
one purpose of this guide is to enable Designers to design inherently more
robust and durable solutions which avoid costly and time consuming remedial
works after the air test after a potential air leakage failure
The general guidance here is firmly focused on the
idea of practical design and detailing, and should be read in conjunction with
other guidance on sustainable design, energy efficiency and air tightness
where necessary to provide an overall design framework. The details provided
have been fully costed, tested and subjected to a Defects Liability insurance
assessment. They are offered as viable alternatives to standard details, and
illustrate the possibilities that exist. It simply remains for you, the
reader, to apply them appropriately in the context of your next project prior
to your air test/air tightness test
Aims
of this Guide
• To highlight benefits of air tightness
which include both energy and cost efficiency, improved comfort and reduced
risks of damage to building fabric
• To improve awareness of the need for air tightness
in construction at design stage
•
To promote detailing and specification solutions which create airtight and
efficient buildings thus reducing the need for remedial works after air leakage
failure- ‘prevention rather than cure’
• To show that new build and remedial air
tightness are achievable without undue cost penalties to construction works
due to multiple air leakage failures, you should be passing after the first
• .and in this way to help to ‘mainstream’
the good and best practice outlined in the document
Target audience
This
Guide will help all those who wish to improve the airtightness/air leakage rate
and energy efficiency of buildings through their construction, e.g:
• clients –building owners and users,
principal and specialist contractors, interior designers architects and
technicians, structural engineers, building service engineers, building
surveyors, quantity surveyors/ cost consultants, maintenance and facilities
managers, project managers , planning officers and building control officers,
funding bodies and their professional advisors, government and non-governmental
agencies
How to use this Guide
This
Guide is divided into six sections. The first two sections provide an overview
of the issues surrounding air tightness. Sections Three, Four and Five describe
the requirements for the design process, the procurement and the air
leakage/air tightness testing involved in designing for airtight buildings.
Section
Six provides a number of representative details which have been optimised in
terms of preparing for the
6.0 The Context
Key Principles
1. Most
2. Airtight building will NOT mean ‘stuffy’ buildings.
Good ventilation is vital for health and comfort - it is the UNPLANNED air
leakage that we are aiming to stem.
3. Legislation is slowly catching up with
best practice in
4. Good and Best Practice Targets of air tightness/air
leakage have been set for many types of buildings and are easily achievable.
6.1 Infiltration, Ventilation and Airtightness
Air infiltration (air leakage) is the uncontrolled
flow of air through gaps in the fabric of buildings; this is all the more
apparent during the air tes
Ventilation, on the other hand, is the intended and
controlled ingress and egress of air through buildings, delivering fresh air,
and exhausting stale air in combination with the designed heating system and
humidity control, and the fabric of the building itself.
Whilst some unwanted air infiltration (air
leakage) will at times aid comfort levels,
it is not reliable and moreover brings with it a range of significant
disadvantages such as high levels of heat loss, reduction in performance of the
installed thermal insulation, poor comfort, poor controllability and risks to
the longevity of the building fabric itself. It cannot be considered an
acceptable alternative to designed ventilation. Air Infiltration (air leakage)
needs to be reduced as much as possible if we are to create efficient,
controllable, comfortable, healthy and durable buildings. This can be achieved
by delivering airtight buildings that pass the air test /air leakage tests
first time.
Air tightness is a term used to describe the
‘leakiness’ of the building fabric. An airtight building will resist most
unwanted air infiltration (air leakage) while satisfying its fresh air
requirements through a controlled ventilation strategy. Most existing
buildings, even those built recently, are far from being airtight and may fail
an air test and because of unwanted air infiltration (air leakage) generate
huge costs to owners and occupants, in environmental, financial and health
terms. One way of overcoming this making sure the building passes the air
test/air leakage
It is important to emphasise the distinction between
infiltration (air leakage) and ventilation, because while the primary purpose
of this document is to show how buildings can be designed and constructed to be
airtight and so pass the air test /air leakage test first time, it is equally
important to stress that good levels of ventilation and a clear ventilation
strategy will be required in every case. As the saying goes:
‘build tight, ventilate right.’
6.2
Why Build Airtight?
Legislation
At
a rather prosaic level, the issue is important because it is now part of the
Building Regulations in
Energy
and Cost Saving
Typically,
the largest heat losses in most buildings are related to levels of thermal
insulation, followed by those related to infiltration (air
leakage), followed by those related to inefficient plant. Quite rightly
therefore, most efforts to save energy and costs have until recently been directed
at increasing thermal insulation levels. But as these levels have risen, so the
relative contribution of infiltration (air leakage) has
increased to the point where it can represent around half of all heat loss in a
building. In highly insulated buildings, the percentage may be higher.
This
is reflected in the fact that total space heating costs in an airtight building
may be as much as 40% less than in a leaky one
We
are at the stage where it is likely that any further increase in thermal
insulation levels would be ineffective until levels of air tightness in
construction have improved considerably, this is where the air test in
invaluable
Space
Heating System Reduction
Clearly
there is potential to reduce the capacity of space heating systems sized to
cope with current levels of heat loss if those levels can be reduced by a half
or more. Ensuring you achieve a low pass rate during the air test
. In addition, airtight buildings are more predictable in terms of
environmental control and the capital cost savings of installing smaller
heating plant may be augmented by reduced plant room sizes in certain cases and
particularly by reduced running costs in the longer term. As well as reducing
the need for heating plant, airtight buildings offer much greater potential to
respond positively to the local external climate through passive, or climate
responsive design strategies such as natural ventilation, day lighting, the use of thermal mass and passive solar gain. Energy
savings, capital and running costs, along with CO2 emissions can
thus be further reduced.
Comfort
and Control
As
noted above, airtight buildings are not as affected by variations in external
conditions. This makes them easier to control from an Engineer or Designer’s
point of view, but it also makes them more comfortable from the point of view
of the occupant.
In
buildings with high levels of infiltration/air leakage those occupants near
draughty windows, for example, will suffer the cold, particularly on windy
days, whereas those elsewhere may well suffer from too much heat locally as the
system tries to raise the temperature overall. Those who try to achieve
comfortable levels through the use of the provided ventilation controls will
find these to be relatively ineffective, whereas in more airtight buildings
greater levels of control and comfort generally are achievable and local
control and variation by occupants can have a more direct effect. In one
example of an existing superstore, the ambient temperature in the store was
raised by 5oC after the store had been sealed after the air test . Complaints by occupants in leaky buildings are
common, and remedial measures are usually difficult and expensive, an air test can
finds all air leakage paths quickly and effectively
Deterioration
of Fabric
Leaky
buildings allow cold air in through the construction causing discomfort,
they also allow warm (and often moist) air out, causing heat loss. This warm
and often moist air can find itself in colder parts of the outer construction
where it can cool, and the moisture in the air can condense, leading to a
buildup of moisture. This in turn can lead to:
• decay of organic
materials such as timber frames
• saturation of
insulating materials thus reducing their insulative effect (which increases
heat loss further)
• corrosion of
metal components
• frost damage
where moisture has accumulated on the cold side of the insulation.
6.3 Legislation
In
Proposals for changes to the Energy standards were
issued to public consultation in March 2006, including guidance that air
tightness testing would be required if the calculation of energy performance
included air permeability rates lower than 10m3/m2h at 50 Pa.
6.4 Measurement
A range of units for measuring air tightness/air
leakage have been used in the past and this can complicate matters. However,
one method only – “air permeability” - is the measure used in European Standards,
the new editions of the various UK Building regulations and in CIBSE’s TM23 Air
Testing methodology and has been used throughout this document. The Air
Permeability is defined as the volume flow in cubic metres of air per hour per
square metre of the total building surface area (including the floor) at 50
Pascals pressure differential, expressed in m3/hr/m2 @ 50
The main difference between the air permeability and
previous practice in the
Of the range of measurements used previously, the
“Average Air Leakage Rate (or Index)” is similar to the “Air Permeability”
except that non-exposed floors are excluded from the measurement. Another
common expression is the “Air Changes per Hour at 50 Pascals (ACH @ 50 Pa).
This is a useful measurement in particular because, when divided by twenty, it
gives an approximate value of the natural infiltration rate of the building at
normal atmospheric pressure, which can then be used to help size heating and
ventilating plant etc.
Yet another measurement is the “Equivalent Air Leakage
Area” (ELA) at 50, 10 and/or 4 Pascals. This figure gives a representation of
the sum of all of the individual cracks, gaps and openings as a single orifice
and helps to visualise the scale of the air leakage problem. The main problem
of changing the measurement technique is the ability to compare data
The standard pressure differential used is 50 Pascals.
This is not in fact a very large pressure differential and corresponds to the
pressure exerted by a column of water 5mm high. Compared to the fact that
buildings can withstand wind induced pressures of at least 500 Pascals, this
seems insignificant, but it is larger than wind induced pressure on a calm day,
and by air testing and quoting air leakage figures at 50 Pascals, inaccuracies
are reduced and repeatability is improved using this air test method.
6.5 Targets
As noted above, the only ‘official’ guidance in the
A number of air tightness experts believe the stated
targets are inadequate when compared with the overwhelming need to address
carbon emission reductions, and the potential to do so through air tightness
measures. For example, the house illustrated to the right was built in 1992 for
the same cost as nearby houses and improved upon the standards noted above by
two thirds.
Building
Type Air Permeability (m3/hr/m2 at 50 Pa)
Good
Practice/ Best Practice
Dwellings
10.0/ 5.0
Dwellings
(with balanced mech. vent.) 5.0/ 3.0
Offices
(naturally ventilated) 7.0/ 3.5
Offices
(with balanced mech. vent.) 3.5/ 2.0
Superstores
3.0/ 1.5
Offices
(low energy) 3.5/ 2.0
Industrial
10.0/ 2.0
Museum
and Archival Storage 1.7/ 1.25
Cold
Storage 0.8/ 0.4
7.0
Designing for Airtightness
7.1 Key Principles
1. A Performance Specification is a crucial
document for establishing the appropriate targets for airtightness, along with
the methodology for achieving it, and the roles and responsibilities of those
involved.
2. Conceiving of a
building in zones and air barriers will help all involved to visualise the task.
3. Air barriers must be impermeable,
continuous, durable and accessible. They should be supported by positive
mechanical seals where possible.
4. The simplest solutions will be the most
buildable and durable.
5. A culture of airtight construction does
not yet prevail and until it does, it may be necessary to follow up targets
with specific details and specifications, along with guidance on the process of
implementing the necessary level of co-ordination and attention to detail.
Unlike design for deconstruction (the subject of the
first in this series of Guides) and the forthcoming guide on chemical-free
design, the design of airtight buildings cannot be left to the specification
and details, at least, not until the industry as a whole recognises the need
and has sufficiently widespread experience, unfortunately this alone would
surely end in an air test failure. For the next few years, it will be necessary
not only to provide careful details and performance specification, but also to
develop thorough inspection and testing regimes, hence the need for Chapters
7.4 and 7.5 of this guide.
7.2 Performance Specification
The Performance specification may be the only document
needed by the Architect / Designer / Client if the building is to be procured
through Design and Build or similar route. However, it is more likely to be
part of a suite of documents including detailed drawings.
The performance specification allows appropriate
targets to be set for the project, along with a description of how the process
is to be conducted, in terms of scheduling, audits and air testing, and
potentially remedial works. Given the increasing use of specialist
subcontractors, particularly in larger projects, it is also critical that the
performance specification sets out both the responsibility for, and
constructive guidance regarding the co-ordination of trades with respect to the
final air permeability of the completed envelope.
Zones and Barriers
Once appropriate targets have been set for the
project, the next task is to identify zones which require greater or lesser
airtightness
For example an industrial unit with office space is
divided into five separate zones, and air barriers are identified as required.
Such a drawing, however diagrammatic initially, helps to conceive of the
subsequent specification and detailing needs, giving an overview of the
problem.
Heated zones need to be kept separate from unheated
zones such as roof voids, delivery bays etc. whilst
service shafts may require particular attention. Boiler rooms with large flues
and intake vents may need to be separated prior to the air test
Entrances are often significant sources of draughts.
Lobbies with doors set apart by around 4m, so that one door closes before the
second is opened, can be effective, whereas in highly trafficked areas
revolving doors are likely to be preferable. Tall buildings, with atria,
stairways and service shafts all of which rise through the building can be
prone to ‘stack effect’ air movement whereby warm air rises, dragging in cooler
air from outside at the lower levels creating more acute air leakage problems.
A number of tactics may be employed to reduce the effects, but in any event
issues of airtightness are likely to be highlighted in these cases prior to the
air test
7.3 Design
With
the zones and air barriers located, it is necessary to design the air barriers
themselves.
To
be effective, the air barrier must:
• be made of suitably air impermeable
materials;
• be continuous around the envelope or zone
• have sufficient strength to withstand any
pressures created by wind, stack effect or air control systems
• be easily installed
• be durable
• be accessible for maintenance /
replacement if appropriate
The
last of these is important since there is evidence that the airtightness of
some constructions will tend to decrease over time and in particular the first
period after completion.
There
are a number of strategic measures which can be employed to simplify the
business of designing an airtight building. Since service penetrations in and
out of a building provide a major source of air leaks, one strategy is to
collect all such penetrations into one accessible area, this will drastically
improve the air test results
In
construction types such as steel and timber frame, it is usually wise to employ
a specific membrane or layer as the air barrier, rather than rely on sealant
between, for example, the sheathing boards. Such a membrane can usually double
up as the vapour barrier if used internally and gives the Designer the
opportunity to consider and address airtightness explicitly, rather than as a
function of other elements. Bear in mind that most membranes are flimsy and
will need support in all areas, although there is minimal air pressure during
the air test it can still move unsupported membranes this can result in an air
test failure.
Another
strategy is to employ service voids. Creating a service void internally allows
for alteration and maintenance of services and finishes without recourse to
penetrations through the air barrier. This allows for long term good
performance in contrast to membranes which are liable to penetration at all
service points, necessitating careful sealing of each and every penetration, in
the short term this will help to reduce the air leakage uk rate and vastly improve your chances
of an air test pass, not only initially, but over the years of alterations and
maintenance to come.
Generally, it is better to conceive of the joints in
airtight layers as ‘positively’ connected, anticipating differential movement
and decay of adhesive or chemical bonds. For example, where different
components of a curtain walling system are liable to differential movement, it
is clear that a joint whereby the two components are held together with a
positive mechanical connection across a compressed gasket is likely to remain
airtight longer that a simple butt joint with a mastic sealant between, this
attention to detail will improve your air leakage uk rate and the chances of an
initial air test pass.
Finally
it is clear that complex solutions to airtightness are likely to be more prone
to poor execution and potentially to greater vulnerability to differential
movement, failure of sealants, dislocation of
components and so on. It is important therefore to aim for the simplest
solutions to providing a robust airtight layer, using the fewest separate
materials, junctions and penetrations, and the easiest installation and
maintenance, this will improve the buildablity and improve the chances of a air
test /leakage pass.
It is worth making a point of considering each and
every specified component with regard not only to its own intrinsic
airtightness
The
following provide a few examples whereby airtightness can be simplified at the
earliest design stages.
However good the workmanship, blockwork on its own can
never be considered airtight. Once plastered, on the other hand, it may be considered extremely
airtight, with concern only for those edges and corners where cracking or gaps
can appear. This may be contrasted with the more common practice of drylining
block walls with plasterboard on battens or dabs, either way when either is
built correctly they can form a excellent air test barrier.
Design
& Detailing for Airtightness
Services Zones or Rooms enable a
range of services to be collected together before exiting the building,
allowing most of the penetrations in the external fabric to be grouped and
sealed effectively prior to the air test .
Service voids enable cables and pipework
to be installed and altered without needing to penetrate the air barrier. Note
however that if they are not run in conduit, protection may be needed against
subsequent fixings, if not undertaken prior to the air test this will result in
an air test failure
Positive physical connections are to be preferred over any
other joint such as one relying on adhesives. In the timber frame example shown
the air barrier membrane is shown lapped and sealed with mastic over a firm
background (boards with stud behind) and with a positive mechanical fix - a
batten - fixed over the top and through to the stud.
Trinsically
non-airtight block wall behind, this form of construction typically gives rise
to a wide range of air leakage uk paths behind the boards and into floor,
partition wall and ceiling cavities. From the perspective of airtightness, dry
lining should be avoided unless great care is taken, otherwise it will result
in a air test failure
Similarly,
timber floors are difficult to seal well without a good deal of care. On the
continent - and to an increasing extent in the UK at large - concrete floor
systems are being used for both ground and first floors (often for other
reasons such as acoustics, fire and the desire for underfloor heating) and
these are easier to make adequately airtight prior to the air test . Hollow
planks however can leak into cavities and require to be sealed at their ends,
this will dramatically improve air tightness and the chances of an air test pass
One important and often quoted example is the timber
first floor connection with a block wall inner leaf. Who is responsible for
ensuring absolute airtightness when the timber joists rest on the wall and are infilled
between with block and mortar? Presumably the bricklayer, but is it then his
fault if the timber is installed at the wrong moisture level and subsequently
twists and warps, leaving cracks around every joint? Is it really feasible to
attempt to tape or mastic seal around them all, and what if the underside of
the ceiling is to be exposed? Far better perhaps, to do away with the
joist-onto-wall detail altogether and replace with joist hangers.
Increasingly, the designer should be seeking solutions which are intrinsically
airtight because of the design, rather than continuing as before while accepting
an increased use of duck tape and mastic on site! Whilst these may get you
through the initial airtightness tests/air tests, they are short term solutions
and not likely to lead to the anticipated energy savings for the Client in the
long term.
A good review of the various materials and components
which allow the Designer to create an air barrier may be found in the
7.4 Detailed Specification
Beyond the performance specification illustrated
earlier, it is important that the issue of air tightness/air tightness testing
becomes embedded within the standard specification vocabulary.
Where an equal or approved alternative may be allowed,
it is critical that an airtightness performance specification is part and
parcel of that equality of performance. For example, it may no longer remain
satisfactory merely to specify a membrane, but in addition to specify the
fairly precise nature of the sealing, overlapping and potentially the
subsequent layers as well. Simply offering a performance specification and
ensuring the responsibility resides with the Contractor is all very well, but
it is important too to offer solutions that will enable a satisfactory outcome
to be achieved and subsequently improve air tightness
Design
& Detailing for Airtightness - Implementing Airtightness
In addition to the intrinsic lack of
airtightness
Timber joists built into a block wall
- a poor detail for airtightness. Far better to use joist
hangars and avoid the problem. Source. Concrete
planks are not free of problems either hollow planks
are often left ungrouted where they meet the external wall, which could lead to
extensive air leakage internally and subsequently an air test failure.
7.5
Implementing Airtightness
Key Principles
1.
The
Contractor or Project Manager must be made responsible for achieving the air
tightness levels set. In particular, this will involve careful co-ordination
between trades; if this doesn’t happen then an air test /air tightness failure
will surely follow
2.
Inspection
remains an integral part of achieving air tightness and passing the
3.
Ideally
at least 2 air tests (air tightness tests) will be undertaken; the first when
the building is weather tight, and the second air test
a couple of weeks or so before handover.
4.
Experience suggest that making one person (or team) responsible for air
tightness is the most effective way to tackle the issue, this will drastically
improve the chances of a air test pass.
5.
Remedial air tightness works to existing properties can reap substantial
benefits without undue disruption and improve the chances of an air test pass.
It is not yet generally possible within the
The responsibility of the Designer in regards to the
airtightness cannot be overestimated, for if airtight buildings are to become
main stream, as they are elsewhere in the world, the techniques must be above
all simple and buildable, with most if not all of the ‘tricky’ areas designed
out from the start. In this way, such techniques can become ‘second nature’ to
Contractors and there is less reliance on potentially adversarial inspection
and air test/air tightness testing failures.
Ideally too, the Designer will understand the issues
sufficient to prepare a sound performance specification – giving achievable
targets for air tests/airtightness as well as a clear description of responsibilities
and procedures, and a clear and practical set of overall and detail drawings,
along with a detailed specification.
In the meantime, and even with good documents, there
is likely to be a need for effort and vigilance by both the Design Team and the
Main Contractor or Project Manager on site. This chapter briefly describes this
effort, while the next describes in more detail the actual air test procedures
and auditing techniques used.
7.5 Plan of Work
The RIBA Plan of Work provides a framework
for the entire design and construction process. The table on the next page
allocates specific tasks relating to airtightness to each Work Stage to enable
a schedule of tasks and responsibilities for the Design team to be prepared
according to each project, this will drastically improve the chances
of a air test pass.
7.6 Roles and Responsibilities on Site
Designer / Design Team
The responsibilities of the Design Team are detailed
on the following page, showing all stages including site works and beyond. Buildings
usually comprise a number of different components, creating a myriad of routes
through which air can escape if not carefully sealed at each and every
junction. The Designer’s role is to simplify these details to reduce
difficulties on site.
It is critical that the purpose of pursuing air
tightness is explained so that all concerned understand why they are being asked to
attend to these issues. The initial briefing of key personnel at mobilisation
stage – whether or not this involves the air tightness specialist – is also
critical in determining the approach to conducting the works, inspection, air
tightness testing and auditing etc, prior to the air test . which will need to be dovetailed into the many other concerns
on site.
On large projects it may be useful for one member of
the Design Team to take special responsibility for the air tightness / air test
issues.
Contractor
The Main Contractor’s principal responsibility is to
deliver the air tightness/air test performance overall and the most likely
task on any but the smallest jobs will be that of co-ordination between the subcontractors.
The Main Contractor must be clear that he carries responsibility for the
overall air tightness and in turn must ensure that all subcontractors are clear
about the extent of their responsibilities. This is important since there may
be some deviation to conventional practice in order for air tightness to be
achieved and a air test pass. It is always prudent to
place someone in the site team who has experience of air leakage/air tests on
their previous projects as their experience may avert a potential air leakage/air
test failure
RIBA Work Stage Design Team Tasks
A Appraisal
Establish appropriate air permeability rate
B
Feasibility / Briefing Note Microclimate
Test existing buildings / building to be
refurbished
Identify procedure for review and air
testing
C Outline
proposals Consider a/t issues in relation to decisions about form of
construction
Identify zones and layers
D
Detailed Proposals Identify requirement of additional consultants / design by
specialists
E Final
Proposals Ensure co-ordination between DT to ensure a/t envelope &
penetrations
Detailed application of airtight materials,
junctions, service penetrations
F Production
Info Select sub-contractors for specialist works (incl. testing)
Careful specification of components,
membranes, materials
Emphasise methods for airtightness on
documentation
Careful specification of components, membranes,
materials
Emphasise responsibilities in specification for
dealing with ‘loose ends’ between sub-contractor interfaces
G Tender
Docum’n Define Contractors’ responsibilities for co-ordinating work sequences
H
Tender Action Ensure selected tenders include adequate airtightness procedures
J Mobilisation
Brief all involved in areas critical to air infiltration before work starts
Preparation of samples, training, testing
and QA procedures
K-L Site
Works Co-ordinate inspection with Building Control if required
Ensure inspection of areas to be covered
Ensure audits and testing schedule is
adhered to
Ensure design changes do not compromise
airtightness performance
M Post
Completion Obtain feedback from concerning comfort and energy consumption
Carry out remedial work as required at end
of
As with the Design Team, experience suggests that the
best (
For Contractors, the issues of airtightness/air
leakage uk and the passing of the air test are intimately linked to issues of
good or bad workmanship in general and this can make the issue both more
sensitive, but also more difficult to control. Even simple buildings are
immensely complex and so the most important aspect of all is the creation of an
overall culture of careful, tidy, accurate and airtight construction,
something which cannot be simply forced through with a performance
specification.
It is easier to specify and draw an airtight detail
than to build it, and so the emphasis on inspection and Contractor
responsibility has not developed from a prejudice against Contractors, but from
a realistic appreciation that this issue cannot be entirely resolved ‘on
paper.’ It is genuinely about a culture shift (at least for many in the
industry) and this is where the real challenge lies. Once this shift is adopted
there will be a far higher percentage of companies passing their air
leakage/air tests
7.7 Inspection
Air leakage
Of the background air leakage subsequently
investigated, the principal air leakage routes the greatest cause of air test failures were noted
as being:
• Plasterboard dry lining on
dabs or battens, often linked to routes behind skirtings
etc.
• Cracks and joints in the
main structure; open perpends, shrinkage & settlement cracks
• Joists penetrating
external walls, esp. inner leaf of cavity walls
• Timber floors, under
skirtings and between boards
• Internal stud walls, at
junctions with timber floors and ceilings
• Service entries and ducts
• Areas of unplastered
masonry walls; intermediate floors, behind baths, inside service ducts
.
It is perhaps worth mentioning that the
The following table lists
many of the most common infiltration problem areas. On larger projects, common
problems include:
·
Incomplete bulkheads at eaves;
·
Gaps where block work abuts to steel columns or beams
·
Uncapped cavity walls, at eaves (right) and mid-points
where cavity walls change to composite panels
·
Gaps along the underside of corrugated roof
linings
·
Gaps between block work and steel,
and uncapped cavity wall at join with composite panels Common Locations for
Inspection (Applicable to all types of Construction)
Foundation / Ground Floor
·
Check wall and floor dpcs form an adequate air
tightness layer, is a separate layer needed?
·
Check gaps at perimeter insulation strips
·
Check potential movement gaps between loadbearing
structure such as columns and adjacent non- loadbearing slab
First and Intermediate Floor Levels
·
Concrete floors: Check joint between the floor and
plasterboard to walls
·
Check gaps between concrete planks, or beam &
blocks are sealed at the wall
·
Check voids under floor finishes and service run
penetrations
·
Timber floors: Check a membrane seal has been
incorporated if required
·
Check any membrane used is supported between joists
Eaves and Verge
·
Check continuity of airtight layer between wall and
roof / ceiling
Ceiling level beneath the roof
·
Check for separation between deliberate roof
ventilation and the conditioned zone
·
Check for service penetrations and hatches which pass
across the airtight layer
Boundaries between different wall envelope
systems
·
Check all systems have a dedicated airtightness layer
assigned, and that these can be constructed to be continuous across dissimilar
elements
Windows and Doors
·
Check that the frame to wall junction is properly
sealed and continuous with the wall airtight layer, particularly at cills
·
Check the windows and doors have appropriate weather
seals between the opening unit and the frame
Services penetrations
·
Check for proper seals at service entry points, and at
points of entry into conditioned zones. These may also require fire protection
Main Entrances
·
Check that the whole entrance area is separated from
the conditioned zone by an inner airtight layer
Lift Shafts, Service Cores, Delivery Areas
/ Car Park
·
Check these have been separated from conditioned zones
with air barriers and draughtproofed access doors
Where profile fillers are
used poor workmanship is common
• Perforated (acoustic)
roofs, where the unsealed mineral fibre acoustic layer bridges the eaves of the
building, constituting a major leakage point
• Gaps where plasterboard or
wall linings are incomplete, commonly above suspended ceilings and to the
underside of beams
• Incomplete door and window
reveals
• Services Penetrations into
the building, and between zones inside the building
Another common issue is porous blockwork, particularly
when internal walls are drylined rather than plastered or painted. Where this
is likely to be unavoidable, it may be worth requiring blockwork to have an
initial air test for air permeability(air leakage), and to have an AP value (by
an accredited lab) that is no more than 50% of the target Air Permeability/air
leakage uk for the overall building.
7.8 Air Testing and Audit Schedule
In many cases to date, an air test /air tightness test
has been carried out a week or so before practical completion. If the result is
poor – a high rate of air leakage – then a great deal of work suddenly needs to
be done, often to areas which have been covered up and the whole business can
be both costly and time consuming, just at the point where in many contracts
there is already considerable pressure on Contractors.
Far better therefore to schedule the air test /air
tightness test
Ideally therefore, two Air tests/air tightness tests at least
should be carried out. The first Air test /air tightness test
.
In this way, the second and final air test /air tightness test
Such air tightness
It is often the case that the envelope is
not sufficiently complete on the due date for the air test /air tightness
testing
On larger projects, more air leakage
7.9 Remedial Airtightness Works
With airtightness testing and a general awareness of
airtightness
Either as a standalone measure or as part of a package
of energy efficiency measures generally, there is scope for remedial works to
most of the existing
As with thermal insulation, there is an extent to
which controlling some of the air leakage merely diverts the flow of air,
inward or outward, to another defect or gap, (this will still result in an air
test failure) but there is such scope for improvement that even fairly basic
efforts are likely to reap substantial environmental, financial and comfort
benefits for owners and occupiers alike.
There are many examples of remedial works described in
the various publications noted in the references. Some of the more successful
measures included carefully sealed secondary glazing installed where old
windows had to be kept for conservation purposes, draughtproofing of doors and
entranceways generally, and installation of lobbies in well trafficked
reception areas, attention to draughtproofing of existing windows and targeted
use of flexible sealants to ill fitting components and joints between different
construction types, will drastically improve your air leakage rate and will
should enable you to pass your air test first
time
8.
Testing Airtightness
Key Principles
1.
Air
test procedure is set out in CIBSE TM 23 and in BS EN 13829: 2001.
2.
An air
test / air tightness test
3.
Air tests
4.
Where
projects comprise large quantities of a single component, component testing in
the laboratory may be appropriate as well as on site element
8.1 Climatic conditions
As mentioned in Chapter 1, the raised pressure
differential of 50 Pascals created during an air test /air leakage test
Air tests uk/ air tightness tests uk require calm days
– i.e. a reading on the Beaufort Scale of 3 or less (3.4 to 5.4 metres per
second wind speed at 10 m above ground) This corresponds to a gentle breeze
with leaves and small twigs in constant motion. In winter conditions and on
exposed sites therefore testing may not be possible, although it is often
possible to make allowances, so long as these are carefully recorded, during
the air test .
8.2 The Air Test itself
Essentially the process is one of pressurising the
inside of the whole building (during the air test ) and of measuring the rate at which air needs
to be blown or sucked to maintain that pressure differential; a building
suffering large amounts of air leakage will equalise readily and require a
greater measurable effort to maintain the 50 Pascal differential, while a air
tight building will easily contain the enforced differential and require little
additional input during the air test , this will be easily recognisable
within the first couple of minutes during the air test / air tightness test
uk
The pressure difference is induced by one or more
calibrated fans that are normally mounted within a suitable doorway. An
adjustable door panel system, sealed around the edges is used which can also be
connected to large external fans via collapsible ductwork if required. The
rate of the fan, or the volume flow of air through the fan can be understood as
the rate of air entering / escaping throughout the remainder of the building
envelope, this is recorded throughout the
Buildings are air tested in such a way as to recreate
‘normal’ conditions. Doors and windows are closed,
trickle ventilators closed, extract fans and such like are closed but not
sealed. Internal doors are wedged open.
If the building is under construction the air test /air leakage testing uk is ideally
undertaken out of working hours, but sometimes this is not practical so some
scheduling of work needs to be thought through in advance. With all external
doors and windows sealed shut, some work becomes impossible (such as work with
solvents requiring ventilation) and internal trades are normally ‘sealed in’
for a short time, where they can carry on undisturbed during air test .
In existing buildings, air tests uk/air tightness
tests are normally carried out when the building is unoccupied if possible
because of the disruption.
8.3 Air Leakage Audits
The air test /air tightness test
In many cases a simple visual inspection may be
sufficient – especially if undertaken by someone with experience of the likely
locations of leakage, this is a good way of lowering the risk of a air test /
air tightness test
However, most leakage routes are difficult or
impossible to spot without visual aids. One common technique is to use smoke
tracers – smoke pencils or smoke machines. These render the air leakage paths
visible in certain situations during the
Another technique, which has certain advantages and
disadvantages compared to smoke tracing, is the use of an infrared camera by
undertaking a thermographic survey, Used either externally or internally, these
thermographic cameras register the radiant heat levels of surfaces and so are
able to ‘see’ for example, where cold air is cooling the fabric around a gap
internally, or conversely where warm air is escaping and heating the colder
materials on the external face.
To work effectively, there needs to be a recognisable
difference between the internal and external ambient temperature, so before
any heating has been installed and on a warm summer’s day thermography /
thermographic surveys may not be effective. Similarly on warm and sunny days,
sunshine on external surfaces can distort the true situation so it is better on
such days to wait until early evening. Conversely, rain on external surfaces
can be equally distorting of the true thermal situation. However, these cameras
are useful in identifying problems at high level or difficult to reach areas,
and are also very helpful in identifying other construction defects such as
poorly installed (or non-existent!) insulation within the fabric.
On larger commercial buildings, air tests
8.4 Component Testing
A
distinct aspect of overall air tightness testing is the individual component
Insitu
element
9. The
Details
Caveat
It
is important to emphasise the scope and purpose of the following drawings and
specifications.
They
are included solely to show practitioners the sort of alterations that can be
made in order to enable buildings to be much more airtight in general.
Their
purpose is not to offer approved details in any sense, but to illustrate the difference between details and specifications which do not
address airtightness issues, and those that do. It is the differences
between the originals and alternatives which is intended to be
illustrative, not necessarily the alternatives themselves.
The
original details have been taken from conventional details and specifications
we believe to be broadly representative of their construction types. We hope
the principles shown, and the specific references made will assist designers in
making similar changes in their own work, but it goes without saying that air test /air testing cannot take responsibility for any work undertaken as a
result of the use of these details.
Specifically,
these details are not intended to show best practice in any sense, nor are they
even intended to be up to date. We have striven in the preparation of these
details and specifications to keep as close to the original as possible. We
have done this in order to show that some quite fundamental alterations – in
terms of airtightness - may be made with the minimum of visual or functional
impact on the original. Where these original details and specifications do not
meet current standards or aspirations, the alternatives given are likely to be
similarly wanting. To re-iterate, the purpose is not to produce approved
details, but to illustrate the process of improvement – in terms of
airtightness only – that may be made.
Consideration
of priorities in airtightness design and specification is potentially
misleading since, in effect, all gaps, cracks or tears let in air and the
sealing of one simply redirects infiltration to somewhere else, this becomes
all the more apparent during the air test . Like thermal insulation, what is
important is the level of continuity generally, not any particular detail on
its own. Nonetheless some prioritisation has been attempted in order to help
Designers to prioritise their own efforts since not all measures may be
necessary.
9.1 Steel
Frame + Concrete Block Cavity Wall
Original Specification
Discussion
Because
of the largely wet trades involved, one might imagine a masonry construction is inherently more
airtight than the dry fixed timber frame and curtain walling construction
types. However, insofar as concrete inevitably shrinks as it dries, as mortar
beds and perpends are often poorly filled, and due to the differential movement
between masonry and the steel frame, the myriad pathways that open up can make
masonry buildings extremely susceptible to infiltration, this can lead to an air
test failure.
To
make things worse, construction such as this does not easily lend itself to a
simple, single airtight layer which can be applied separately and therefore the
need for vigilance, and some care and attention to a number of small but
potentially time consuming sealing jobs is high, however these must be
undertaken if you are to pass an air test on the first attempt.
It
would be possible to form an airtight layer internally through the use of an
applied membrane and the adoption of a service void. This would have the
advantage of allowing for changes in the service or fit-out provision without
the risk of damage of compromise of the airtight membrane,(this will lead to a
air test failure) and for those inclined to this solution.
A
parge coat and service void could have a similar effect, but the use of plaster
internally is a common and effective technique for creating an airtight layer
and is preferable in this instance as it is closer to the original detail and
will improve the overall air test results
HIGH PRIORITY
·
Wet Plaster Finish or Wet plaster coat costs more but
provides a better finish overall, as well as significantly improved
airtightness across the masonry leaf. Plaster should be extended to all wall
areas and not left off in areas which will not be seen,(
such as suspended ceilings.
·
Membranes Lapped & Sealed2 lines of tape and a
positive mechanical fixing by batten ensure laps are sealed for the long term
·
Mastic to Skirtings, Linings etc.
·
Critical in this detail since the plaster cannot form
a continuous layer at these junctions
·
Sealed Cavity Closer: Gaps around openings are common so
care is needed here to prevent infiltration around the frame and into the
cavity
·
Vapour Barrier Seal at Eaves: Important here since no
effective seal is noted on the original which could lead to excessive airflow
at this vulnerable point.
Costs
The
most significant cost implication is associated with the addition of the wet
plaster coat to the inner leaf of blockwork. This results in approximately a
60% increase in cost, although the quality of the blockwork is not as critical.
This item is also significant in that is changes the ‘look’ of the detail but
is probably the highest priority.
Otherwise,
most of the costs are associated with the additional time, effort and care
implicated within the specification and details. Of these, the most significant
is the additional labour and materials required for the joining of the vapour
barrier in the roof, and sealing it around the perimeter. This work almost
certainly more than doubles the cost of the vapour barrier in the original
detail, but again, represents a critical factor in reducing air leakage and saves
the cost of multiple air test failures.
A
number of the measures described represent no more than a re-iteration of good
practice, such as the sealing of perpends, lapping and sealing of membranes,
draught stripping of windows and so on. These may assumed to incur no cost
implication, but perhaps one of attention to details (this usually results in
first time air test passes) on site.
The
mastic sealant to skirtings, cills and the like would add about 50% to the
costs of these items, though these items represent only a small fraction of the
overall costs.
Taping
of the insulation boards would depend largely on the board type, but might
realistically attract only a marginal cost increase, as would the use of compressible
foam around the steelwork.
MEDIUM PRIORITY
·
Concrete Slab Floors:Concrete
slabs form an airtight layer but joints with penetrations such as perimeter
blockwork, insulation or structural columns must be sealed.
·
Cill to Window Sealing: Double sealed detail which increases
the chance of securing an airtight seal at this often overlooked junction
·
Compressible Foam between Steel and Blockwork: Potential solution to the
inevitable gap which will form here, also sealable with mastic oninside face
only.
·
Perpends Fully Filled: Not critical if a wet plaster finish
is applied internally, but high priority if they are not.
·
T&G and Taped Insulation:Not
technically part of the airtight layer, but gaps here simply increase the
likelihood of infiltration (air leakage) and are relatively easily sealed.
·
Expanding Foam to Gap at Eaves: Not part of the airtight
layer but by sealing a large gap in the fabric, this reduces the wind pressure
driven airflow within the cavity thus reducing the risk of infiltration (air
leakage) indirectly.
9.2 Index
a) Perpends fully filled
A common problem with blockwork and
brickwork buildings is that perpends are not completely
filled and this leads to air flow (air leakage) through the wall this becomes
apparent during the
b) Blockwork Maximum Air Permeability by
Component Test
An alternative to wet plastering the blockwork on the
inner leaf is to require a component air test of the blockwork to satisfy a
maximum air permeability of, say, 5m3/hr/m2 or
less. On larger
projects, or where wet plastering is unlikely to be
effective or desirable, this is one method of
ensuring a reasonable degree of airtightness from the
blockwork leaf. These conditions may also be used for the outer leaf but is not
as important because it is the inner leaf which is providing the main air
barrier for the
c) Membrane Lapped and Sealed
Typically membranes are lapped and stapled
or tacked, but in order to create airtight layers, it is
important that these laps are rigorously sealed.
Best practice in this regard - beyond the correct
use of Manufacturers’ overlap dimensions,
proprietary tapes and other accessories - is to run a
layer of double sided tape between the membranes
at the overlap and run a tape over the
leading edge of the outer sheet. In addition,
since many tapes tend not to last too well, it is
advisable to ensure that laps are made directly over
supported areas (i.e. with studs or dwangs
directly behind) and are held down positively with
battens fixed through forming a mechanically
tight, as well as an adhesive seal, this will provide an
especially strong air tightness seal and will improve the chances of an air
test pass
d) T&G or Shiplap and Taped Non-Mineral
Fibre Insulation
Mineral fibre is permeable to air movement
and cannot be counted upon to help in reducing air
leakage. Extruded polystyrene and other closed
cell plastic insulation materials do not suffer from
this and so have the potential to reduce air
leakage and improve air tightness in and out of the
building(improving the chance of passing the
e) Wet Plaster Finish Internally
An alternative to arranging component tests
for the blockwork. A simple block finish with 2 coats
of paint which in terms of airtightness is an
improvement on a uncoated block wall but is not
sufficient to consider the blockwork airtight in the
least. Wet plastering of the blockwork is more
expensive but ensures an airtight masonry leaf, this will
improve the chances of a
An alternative which would have a similar
effect would be to use a parge coat over the blockwork,
before application of a service void and separate
finish layer. The simple wet plaster finish is
closer to the original.
f) Mastic Both Edges to Skirtings, Reveal
Linings etc.
Where the corner junction behind has been
carefully sealed then this measure may not be
required, but in the examples shown on this
construction, this particular detail is critical since it
forms an integral part of the airtight layer,
particularly where the plaster has to be discontinuous.
g) Concrete Slab Floors
Unchanged from the original detail, this is simply to
note that concrete slabs form an airtight
barrier and may therefore be considered good practice in this
regard. However, no note is made
of the need for care to be taken where the slab meets
elements of structure which pass through,
steel columns, for example. At these junctions, a
compressible foam strip may be laid around the
steel prior to pouring the concrete if practicable, or a
mastic sealant may be used subsequent to
the pour to seal the inevitable shrinkage cracks which will form and
become air leakage
h) Insulated and Robust Cavity Closer
A robust and insulated cavity closer
enables the cavity to be effectively closed, the gap to be
bridged with insulation without risk of moisture
flow between inner and outer and the window to
be securely fixed at the head and jambs if
required. The gap between window frames and the
main wall is a notorious place for infiltration (air leakage
i) Proprietary Cill with Foam Sealant
Internally and Mastic Sealant Externally
In addition to the mechanical fixing of the
window frame through the cill piece, it is important that
this fixing is made through a compressible foam
strip which is then sealed against air leakage
from outside with a mastic type sealant. This
gives the Contractors two opportunities to ensure a
completely airtight seal at this particularly
vulnerable point, prior to the
k) Compressible Foam Strip beneath Steel
Beam to Blockwork Top
For reasons of both initial shrinkage and
subsequent structural movement, it is to be expected
that a direct connection between a steel beam
(or column) and a block wall will open up over time
to form a potential route for infiltration
(air leakage). One way to try and reduce this inevitable gap
is to build the blockwork against a
compressible foam strip which immediately expands to fill the
gap between and remains flexible thus
continuing to fill the gap even after shrinkage and
movement. Since compressible foam strips are not
intrinsically airtight, mastic sealant should be
used in addition to form a neat internal joint
which should further seal the connection, thus
drastically improving air tightness
l) Vapour Barrier Detail at Eaves
Here the vapour barrier is positively
sealed to the steel perimeter beam to properly seal the
ceiling vapour - and air - barrier along its edge.
Assuming that the steel beam is without
penetrations (a specification note has been added to
ensure that this is checked) then as long as
the plaster seal to the underside of the beam
is adequate, an airtight layer has been formed
which may be discontinuous in materials but continuous in
terms of airtightness
m) Expanding Foam to Large Gap at Eaves
Whilst not strictly part of the airtight
layer, this measure reduces the potential wind pressures on
the cavity which in turn reduces the risk of air
infiltration through the airtight layer itself. Note also the introduction of
a ply layer above to support the insulation (nothing is noted as doing so in
the
original detail) but significantly against which
the foam can create a firm seal which should
drastically improve air tightness
n) Mastic Sealant to Joints
Additional notes to seal connections
between dissimilar materials which are likely to provide
routeways for airflow (air leakage) unless
conscientiously sealed.
o) Draughtstripping to Windows and Doors
Most commercially available joinery, metal
or plastic windows and doors will be adequately
draughtstripped but it is important to explicitly ensure that
this is the case, and that seals
(preferably
tubular rubber / epdm type) are accessible and can be easily replaced should
they
begin to fail to adequately seal when closed.
9.3 Timber
Frame with Concrete Block Outer Leaf
Original Specification
Discussion
Despite
the inherently dry fixed nature of timber frame construction, it offers good
opportunities to ensure airtightness
Although
there are a large number of small adjustments to conventional practice
outlined, none of these, except perhaps the addition of the service void and
backing board involve any major shift in construction process. Experience
suggests that such changes are readily made and subsumed within the standard
details and specification clauses of the practice.
More tricky is the need to convey the need for greater effort,
co-ordination, care and vigilance to Contractors for whom there is little to be
gained from the good practice noted, and quite a lot to be lost in terms of
potentially time consuming additional tasks. In the short term it is important
to emphasise the additional co-ordination and tasks to Contractors at the time
of tendering so that these are not overlooked and the extra effort can be
adequately assessed.
HIGH PRIORITY
a) Continuity of Layer / Co-ordination of Trades
General measures to ensure tradesmen are aware of the
need for air tightness that all involved are conscientious and rigorous, and
that someone is responsible for co-ordination between trades prior to the air test
b) Service Void
Use of a service void means most if not all
penetrations through the vapour control and airtight layer can be avoided.
c) Joist Hangers
Use of Joist hangers avoids the common problems of air
infiltration where joists are built into the inner leaf
d) Membrane to Floor Perimeter Beams
Slightly awkward solution for solving the
problems of discontinuity at this area which is nearly impossible to solve
otherwise.
e) Flexible Foam around Joinery
Gaps around openings are common and neat, effective
solutions can be difficult, careful use of flexible foam enables effective and
durable seals to be formed.
f) Continuous Layer Over
Partitions
High priority because of the high potential
exfiltration rates and condensation risks at this point
g) Backing Boards
Use of backing boards makes installation of the
membrane easy and thus less prone to poor workmanship and subsequent air test failure.
MEDIUM PRIORITY
a) Membranes Lapped & Sealed
2 lines of tape and a positive mechanical fixing by
batten ensure laps are sealed for the long term
Costs
Not
surprisingly, the addition of the service voids adds considerably to the costs
of both the walls and ceilings. Of course, such costs say nothing of the
increased ease of services installation, nor of the long term benefits of a
much greater access for upgrading and alterations.
Nonetheless,
the addition of the OSB and battens forming the service void in the walls adds
approximately 35% to the cost of the external wall. Mechanically fixing the
vapour barrier to the floor and taping would add approximately 4% to the
overall wall cost in addition.
Adding
the service void to the ceiling would represent an approximate 130% increase in
cost over just the 2 layers of plasterboard. But again, services installation
would be easier.
The
additional work associated with the breather membrane would incur a similar
additional cost, but may not be a priority if the internal vapour barrier is
well installed.
The
mastic sealing of the skirting boards would increase the cost of their
installation by about 50%, although these represent only small costs overall,
the use of polythene strips at the floor and eaves, and the use of foam around
the windows would attract only a marginal cost increase.
The
use of flexible insulation need not attract any increase in cost if a common,
economical type was chosen. Remember all of the above can be far more
economical that not passing the air test and therefore resulting in costly
LED’s
MEDIUM PRIORITY
a) Joinery Draughtstripping
Tubular seals are probably the best option.it is
important that they can be easily accessed for maintenance and replacement.
b) Continuity at Openings
Continuity between the framing sealant (m) and the
membrane can be tricky and care is needed to ensure a good, durable seal.
c) Seal Loft Hatches
Unsealed loft hatches may contribute to air leakage,
so worth some care.
d) Plasterboard Penetrations
If the airtight layers are sound then this should not
matter, but still worth attention.
e) Flexible Not Rigid Insulation
Flexible Insulation provides a better fill between
studs, rafters etc.
a) Continuity Behind Lintols
An extra strip of membrane to form a continuous
layer when the main one is lapped over the cavity barrier, also fill behind
lintol.
b) Mastic to Skirting’s, Linings, Cornices
Not necessary if the airtight layer is sound
c) Air Barrier to Ceiling
High Priority in separating floors
d) Laying Tape to Plasterboard Junctions
e) Wall Tie Fixings
f) Top Runner Strip Seal
g) Airtight Service Boxes
h) Corrosion Resistant Fixings
9.3
Index
e) Wall Tie Fixings to Timber Frame
The breather membrane is not the main air barrier, but it is
nonetheless a useful ally in reducing air leakage
(b) Use of Corrosion Resistant Staples or Fixings
Non-corrosion resistant fixings to external breather membrane can
corrode to a point where they fail, allowing the membrane to come loose, often
creating a small hole in the membrane and reducing the effectiveness of the
membrane as an airtight layer, this will allow for air leakage uk and a
probable air test failure. Copper is non-corrosive but can affect polyethylene,
whereas stainless steel has no effect on polyethlene and so should be
preferred.
(c) Membranes to be Lapped and Sealed
Typically both internal and external membranes are lapped and stapled
or tacked, but in order to create airtight layers, it is important that these
laps are rigorously sealed, this will ensure a air
test pass. Best practice in this regard - beyond the correct use of
Manufacturers’ overlap dimensions, proprietary tapes and other accessories -
is to run a layer of double sided tape between the membranes at the overlap and
run a tape over the leading edge of the outer sheet. In addition, since many
tapes tend not to last too well, it is advisable to ensure that laps are made
directly over supported areas (i.e. with studs directly behind) and are held
down positively with battens fixed through forming a mechanically tight, as
well as an adhesive seal. This requires consideration of lap positions early on
if extra framing or subsequent battening is needed.
(d) Ensure Continuity of Membrane / Co-ordination of Trades
Whilst this is easy to achieve across large, flat areas, it is more
difficult at the many awkward angles, junctions, corners and so on a typical
site. There is no specific guidance except to ensure that those responsible for
installation of the membrane are rigorous and conscientious in their attention
to all of the inevitable nooks and crannies, and that the person responsible
for co-ordination is equally attentive, particularly when the junctions are
between separate forms of joint and separate trades. Having someone experienced
on previous projects that required air tightness uk
tests is a huge advantage and dramatically improves the chance of a air test pass
(e) Ensure Membrane is taken into Opening Reveals, Taped and Sealed and
Made Continuous with Opening Seals
it is typical at openings in timber frame buildings to allow the
membrane to run across the opening initially, then form a star cut into the
opening, folding over the sections of membrane and trimming as necessary. In
these cases, there are inevitable gaps in the airtight layer at the corners of
the opening, and it is important to ensure that these are made good before subsequent installation of joinery etc and the
following
(f) Fix Airtight Membranes to Firm Backing Boards
In conventional timber frame construction, vapour barriers are fixed
across studwork, usually after the installation of insulation and prior to the
fixing of the internal lining. Equally external breather membranes are
sometimes installed across gaps between rafters or studs. In both cases
membranes are susceptible to pressures from both sides, leading to the membrane
breaking free of its fixing and creating holes in the airtight layer. Ideally,
membranes should be fixed against a firm backing board by way of protection
against damage of this nature, this in turn will improve the chances of passing
the initial air test .
(g) Service Void
The principal advantage of a service void is related to functionality
and maintenance over time, but a secondary advantage which relates directly to
airtightness uk is that since all services may be incorporated within, that
is, on the inside of the vapour control layer, there is no need to penetrate
the layer at each and every service installation, thus significantly cutting
down on the myriad potential gaps that are typically formed and either left, as
this will surely lead to a air test failure or made good which is time
consuming and costly.
(h) Laying Tape at Plasterboard Junctions
Using laying tape at junctions makes the formation of an airtight
junction both conscious and relatively easy, even allowing for subsequent
shrinkage and cracking of the skim layer.
(i) Airtight Service Boxes
Developed in Canada where airtight construction is more advanced, these
service boxes are fitted with gaskets and a flange surround allowing for an
airtight seal at all openings in the lining.
(j) Mastic Both Edges to
Skirtings, Reveal Linings, Cornices etc.
Where the corner junction behind has been carefully sealed then this
measure may not be required. In addition to the nail or screw fixing, a mastic
seal both edges aid’s efforts to guard against infiltration/air leakage (which
will increase the chances of passing a
(k) Ensure Continuity of Membrane behind and around Lintols
It is likely that to achieve this requires two separate measures. First
the breather membrane needs to be continuous and extend into the opening, thus
a second strip should be affixed to the wall and lapped and sealed to the main
membrane which must lap over the lintol or cavity barrier etc. Second, it is
likely that gaps could form between the top, outer edge of the joinery and the
lower, inner edge of the lintol, leading to a cavity behind the lintol. This
cavity should be filled with expanding foam or mineral wool and if possible the
gap filled, probably with a mastic sealant, this will drastically improve the
chances of passing your air test at the first attempt
(l) Flexible Foam Sealant around Joinery Insertions
Gaps around openings are one of the most common of infiltration paths.
They range from 0 to 20mm, which is too large to be filled by mastic.
Compressible flexible foams are ideal for this application. Ensure that the
airtight membrane meets the seal on both sides to maintain the airtight layer
overall, and subsequently pass the
(m) Draughtstripping of Openings in Joinery
Draughtstripping of joinery comes in many forms. It appears that
synthetic rubber or elastomeric tubular seals work well, creating good seals
with minimal compression, depending on the size of the gap. It is important
that seals are unaffected by paintwork and subsequent decoration, or are easily
accessible and removable. This is important so that seals can be replaced as
they start to fail to maintain the airtight layer.
(n) Seal all Penetrations in Plasterboard / Internal Lining
Even with the use of airtight outlet boxes there will be inevitable
penetrations such as ceiling pendants, pull cords, recessed fittings etc. which
must be made good manually, typically with mastic, otherwise you may end up
failing your air test
(o) Seal Loft Hatches
Generally, this involves a continuous bead of mastic to the underside
flange, and, depending on the design, the use of compressed and flexible foam,
or mineral fibre etc. above. Please note in our experience this is a common
area for air leakage, and a major cause of air test failures
(p) Use of Joist Hangars as Opposed to Built-in Joists
The original specification here is already good practice, that is, the
use of joist hangars which sidestep the problems of joist movement and
shrinkage allowing infiltration and airflow within the floor voids, another
major cause of air test failures
(q) Membrane Strip to Inner Face of Floor perimeter Beams
100 gauge polythene or similar fixed to the inner face of the perimeter
beams early on in the framing process can lapped and sealed to the internal
vapour control layer typically installed a good deal later, so that a
continuous internal vapour control and airtight layer may be effectively
created.
(r) Continuity of Membrane to Ceiling over Partition Walls
ideally this would comprise a continuous membrane affixed
before the partitions are installed. However it is more likely that partitions
are installed before, therefore such a layer would require strips to be fixed
to the partition top runners to be later lapped and sealed to the ceiling
vapour control layer.
(s) Flexible, Rather than Rigid Insulation
Rigid insulation between joists, studs or trusses generally has to be
cut to fit and this is never 100% accurate, leading to myriad gaps and routes
for airflow. Flexible insulation avoids this problem and improves the chances
of passing your air test at the first attempt.
(t) Top Runner Strip Seal
The use of this strip, lapped and sealed with subsequent membranes both
sides prevents air infiltration into the wall itself from the ventilated eaves
area, thus ensuring continuity of the airtight layer, which should help you to
achieve an air test
(t) Air Barrier to Ceilings
In ceilings within dwellings of the same occupancy, this is unlikely to
be useful, but in separating floors, it is extremely important that an air
barrier is included in the floor and ceiling make-up. Noted
here by way of a reminder.
9.4 Steel Frame + Glazed
Façade
Discussion
It
is important to be confident that the curtain walling manufacturer, supplier
and installers all share an explicit commitment to producing an airtight wall
overall, as it will be very difficult for the Main Contractor to ensure a
continuous airtight fabric if this element is not firmly ‘tied down’ before the
start on site, this is one of the main causes for air test failure
The
focus of concern then falls to all the various corers and perimeters where the
system meets other construction elements and here both Designer and Contractor
need to have carefully considered in detail each occurrence and made adequate
provision, to avoid large amounts of ad hoc remedial work, during the
The
roof membrane must be carefully sealed and the perimeter condition considered
so that a continuous and positive connection can be made. Note this is another
major cause of air test failure
HIGH PRIORITY
(a) Curtain Walling Performance Spec.
Since this represents the largest area exposed to wind
it is important that the performance specification is adequate and that the
components are conscientiously installed
(b) Mastic Perimeter Seals
With the main curtain walling components installed and
airtight, the next most significant air leakage route is likely to be the perimeter
seals. Both mastic and membrane seals are valuable in this regard. Note this is another major cause of air
test failure
(c) Membrane Perimeter Seals
With the main curtain walling components installed and
airtight, the next most significant air leakage route is likely to be the perimeter
seals. Both mastic and membrane seals are valuable in this regard. Note as above this is another major cause
of air test failure
(e) Roof Membrane Sealing
Any leakage in the roof membrane or at the roof / wall
junction could be serious in terms of both energy waste and risk of moisture
related damage to the roof build-up, so this detail is important. By properly
preparing for your air test this will alleviate any of these future problems
MEDIUM PRIORITY
(h) Plates Added to Beam
Because of the difficulty in forming an adequate seal
to protruding beams, this is likely to be a major source of air leakage in the
long term so designed, rather than ad hoc site measures to reduce air
infiltration are important.
(f) T&G and Taped Insulation
Potentially a minor issue, but given higher priority
becasue it is relatively easy to solve and reduce airtightness/air test
Costs
It
is difficult to ascertain any meaningful cost implications with this detail
because of the variety of curtain walling systems available.
The
measures outlined are fairly standard in most installations and should in all
cases represent no more than a re-iteration of good or best practice. However,
they could attract an additional cost where one particular system did not
address airtightness and the subsequent air test in one way or another.
Measures
such as the additional efforts associated with air barriers at the separating
floor, eaves and flor / wall junction might attract additional costs over that
aspect of the original detail by approximately 30% largely because of the
additional labour and attention required. However these costs are still cheaper
than suffering costly LED’s for not passing the
(g) Membrane Seal between Floors
The existing detail should provide a reasonable
degree of airtightness, but this measure will make the task conscious and
affect a greater degree of separation.
(d) Foam Filler
Should not be required if the measures in (b) and (c)
are completed, but an additional measure that also has value in providing a
backing to a continuous mastic seal internally.
9.4 Index
(a) Airtight Performance Specification for Curtain Walling
The de facto standard for curtain walling air permeability that most
curtain walling manufacturers comply with is the CWCT (Centre for window and
cladding technology) ‘Standard and Guide to Good Practice for Curtain Walling’.
This specifies a maximum air permeability of 1.5 m3 / hour/m2 @600pascals for
an area of fixed glazing, and 2m3/hr/linear metre of joint for opening panels.
This is the same as the British Standard BS EN 12152:2002, category A4.
However, the BS has a further category, AE that achieves 1.5m3 /hour /m2 at a
pressure differential of more than 600pascals. Specification of this
‘exceptional category may be possible but it may mean a reduction in choice as
this is a more stringent level of
Maximising airtightness can be done by having vulcanised welded joints
to gaskets within the curtain wall frame, instead of usual mitred ones. This
should ensure that the unit its self is airtight, although it is an expensive
option, it will help you to pass the
(b) Mastic Bedded Fixings
Where membranes and components are connected, it is often possible for
thin - and often more or less invisible gaps to be left between the joint. A
continuous mastic seal used along the line of any such mechanical fixing
ensures that any minor cracks like this are completely sealed.
(c) Additional Membrane Seal at Junction
Some Manufacturers (eg Schuco) supply as part of their system an EPDM
perimeter gasket seal that should be tied into vertical DPM. Angle at jambs and
loose dpm to wrap ensure good seal with EPDM. This is
a particularly good way to ensure airtightness and the chances of a air test pass. At these critical junctions because it
requires a conscious task (sealing the membrane) to ensure all ‘loose ends’ are
firmly fixed, as opposed to leaving the airtightness to be achieved through the
use of applied sealants.
(d) Foam Filler to Internal Joint
Assuming that the seal mentioned above is installed correctly this
should not be required, but such a seal acts as an additional check against air
leakage uk and could be used as a backing strip against which to seal a
continuous mastic seal internally.
(e) Membranes to be Lapped and Sealed
Best practice in this regard - beyond the correct use of Manufacturers’
overlap dimensions, proprietary tapes and other accessories - is to run a layer
of double sided tape between the membranes at the overlap and run a tape over
the leading edge of the outer sheet. In addition, since many tapes tend not to
last too well, it is advisable to ensure that laps are made directly over
supported areas (i.e. with solid materials directly behind) and are held down
positively with battens fixed through, or some other ‘positive’ connection
forming a mechanically tight, as well as an adhesive seal. This may require
consideration of lap positions early on.
(f) T&G Jointed and Taped Rigid Insulation
Butt jointed insulation, even if installed firmly may be subject to
movement during the course of construction and over time, and is unlikely to
offer a continuous insulation layer in the long term. Using t&g slabs
overcomes some of this problem and taping the slabs ensures that air leakage
paths cannot form between the minute, but inevitable cracks between the units,
which can later lead to air test failure
(g) Additional Airtight Membrane
It is likely that the fireproof stopping will not be
able to create an adequately airtight seal and so this measure ensures the task
is performed consciously. Using a simple polythene membrane and forming
positive connections to the underside of the slab and the top of the curtain
walling ensures an airtight seal between floors, and at the vulnerable
connection of curtain walling to spandrel panels. One of the main causes of air
test failures
(h) Localised Welding of Plates
of Beam
It is practically very difficult to form an airtight seal perpendicular
to an ‘I’ beam or similar, expanding foam tends to be used because no ‘built’
connection appears workable, nor cost effective. Such
ad hoc seals are unlikely to last in the long term.
Ideally plates should be welded to the beam such that
there is no air route along the length of the beam (a plate welded
perpendicular to the web and extending between the two flanges) and such that
airtight seals are easily formed around the beam as it passes the airtight
layer. Side plates fixed between flanges form a sort of localised rectangular
section which is more easily sealed. This makes the task more readily achieved
on site, and more durable in the long term
9.5 Refurbishment of
Discussion
If
the existing masonry fabric of a refurbished building is in good condition, it
is potentially simple to render it relatively airtight if the details proposed
- particularly the use of service voids - are followed. All the work can be
carried out internally and is simple to install and check and will drastically
improve the chances of a air test pass
In
addition there is no cavity in this form of construction and this means there
are fewer opportunities for undetected airways.
It
goes without saying that any cracks or damage to the existing fabric should be
made good before installation of the internal frame, otherwise this may help
lead to a air test failure
If
there is enough space, it might be best to retain all existing lath and plaster
on ceilings and walls, ensure that it is effectively sealed, and work inwards
from there. Experience suggests that lath and plaster itself is fairly airtight
and removing it merely creates more waste. One potential disadvantage is that
in keeping the existing plaster, it may not be possible to access the gaps
behind which may run into floor voids and partitions creating air leakage
A
number of reviewers of this Guide commented that it is more common to maintain
a cavity between the existing wall and any new-build internal leaf. The
alternative proposed keeps to the same format as the original, but the
advantages of the use of a cavity are well understood.
HIGH PRIORITY
(a) Membranes Lapped & Sealed
With only one membrane to ensure airtightness it is
crucial that laps and junctions are conscientiously sealed.
(b) Service Void
Use of a service void means most if not all
penetrations through the vapour control and airtight layer can be avoided.
(c) Joinery Edge Sealing Batten
If the membrane generally is well sealed, the only
other major area for air infiltration
(d) Joinery Draughtstripping
A particular issue with sash and case
windows. It is important that seals can be easily accessed for maintenance and
replacement.
MEDIUM PRIORITY
(e) Flexible Foam around Joinery
Gaps around openings are common and neat, effective
solutions can be difficult, careful use of flexible foam enables effective and
durable seals to be formed. If this can be effectively achieved with the sash
and case window then (f) is not necessary.
(f) Continuity at Openings
Continuity between the framing sealant (m) and the
membrane can be tricky and care is needed to ensure a good, durable seal.
(g) Backing Boards
Use of backing boards makes installation of the
membrane easy and thus less prone to poor workmanship and subsequent failure.
Costs
The
retention of the ceiling lath and plaster saves approximately 24% of the costs
of that element, while the addition of the service void and vapour check
represents a 18% cost increase, thus, without the addition of the breather
membrane over the ceiling joists (a medium priority measure) there is a cost
saving to complement the increase in ease and cost of services installation.
The
breather membrane represents a 13% increase in cost and therefore tips the
balance of the ceiling cost overall. However this is still cheaper than
repercussions due to failing your
The
addition of the OSB backing board and service void to the walls constitutes
around a 46% increase in cost of the wall, the majority of which (33%) is made
up by the OSB, so perhaps a cheaper, yet firm backing board might alleviate
the cost burden. The addition of the OSB layer to form a service void beneath
the floor boards would add approximately one third to the cost of the original
detail. Again this is still cheaper than repercussions due to failing your
Double
mastic sealing of the skirting boards adds approximately 50% to their
installation cost, although their overall costs are small in the overall
picture.The sealing of the vapour control layer above and below the
intermediate floor should not attract any additional cost if assumed to be part
of a standard, if careful installation. Measures to help seal around the window
would add marginally to a standard installation cost.
MEDIUM PRIORITY
(a) Batten Seal at Corners
A version of (b) but worth particular mention
as these junctions are particularly important to seal well.
(b) Keep Existing Lath and Plaster
Really a version of (e) except in this case we suggest
retaining the existing firm base of lath and plaster against which to affix the
vapour control layer.
(c) Plasterboard Penetrations
If the airtight layers are sound then this should not
matter, but still worth attention.
(d) Membrane Over Roof
Insulation
Protects installed insulation from disruption and
provides a secondary layer at this important area.
(a) Silicone to Joinery Externally
Should be standard practice, but forms
useful role in airtightness as well as weatherproofing.
(b) Mastic to Skirtings, Linings, Cornices
Not necessary if the airtight layer is sound but worth
attention in these examples.
9.6
Index
(a) Breather Membrane over Insulation
In well ventilated loft areas, loose insulation may become dislodged by
air movement. This precautionary measure ensures that the initial fully
fitting installation of batts against joists etc is maintained over time,
reduces dirt and debris entering and provides an additional airtight layer
(which is useful since the loft is ventilated) whilst allowing for vapour
egress into the ventilated space.
(b) Membranes to be Lapped and Sealed
In order to create airtight layers, it is important that laps are
rigorously sealed. Best practice in this regard - beyond the correct use of
Manufacturers’ overlap dimensions, proprietary tapes and other accessories - is
to run a layer of double sided tape between the membranes at the overlap and
run a tape over the leading edge of the outer sheet. In addition, since many
tapes tend not to last too well, it is advisable to ensure that laps are made
directly over supported areas (i.e. with studs or dwangs directly behind) and
are held down positively with battens fixed through forming a mechanically
tight, as well as an adhesive seal. This requires consideration of lap
positions early on if extra framing or subsequent battening is needed.
(c) Vapour Control Layer over Existing Lath and Plaster
Rather than remove the existing lath and plaster ceiling, this detail
saves a little money, time and resources by reusing the existing ceiling as a
backing to the installation of the vapour control layer (refer also (e))
Plaster need not be repaired if damage is localised and does not threaten the
integrity of the vapour control layer.
(d) Service Void
The principal advantage of a service void is related to functionality
and maintenance of services over time, but a secondary advantage which relates
directly to airtightness is that since all services may be incorporated within,
that is, on the inside of the vapour control layer, there is no need to
penetrate the layer at each and every service installation, thus significantly
cutting down on the myriad potential gaps that are typically formed and either
left, or made good which is time consuming and costly, and could lead to a air
test failure
(e) Fix Airtight Membranes to Firm Backing Boards
In many situations membranes required for vapour control and
airtightness are installed unsupported and are thus susceptible to pressures
from both sides, leading to the membrane breaking free of its fixing and
creating holes in the airtight layer. Ideally, membranes should be fixed
against a firm backing board by way of protection against damage of this
nature.
(f) Mastic Both Edges to Skirtings, Reveal Linings etc.
Where the corner junction behind has been carefully sealed then this
measure may not be required, In the examples shown on this construction, this
particular detail is not critical but is nonetheless valuable in helping to
ensure a good seal at all points.
(g) Airtight Layer Taken Behind Batten at Corners
As noted in (b) above, the best airtight seal is a positive and
mechanical one such as shown here whereby at corners and edges, a membrane is
not only lapped and taped against the adjoining surface, but held firm by a
batten fixed through. This overcomes any potential adhesive failures or tears
in staples or tacks etc. In the ceiling junction where the plasterboard layer
must be continuous for reasons of fire spread prevention, it is also advisable
to install laying tape at the junction between the plasterboard and the wall to
ensure an airtight seal here also, thus improving the chances of a air test pass
(h) Seal all Penetrations in Plasterboard / Internal Lining
Even with the use of airtight outlet boxes there will be inevitable
penetrations such as ceiling pendants, pull cords, recessed fittings etc.
which must be made good manually, typically with mastic, and in this case, with
a suitably fireproof mastic to maintain the fire barrier.
(i) Ensure Membrane is taken into Opening Reveals, Taped and Sealed and
Made Continuous with Opening Seals, it is typical at openings to allow the
membrane to run across the opening initially, then form a star cut into the
opening, folding over the sections of membrane and trimming as necessary. In
these cases, there are inevitable gaps in the airtight layer at the corners of
the opening, and it is important to ensure that these are made good before
subsequent installation of joinery etc.
(j) Sealing Batten
This detail may be considered as an alternative, or ideally as an
additional measure with (k). Since it is possible that replacement sash and
case windows cannot be easily sealed around their perimeter (they are often
‘open’ around the outer edge) it may be necessary to use this detail which
creates the airtight seal on the inside of the frame rather than ‘in line’ with
the frame as noted below.
(k) Flexible Foam Sealant around Joinery Insertions
Gaps around openings are one of the most common of air infiltration
(l) Draughtstripping of Openings in Joinery
Draughtstripping of joinery comes in many forms. It appears that
synthetic rubber or elastomeric tubular seals work well, creating good seals
with minimal compression. It is important that seals are unaffected by
paintwork and subsequent decoration, or are easily accessible and removable.
This is important so that seals can be replaced as they start to fail to
maintain the airtight layer. Brush seals are likely to be used in sash and case
windows.
(m) Silicone Sealant to External Window Surround
Some form of neat and potentially paintable edge seal will be required
externally
.
(n) Breather Membrane Instead of Netlon
Notwithstanding the air barrier placed above, mineral wool is permeable
to air movement and so replacing the netlon with a vapour permeable but
airtight breather membrane reduces air movement in the insulation, improving
insulation levels and reducing the risk of air leakage
9.7 Concrete Frame and
Panel
Discussion
Concrete
panel construction represents a potentially good airtight form of construction.
This is because the panels themselves are essentially airtight and being large,
have fewer gaps which must be sealed. Being fairly predictable in terms of
thermal and structural movement they are easy to seal well, and the only areas
of concern then are the service penetrations and junctions with openings. With
care and attention in these areas, a very good overall airtight external
envelope is easily within reach. Having said that, in some early examples of
this building type, the sealants between panels have failed, highlighting the
vulnerability of the system to such air test failure and the importance of
correct specification and application.
A
number of systems are available but the principles outlined for the improvement
of the system chosen are widely applicable. Where two leafs of concrete panel
are used, it is unlikely that the outer layer will be used as a rain screen
layer, but this is sometimes done, and in these cases the airtightness of the
internal layer of panels becomes critical, and may be augmented by the application
of a vapour control and airtight membrane on the inner face of the insulation,
applied to the panels before the insulation is installed. Guidance on the
application of this membrane, and on potentially more airtight forms of
insulation may be found
In
HIGH PRIORITY
(a) Integral Beam and Internal Panel
Important because this reduces the number of joints
and simplifies construction.
(b) Sealing of all Penetrations
Care and attention to detail at all services and other
penetrations is vital, most pressure tested panel buildings suffer air leakage
uk at these locations, if these are not sealed it will result in a air test failure
(d) Sealing around Windows
The other major source of air leakage
(c) Screed Edge Strip and Seal
Ensures that air does not leak between floors around
the perimeter and at other floor penetrations and breaks in the screed, another
major cause of air test failure
MEDIUM PRIORITY
(a) Accessible Draughtstripping
It is important that the draughtstripping is
accessible since it is likely that it will not last as long as the windows
themselves and require replacement.
(b) Membrane around Windows
Required for vapour and air leakage control, this
also required attention and inspection and can be seen as complementary to the
mastic / silicon sealants
(c) Double Silicon Seal to External Panels
Double silicon sealant lines in the external panels is
normally standard practice, and is typically good enough to ensure that the
outer panels provide an effective airtight seal throughout, and help towards
passing your air test
Costs
The
alternative specification highlights best practice installation and should not
incur any additional costs. The design of the panel construction system itself
would dictate any cost difference.
(h) Membrane Under Roof
Insulation
May not be required if the screed below is fully
sealed against vapour and air flow, but given the typical number of
penetrations in a commercial roof screed, the addition of a dedicated membrane
may be considered advisable
1.0 Key Principles
1.
The air test procedure is set out in CIBSE TM 23 and the ATTMA TS1
2. A air test involves sealing all ‘normal’ gaps such as vents
and pressurising or depressurising the building. The level of fanpower required
to maintain the pressure differential indicates the ‘leakiness’ or
‘permeability’ of the building.
3.
Air test are typically followed by an air test audit (using smoke pencils, for
example) to expose and make visible the various air test leakage routes.
4.
Where projects comprise large quantities of a single component, component
2.0 Climatic conditions
As
mentioned in Chapter 1, the raised pressure differential of 50 Pascals created
during an air test is quite small. Whilst this is adequate to overcome most of
the common pressure differential anomalies, such a small differential is
vulnerable to larger pressure differences created by climatic conditions.
Air
tests uk require calm days – i.e. a reading on the Beaufort Scale of 3 or less
(3.4 to 5.4 metres per second wind speed at 10 m above ground) This corresponds
to a gentle breeze with leaves and small twigs in constant motion. In winter
conditions, and on exposed sites therefore an air test may not be possible,
although it is often possible to make allowances, so long as these are
carefully recorded during the air tests uk .
3.0 The Test itself
Guidance
on testing buildings for an air test is contained in CIBSE Technical Memorandum
TM23 Testing Buildings for Airtightness and in BS EN 13829: 2001. All
accredited air tightness testers test to the guidelines contained in the BS EN
and within the ATTMA TS1
Essentially
the air test process is one of pressurising or (less commonly) de-pressurising
the inside of the whole building, and of measuring the rate at which air needs
to be blown or sucked to maintain that pressure differential; a building
suffering from high amounts of air leakage uk during the air test will equalise
readily and require a greater measurable effort to maintain the 50 Pascal
differential, while a air tight building will easily contain the enforced
differential and require little additional input during the air test .
The
pressure difference is induced by one or more calibrated fans that are normally
mounted within a suitable doorway. An adjustable door panel system, sealed
around the edges is used which can also be connected to large external fans via
collapsible ductwork if required. The rate of the fan or the volume flow of air
through the fan during the air test can be understood as the rate of air
entering / escaping throughout the remainder of the building envelope.
Buildings are air tested/air leakage tested in such a way as to recreate
‘normal’ conditions. Doors and windows are closed; trickle ventilators closed,
extract fans and such like are closed but not sealed. Internal doors are wedged
open. All of this must be actioned prior to the air test /air tightness test
If
the building is under construction the air test /air leakage testing uk can
be undertaken during working hours, but sometimes this is not
practical so some scheduling of work needs to be thought through in advance.
With all external doors and windows sealed shut, some work becomes impossible
(such as work with solvents requiring ventilation) and internal trades are
normally ‘sealed in’ for a short time, where they can carry on undisturbed
throughout the air test /air tightness test uk
In
existing buildings the air tests
4.0 Air Leakage Audits
The
air test /air tightness test quantifies the rate of air leakage through the envelope as a whole, but
it cannot locate the air leakage paths. Where remedial work is required
therefore, air test /air tightness tests are followed by a range of auditing
techniques designed to identify the specific places where air leakage is apparent through the
building envelope.
In
many cases a simple visual inspection may be sufficient – especially if
undertaken by someone with experience of the likely locations of air leakage.
However,
most air leakage routes are difficult or impossible to spot without visual aids
during the air test /air tightness test. One common technique is to use smoke
tracers – smoke pencils or smoke machines. These render the air paths visible
in certain situations during the
Another
technique, which has certain advantages and disadvantages compared to smoke
tracing along with an air test /air tightness test, is the use of an infrared
camera during a Thermographic test. Used either externally or internally, these
thermographic cameras register the radiant heat levels of surfaces and so are
able to ‘see’ for example, where cold air is cooling the fabric around a gap
internally, or conversely where warm air is escaping and heating the colder
materials on the external face.
To
work effectively, there needs to be a recognisable difference between the
internal and external ambient temperature, so before any heating has been
installed and on a warm summer’s day thermography testing may not be effective.
Similarly on warm and sunny days, sunshine on external surfaces can distort the
true situation so it is better on such days to wait until early evening. Conversely,
rain on external surfaces can be equally distorting of the true thermal
situation. However the
Thermographic cameras are useful in identifying problems at high
level or difficult to reach areas, and are also very helpful in identifying
other construction defects such as poorly installed (or non-existent!)
insulation within the fabric.
On
larger commercial buildings, the air test /air leakage testing may be
undertaken at the same time as ‘standard’ ventilation system commissioning and
associated studies, but these are not discussed as part of this guide.
5.0 Component Testing
A
distinct aspect of overall air test /air tightness testing is the individual
component test. This may be undertaken quite separately, in the laboratory or
by the manufacturer of a particular component. Such air testing /air
tightness sampling tests may be deemed necessary on a large project where large
areas of one particular type of component, for example curtain walling, are to
be specified, Insitu element air testing involves isolating the area within a
temporary sealed compartment, which is then pressurized during the air test ,
and the air leakage related to the area of interest assessed. In this way
sample areas of a building may be air test /air tightness tested using
smaller fans as required.
5.1 Concrete frame and panel
1
Introduction
As thermal insulation levels have risen in the last
few years the proportion of energy lost to draughts has increased to the extent
that now in some cases around half of all heat losses are due to air leakage
across the building fabric. Given that approximately half of all energy used in
the
The situation is such that further increasing thermal
insulation levels would be largely unproductive unless air tightness is conscientiously
addressed. Air leakage has been shown to reduce the effectiveness of thermal insulation
by up to 70% and so it is clear that if energy efficiency is to be improved in
buildings, the next efforts will have to focus on air test /air leakage
testing.
Many people make the mistake of thinking that an
airtight building is necessarily a ‘stuffy’ building. This is not the case. All
buildings have to be ventilated for health and comfort and airtight buildings
are no different. An adequate ventilation system (which may well include open
able windows as well as fans etc.) has to be planned for every building. The
difference will be that a great deal of unplanned air leakage needs to be
stemmed, this can be ascertained during the
The additional costs of creating an adequately
airtight building can be negligible, but even where costs are increased e.g.
for the air test , these can be more than offset by a reduction in the
capital cost of heating and ventilation equipment, not to mention the long term
savings in energy.
Given that the vast majority of building stock is
existing, a great deal of attention will need to be given, in the foreseeable
future, to remedial works to existing buildings, all existing air leakage paths
can quickly be found during a air test . This guide specifically includes
examples of good and best practice remedial work in terms of air tightness and
shows that such works can offer substantial benefits without undue disruption
or cost, an air test will have a low impact on site works
Air Pressure Testing hopes to provide practical
guidance on how to save energy and costs and protect building fabric. On the
basis that prevention is cheaper and easier than cure,
one purpose of this guide is to enable Designers to design inherently more
robust and durable solutions which avoid costly and time consuming remedial
works after the air test after a potential air leakage failure
The general guidance here is firmly focused on the
idea of practical design and detailing, and should be read in conjunction with
other guidance on sustainable design, energy efficiency and air tightness
where necessary to provide an overall design framework. The details provided
have been fully costed, tested and subjected to a Defects Liability insurance
assessment. They are offered as viable alternatives to standard details, and
illustrate the possibilities that exist. It simply remains for you, the
reader, to apply them appropriately in the context of your next project prior
to your air test/air tightness test
Aims
of this Guide
• To highlight benefits of air tightness
which include both energy and cost efficiency, improved comfort and reduced
risks of damage to building fabric
• To improve awareness of the need for air
tightness in construction at design stage
•
To promote detailing and specification solutions which create airtight and
efficient buildings thus reducing the need for remedial works after air leakage
failure- ‘prevention rather than cure’
• To show that new build and remedial air
tightness are achievable without undue cost penalties to construction works due
to multiple air leakage failures, you should be passing after the first
• .and in this way to help to ‘mainstream’
the good and best practice outlined in the document
Target audience
This
Guide will help all those who wish to improve the airtightness/air leakage rate
and energy efficiency of buildings through their construction, e.g:
• clients –building owners and users,
principal and specialist contractors, interior designers architects and
technicians, structural engineers, building service engineers, building
surveyors, quantity surveyors/ cost consultants, maintenance and facilities
managers, project managers , planning officers and building control officers,
funding bodies and their professional advisors, government and non-governmental
agencies
How to use this Guide
This
Guide is divided into six sections. The first two sections provide an overview
of the issues surrounding air tightness. Sections Three, Four and Five describe
the requirements for the design process, the procurement and the air
leakage/air tightness testing involved in designing for airtight buildings.
Section
Six provides a number of representative details which have been optimised in
terms of preparing for the
6.0 The Context
Key Principles
1. Most
2. Airtight building will NOT mean ‘stuffy’ buildings.
Good ventilation is vital for health and comfort - it is the UNPLANNED air
leakage that we are aiming to stem.
3. Legislation is slowly catching up with
best practice in
4. Good and Best Practice Targets of air tightness/air
leakage have been set for many types of buildings and are easily achievable.
6.1 Infiltration, Ventilation and Airtightness
Air infiltration (air leakage) is the uncontrolled
flow of air through gaps in the fabric of buildings; this is all the more
apparent during the air tes
Ventilation, on the other hand, is the intended and
controlled ingress and egress of air through buildings, delivering fresh air,
and exhausting stale air in combination with the designed heating system and
humidity control, and the fabric of the building itself.
Whilst some unwanted air infiltration (air
leakage) will at times aid comfort levels,
it is not reliable and moreover brings with it a range of significant
disadvantages such as high levels of heat loss, reduction in performance of the
installed thermal insulation, poor comfort, poor controllability and risks to
the longevity of the building fabric itself. It cannot be considered an
acceptable alternative to designed ventilation. Air Infiltration (air leakage)
needs to be reduced as much as possible if we are to create efficient,
controllable, comfortable, healthy and durable buildings. This can be achieved
by delivering airtight buildings that pass the air test /air leakage tests
first time.
Air tightness is a term used to describe the
‘leakiness’ of the building fabric. An airtight building will resist most
unwanted air infiltration (air leakage) while satisfying its fresh air
requirements through a controlled ventilation strategy. Most existing
buildings, even those built recently, are far from being airtight and may fail
an air test and because of unwanted air infiltration (air leakage) generate
huge costs to owners and occupants, in environmental, financial and health
terms. One way of overcoming this making sure the building passes the air
test/air leakage
It is important to emphasise the distinction between
infiltration (air leakage) and ventilation, because while the primary purpose
of this document is to show how buildings can be designed and constructed to be
airtight and so pass the air test /air leakage test first time, it is equally
important to stress that good levels of ventilation and a clear ventilation
strategy will be required in every case. As the saying goes:
‘build tight, ventilate right.’
6.2
Why Build Airtight?
Legislation
At
a rather prosaic level, the issue is important because it is now part of the
Building Regulations in
Energy
and Cost Saving
Typically,
the largest heat losses in most buildings are related to levels of thermal
insulation, followed by those related to infiltration (air
leakage), followed by those related to inefficient plant. Quite rightly
therefore, most efforts to save energy and costs have until recently been directed
at increasing thermal insulation levels. But as these levels have risen, so the
relative contribution of infiltration (air leakage) has
increased to the point where it can represent around half of all heat loss in a
building. In highly insulated buildings, the percentage may be higher.
This
is reflected in the fact that total space heating costs in an airtight building
may be as much as 40% less than in a leaky one
We
are at the stage where it is likely that any further increase in thermal
insulation levels would be ineffective until levels of air tightness in
construction have improved considerably, this is where the air test in
invaluable
Space
Heating System Reduction
Clearly
there is potential to reduce the capacity of space heating systems sized to
cope with current levels of heat loss if those levels can be reduced by a half
or more. Ensuring you achieve a low pass rate during the air test
. In addition, airtight buildings are more predictable in terms of
environmental control and the capital cost savings of installing smaller
heating plant may be augmented by reduced plant room sizes in certain cases and
particularly by reduced running costs in the longer term. As well as reducing
the need for heating plant, airtight buildings offer much greater potential to
respond positively to the local external climate through passive, or climate
responsive design strategies such as natural ventilation, day lighting, the use of thermal mass and passive solar gain. Energy
savings, capital and running costs, along with CO2 emissions can
thus be further reduced.
Comfort
and Control
As
noted above, airtight buildings are not as affected by variations in external
conditions. This makes them easier to control from an Engineer or Designer’s
point of view, but it also makes them more comfortable from the point of view
of the occupant.
In
buildings with high levels of infiltration/air leakage those occupants near
draughty windows, for example, will suffer the cold, particularly on windy
days, whereas those elsewhere may well suffer from too much heat locally as the
system tries to raise the temperature overall. Those who try to achieve
comfortable levels through the use of the provided ventilation controls will
find these to be relatively ineffective, whereas in more airtight buildings
greater levels of control and comfort generally are achievable and local
control and variation by occupants can have a more direct effect. In one
example of an existing superstore, the ambient temperature in the store was
raised by 5oC after the store had been sealed after the air test . Complaints by occupants in leaky buildings are
common, and remedial measures are usually difficult and expensive, an air test can
finds all air leakage paths quickly and effectively
Deterioration
of Fabric
Leaky
buildings allow cold air in through the construction causing discomfort,
they also allow warm (and often moist) air out, causing heat loss. This warm
and often moist air can find itself in colder parts of the outer construction
where it can cool, and the moisture in the air can condense, leading to a
buildup of moisture. This in turn can lead to:
• decay of organic
materials such as timber frames
• saturation of
insulating materials thus reducing their insulative effect (which increases
heat loss further)
• corrosion of
metal components
• frost damage
where moisture has accumulated on the cold side of the insulation.
6.3 Legislation
In
Proposals for changes to the Energy standards were
issued to public consultation in March 2006, including guidance that air
tightness testing would be required if the calculation of energy performance
included air permeability rates lower than 10m3/m2h at 50 Pa.
6.4 Measurement
A range of units for measuring air tightness/air
leakage have been used in the past and this can complicate matters. However,
one method only – “air permeability” - is the measure used in European Standards,
the new editions of the various UK Building regulations and in CIBSE’s TM23 Air
Testing methodology and has been used throughout this document. The Air
Permeability is defined as the volume flow in cubic metres of air per hour per
square metre of the total building surface area (including the floor) at 50 Pascals
pressure differential, expressed in m3/hr/m2 @ 50
The main difference between the air permeability and
previous practice in the
Of the range of measurements used previously, the
“Average Air Leakage Rate (or Index)” is similar to the “Air Permeability”
except that non-exposed floors are excluded from the measurement. Another
common expression is the “Air Changes per Hour at 50 Pascals (ACH @ 50 Pa).
This is a useful measurement in particular because, when divided by twenty, it
gives an approximate value of the natural infiltration rate of the building at
normal atmospheric pressure, which can then be used to help size heating and
ventilating plant etc.
Yet another measurement is the “Equivalent Air Leakage
Area” (ELA) at 50, 10 and/or 4 Pascals. This figure gives a representation of
the sum of all of the individual cracks, gaps and openings as a single orifice
and helps to visualise the scale of the air leakage problem. The main problem
of changing the measurement technique is the ability to compare data
The standard pressure differential used is 50 Pascals.
This is not in fact a very large pressure differential and corresponds to the
pressure exerted by a column of water 5mm high. Compared to the fact that
buildings can withstand wind induced pressures of at least 500 Pascals, this
seems insignificant, but it is larger than wind induced pressure on a calm day,
and by air testing and quoting air leakage figures at 50 Pascals, inaccuracies
are reduced and repeatability is improved using this air test method.
6.5 Targets
As noted above, the only ‘official’ guidance in the
A number of air tightness experts believe the stated
targets are inadequate when compared with the overwhelming need to address
carbon emission reductions, and the potential to do so through air tightness
measures. For example, the house illustrated to the right was built in 1992 for
the same cost as nearby houses and improved upon the standards noted above by
two thirds.
Building
Type Air Permeability (m3/hr/m2 at 50 Pa)
Good
Practice/ Best Practice
Dwellings
10.0/ 5.0
Dwellings
(with balanced mech. vent.) 5.0/ 3.0
Offices
(naturally ventilated) 7.0/ 3.5
Offices
(with balanced mech. vent.) 3.5/ 2.0
Superstores
3.0/ 1.5
Offices
(low energy) 3.5/ 2.0
Industrial
10.0/ 2.0
Museum
and Archival Storage 1.7/ 1.25
Cold
Storage 0.8/ 0.4
7.0
Designing for Airtightness
7.1 Key Principles
1. A Performance Specification is a crucial
document for establishing the appropriate targets for airtightness, along with
the methodology for achieving it, and the roles and responsibilities of those
involved.
2. Conceiving of a
building in zones and air barriers will help all involved to visualise the
task.
3. Air barriers must be impermeable,
continuous, durable and accessible. They should be supported by positive
mechanical seals where possible.
4. The simplest solutions will be the most
buildable and durable.
5. A culture of airtight construction does
not yet prevail and until it does, it may be necessary to follow up targets
with specific details and specifications, along with guidance on the process of
implementing the necessary level of co-ordination and attention to detail.
Unlike design for deconstruction (the subject of the
first in this series of Guides) and the forthcoming guide on chemical-free design,
the design of airtight buildings cannot be left to the specification and
details, at least, not until the industry as a whole recognises the need and
has sufficiently widespread experience, unfortunately this alone would surely
end in an air test failure. For the next few years, it will be necessary not
only to provide careful details and performance specification, but also to
develop thorough inspection and testing regimes, hence the need for Chapters
7.4 and 7.5 of this guide.
7.2 Performance Specification
The Performance specification may be the only document
needed by the Architect / Designer / Client if the building is to be procured
through Design and Build or similar route. However, it is more likely to be
part of a suite of documents including detailed drawings.
The performance specification allows appropriate
targets to be set for the project, along with a description of how the process
is to be conducted, in terms of scheduling, audits and air testing, and
potentially remedial works. Given the increasing use of specialist
subcontractors, particularly in larger projects, it is also critical that the
performance specification sets out both the responsibility for, and
constructive guidance regarding the co-ordination of trades with respect to the
final air permeability of the completed envelope.
Zones and Barriers
Once appropriate targets have been set for the
project, the next task is to identify zones which require greater or lesser
airtightness
For example an industrial unit with office space is
divided into five separate zones, and air barriers are identified as required.
Such a drawing, however diagrammatic initially, helps to conceive of the
subsequent specification and detailing needs, giving an overview of the
problem.
Heated zones need to be kept separate from unheated
zones such as roof voids, delivery bays etc. whilst
service shafts may require particular attention. Boiler rooms with large flues
and intake vents may need to be separated prior to the air test
Entrances are often significant sources of draughts.
Lobbies with doors set apart by around 4m, so that one door closes before the
second is opened, can be effective, whereas in highly trafficked areas
revolving doors are likely to be preferable. Tall buildings, with atria,
stairways and service shafts all of which rise through the building can be
prone to ‘stack effect’ air movement whereby warm air rises, dragging in cooler
air from outside at the lower levels creating more acute air leakage problems.
A number of tactics may be employed to reduce the effects, but in any event
issues of airtightness are likely to be highlighted in these cases prior to the
air test
7.3 Design
With
the zones and air barriers located, it is necessary to design the air barriers
themselves.
To
be effective, the air barrier must:
• be made of suitably air impermeable
materials;
• be continuous around the envelope or zone
• have sufficient strength to withstand any
pressures created by wind, stack effect or air control systems
• be easily installed
• be durable
• be accessible for maintenance /
replacement if appropriate
The
last of these is important since there is evidence that the airtightness of
some constructions will tend to decrease over time and in particular the first
period after completion.
There
are a number of strategic measures which can be employed to simplify the
business of designing an airtight building. Since service penetrations in and
out of a building provide a major source of air leaks, one strategy is to
collect all such penetrations into one accessible area, this will drastically
improve the air test results
In
construction types such as steel and timber frame, it is usually wise to employ
a specific membrane or layer as the air barrier, rather than rely on sealant
between, for example, the sheathing boards. Such a membrane can usually double
up as the vapour barrier if used internally and gives the Designer the
opportunity to consider and address airtightness explicitly, rather than as a
function of other elements. Bear in mind that most membranes are flimsy and
will need support in all areas, although there is minimal air pressure during
the air test it can still move unsupported membranes this can result in an air
test failure.
Another
strategy is to employ service voids. Creating a service void internally allows
for alteration and maintenance of services and finishes without recourse to
penetrations through the air barrier. This allows for long term good
performance in contrast to membranes which are liable to penetration at all
service points, necessitating careful sealing of each and every penetration, in
the short term this will help to reduce the air leakage uk rate and vastly improve your chances
of an air test pass, not only initially, but over the years of alterations and
maintenance to come.
Generally, it is better to conceive of the joints in
airtight layers as ‘positively’ connected, anticipating differential movement
and decay of adhesive or chemical bonds. For example, where different
components of a curtain walling system are liable to differential movement, it
is clear that a joint whereby the two components are held together with a
positive mechanical connection across a compressed gasket is likely to remain
airtight longer that a simple butt joint with a mastic sealant between, this
attention to detail will improve your air leakage uk rate and the chances of an
initial air test pass.
Finally
it is clear that complex solutions to airtightness are likely to be more prone
to poor execution and potentially to greater vulnerability to differential
movement, failure of sealants, dislocation of
components and so on. It is important therefore to aim for the simplest
solutions to providing a robust airtight layer, using the fewest separate
materials, junctions and penetrations, and the easiest installation and
maintenance, this will improve the buildablity and improve the chances of a air
test /leakage pass.
It is worth making a point of considering each and
every specified component with regard not only to its own intrinsic
airtightness
The
following provide a few examples whereby airtightness can be simplified at the
earliest design stages.
However good the workmanship, blockwork on its own can
never be considered airtight. Once plastered, on the other hand, it may be considered extremely
airtight, with concern only for those edges and corners where cracking or gaps
can appear. This may be contrasted with the more common practice of drylining
block walls with plasterboard on battens or dabs, either way when either is
built correctly they can form a excellent air test barrier.
Design
& Detailing for Airtightness
Services Zones or Rooms enable a
range of services to be collected together before exiting the building,
allowing most of the penetrations in the external fabric to be grouped and
sealed effectively prior to the air test .
Service voids enable cables and pipework
to be installed and altered without needing to penetrate the air barrier. Note
however that if they are not run in conduit, protection may be needed against
subsequent fixings, if not undertaken prior to the air test this will result in
an air test failure
Positive physical connections are to be preferred over any
other joint such as one relying on adhesives. In the timber frame example shown
the air barrier membrane is shown lapped and sealed with mastic over a firm
background (boards with stud behind) and with a positive mechanical fix - a
batten - fixed over the top and through to the stud.
Trinsically
non-airtight block wall behind, this form of construction typically gives rise
to a wide range of air leakage uk paths behind the boards and into floor,
partition wall and ceiling cavities. From the perspective of airtightness, dry
lining should be avoided unless great care is taken, otherwise it will result
in a air test failure
Similarly,
timber floors are difficult to seal well without a good deal of care. On the
continent - and to an increasing extent in the UK at large - concrete floor
systems are being used for both ground and first floors (often for other
reasons such as acoustics, fire and the desire for underfloor heating) and
these are easier to make adequately airtight prior to the air test . Hollow
planks however can leak into cavities and require to be sealed at their ends,
this will dramatically improve air tightness and the chances of an air test pass
One important and often quoted example is the timber
first floor connection with a block wall inner leaf. Who is responsible for
ensuring absolute airtightness when the timber joists rest on the wall and are
infilled between with block and mortar? Presumably the bricklayer, but is it
then his fault if the timber is installed at the wrong moisture level and
subsequently twists and warps, leaving cracks around every joint? Is it really
feasible to attempt to tape or mastic seal around them all, and what if the
underside of the ceiling is to be exposed? Far better perhaps, to do away with
the joist-onto-wall detail altogether and replace with joist hangers.
Increasingly, the designer should be seeking solutions which are intrinsically
airtight because of the design, rather than continuing as before while accepting
an increased use of duck tape and mastic on site! Whilst these may get you
through the initial airtightness tests/air tests, they are short term solutions
and not likely to lead to the anticipated energy savings for the Client in the
long term.
A good review of the various materials and components
which allow the Designer to create an air barrier may be found in the
7.4 Detailed Specification
Beyond the performance specification illustrated earlier,
it is important that the issue of air tightness/air tightness testing becomes
embedded within the standard specification vocabulary.
Where an equal or approved alternative may be allowed,
it is critical that an airtightness performance specification is part and
parcel of that equality of performance. For example, it may no longer remain
satisfactory merely to specify a membrane, but in addition to specify the
fairly precise nature of the sealing, overlapping and potentially the
subsequent layers as well. Simply offering a performance specification and
ensuring the responsibility resides with the Contractor is all very well, but
it is important too to offer solutions that will enable a satisfactory outcome
to be achieved and subsequently improve air tightness
Design
& Detailing for Airtightness - Implementing Airtightness
In addition to the intrinsic lack of
airtightness
Timber joists built into a block wall
- a poor detail for airtightness. Far better to use joist
hangars and avoid the problem. Source. Concrete
planks are not free of problems either hollow planks
are often left ungrouted where they meet the external wall, which could lead to
extensive air leakage internally and subsequently an air test failure.
7.5
Implementing Airtightness
Key Principles
5.
The
Contractor or Project Manager must be made responsible for achieving the air
tightness levels set. In particular, this will involve careful co-ordination
between trades; if this doesn’t happen then an air test /air tightness failure
will surely follow
6.
Inspection
remains an integral part of achieving air tightness and passing the
7.
Ideally
at least 2 air tests (air tightness tests) will be undertaken; the first when
the building is weather tight, and the second air test
a couple of weeks or so before handover.
8.
Experience suggest that making one person (or team) responsible for air
tightness is the most effective way to tackle the issue, this will drastically
improve the chances of a air test pass.
5.
Remedial air tightness works to existing properties can reap substantial
benefits without undue disruption and improve the chances of an air test pass.
It is not yet generally possible within the
The responsibility of the Designer in regards to the
airtightness cannot be overestimated, for if airtight buildings are to become
main stream, as they are elsewhere in the world, the techniques must be above
all simple and buildable, with most if not all of the ‘tricky’ areas designed
out from the start. In this way, such techniques can become ‘second nature’ to
Contractors and there is less reliance on potentially adversarial inspection
and air test/air tightness testing failures.
Ideally too, the Designer will understand the issues
sufficient to prepare a sound performance specification – giving achievable
targets for air tests/airtightness as well as a clear description of responsibilities
and procedures, and a clear and practical set of overall and detail drawings,
along with a detailed specification.
In the meantime, and even with good documents, there
is likely to be a need for effort and vigilance by both the Design Team and the
Main Contractor or Project Manager on site. This chapter briefly describes this
effort, while the next describes in more detail the actual air test procedures
and auditing techniques used.
7.5 Plan of Work
The RIBA Plan of Work provides a framework
for the entire design and construction process. The table on the next page
allocates specific tasks relating to airtightness to each Work Stage to enable
a schedule of tasks and responsibilities for the Design team to be prepared
according to each project, this will drastically improve the chances
of a air test pass.
7.6 Roles and Responsibilities on Site
Designer / Design Team
The responsibilities of the Design Team are detailed
on the following page, showing all stages including site works and beyond. Buildings
usually comprise a number of different components, creating a myriad of routes
through which air can escape if not carefully sealed at each and every
junction. The Designer’s role is to simplify these details to reduce
difficulties on site.
It is critical that the purpose of pursuing air
tightness is explained so that all concerned understand why they are being asked to
attend to these issues. The initial briefing of key personnel at mobilisation
stage – whether or not this involves the air tightness specialist – is also
critical in determining the approach to conducting the works, inspection, air
tightness testing and auditing etc, prior to the air test . which will need to be dovetailed into the many other
concerns on site.
On large projects it may be useful for one member of
the Design Team to take special responsibility for the air tightness / air test
issues.
Contractor
The Main Contractor’s principal responsibility is to
deliver the air tightness/air test performance overall and the most likely
task on any but the smallest jobs will be that of co-ordination between the subcontractors.
The Main Contractor must be clear that he carries responsibility for the
overall air tightness and in turn must ensure that all subcontractors are clear
about the extent of their responsibilities. This is important since there may
be some deviation to conventional practice in order for air tightness to be
achieved and a air test pass. It is always prudent to
place someone in the site team who has experience of air leakage/air tests on
their previous projects as their experience may avert a potential air leakage/air
test failure
RIBA Work Stage Design Team Tasks
A Appraisal
Establish appropriate air permeability rate
B
Feasibility / Briefing Note Microclimate
Test existing buildings / building to be
refurbished
Identify procedure for review and air
testing
C Outline
proposals Consider a/t issues in relation to decisions about form of
construction
Identify zones and layers
D
Detailed Proposals Identify requirement of additional consultants / design by
specialists
E Final
Proposals Ensure co-ordination between DT to ensure a/t envelope &
penetrations
Detailed application of airtight materials,
junctions, service penetrations
F Production
Info Select sub-contractors for specialist works (incl. testing)
Careful specification of components,
membranes, materials
Emphasise methods for airtightness on
documentation
Careful specification of components,
membranes, materials
Emphasise responsibilities in specification for
dealing with ‘loose ends’ between sub-contractor interfaces
G Tender
Docum’n Define Contractors’ responsibilities for co-ordinating work sequences
H
Tender Action Ensure selected tenders include adequate airtightness procedures
J Mobilisation
Brief all involved in areas critical to air infiltration before work starts
Preparation of samples, training, testing
and QA procedures
K-L Site
Works Co-ordinate inspection with Building Control if required
Ensure inspection of areas to be covered
Ensure audits and testing schedule is
adhered to
Ensure design changes do not compromise
airtightness performance
M Post
Completion Obtain feedback from concerning comfort and energy consumption
Carry out remedial work as required at end
of
As with the Design Team, experience suggests that the
best (
For Contractors, the issues of airtightness/air
leakage uk and the passing of the air test are intimately linked to issues of
good or bad workmanship in general and this can make the issue both more
sensitive, but also more difficult to control. Even simple buildings are
immensely complex and so the most important aspect of all is the creation of an
overall culture of careful, tidy, accurate and airtight construction,
something which cannot be simply forced through with a performance
specification.
It is easier to specify and draw an airtight detail
than to build it, and so the emphasis on inspection and Contractor
responsibility has not developed from a prejudice against Contractors, but from
a realistic appreciation that this issue cannot be entirely resolved ‘on
paper.’ It is genuinely about a culture shift (at least for many in the
industry) and this is where the real challenge lies. Once this shift is adopted
there will be a far higher percentage of companies passing their air
leakage/air tests
7.7 Inspection
Air leakage
Of the background air leakage subsequently
investigated, the principal air leakage routes the greatest cause of air test failures were noted
as being:
• Plasterboard dry lining on
dabs or battens, often linked to routes behind
skirtings etc.
• Cracks and joints in the
main structure; open perpends, shrinkage & settlement cracks
• Joists penetrating
external walls, esp. inner leaf of cavity walls
• Timber floors, under
skirtings and between boards
• Internal stud walls, at
junctions with timber floors and ceilings
• Service entries and ducts
• Areas of unplastered
masonry walls; intermediate floors, behind baths, inside service ducts
.
It is perhaps worth mentioning that the
The following table lists
many of the most common infiltration problem areas. On larger projects, common
problems include:
·
Incomplete bulkheads at eaves;
·
Gaps where block work abuts to steel columns or beams
·
Uncapped cavity walls, at eaves (right) and mid-points
where cavity walls change to composite panels
·
Gaps along the underside of corrugated roof linings
·
Gaps between block work and steel,
and uncapped cavity wall at join with composite panels Common Locations for
Inspection (Applicable to all types of Construction)
Foundation / Ground Floor
·
Check wall and floor dpcs form an adequate air
tightness layer, is a separate layer needed?
·
Check gaps at perimeter insulation strips
·
Check potential movement gaps between loadbearing
structure such as columns and adjacent non- loadbearing slab
First and Intermediate Floor Levels
·
Concrete floors: Check joint between the floor and
plasterboard to walls
·
Check gaps between concrete planks, or beam &
blocks are sealed at the wall
·
Check voids under floor finishes and service run
penetrations
·
Timber floors: Check a membrane seal has been
incorporated if required
·
Check any membrane used is supported between joists
Eaves and Verge
·
Check continuity of airtight layer between wall and
roof / ceiling
Ceiling level beneath the roof
·
Check for separation between deliberate roof
ventilation and the conditioned zone
·
Check for service penetrations and hatches which pass
across the airtight layer
Boundaries between different wall envelope
systems
·
Check all systems have a dedicated airtightness layer
assigned, and that these can be constructed to be continuous across dissimilar
elements
Windows and Doors
·
Check that the frame to wall junction is properly
sealed and continuous with the wall airtight layer, particularly at cills
·
Check the windows and doors have appropriate weather
seals between the opening unit and the frame
Services penetrations
·
Check for proper seals at service entry points, and at
points of entry into conditioned zones. These may also require fire protection
Main Entrances
·
Check that the whole entrance area is separated from
the conditioned zone by an inner airtight layer
Lift Shafts, Service Cores, Delivery Areas
/ Car Park
·
Check these have been separated from conditioned zones
with air barriers and draughtproofed access doors
Where profile fillers are
used poor workmanship is common
• Perforated (acoustic)
roofs, where the unsealed mineral fibre acoustic layer bridges the eaves of the
building, constituting a major leakage point
• Gaps where plasterboard or
wall linings are incomplete, commonly above suspended ceilings and to the
underside of beams
• Incomplete door and window
reveals
• Services Penetrations into
the building, and between zones inside the building
Another common issue is porous blockwork, particularly
when internal walls are drylined rather than plastered or painted. Where this
is likely to be unavoidable, it may be worth requiring blockwork to have an
initial air test for air permeability(air leakage), and to have an AP value (by
an accredited lab) that is no more than 50% of the target Air Permeability/air
leakage uk for the overall building.
7.8 Air Testing and Audit Schedule
In many cases to date, an air test /air tightness test
has been carried out a week or so before practical completion. If the result is
poor – a high rate of air leakage – then a great deal of work suddenly needs to
be done, often to areas which have been covered up and the whole business can
be both costly and time consuming, just at the point where in many contracts
there is already considerable pressure on Contractors.
Far better therefore to schedule the air test /air
tightness test
Ideally therefore, two Air tests/air tightness tests at least
should be carried out. The first Air test /air tightness test
.
In this way, the second and final air test /air tightness test
Such air tightness
It is often the case that the envelope is
not sufficiently complete on the due date for the air test /air tightness
testing
On larger projects, more air leakage
7.9 Remedial Airtightness Works
With airtightness testing and a general awareness of
airtightness
Either as a standalone measure or as part of a package
of energy efficiency measures generally, there is scope for remedial works to
most of the existing
As with thermal insulation, there is an extent to
which controlling some of the air leakage merely diverts the flow of air,
inward or outward, to another defect or gap, (this will still result in an air
test failure) but there is such scope for improvement that even fairly basic
efforts are likely to reap substantial environmental, financial and comfort
benefits for owners and occupiers alike.
There are many examples of remedial works described in
the various publications noted in the references. Some of the more successful
measures included carefully sealed secondary glazing installed where old
windows had to be kept for conservation purposes, draughtproofing of doors and
entranceways generally, and installation of lobbies in well trafficked
reception areas, attention to draughtproofing of existing windows and targeted
use of flexible sealants to ill fitting components and joints between different
construction types, will drastically improve your air leakage rate and will
should enable you to pass your air test first
time
9.
Testing Airtightness
Key Principles
5.
Air
test procedure is set out in CIBSE TM 23 and in BS EN 13829: 2001.
6.
An air
test / air tightness test
7.
Air tests
8.
Where
projects comprise large quantities of a single component, component testing in
the laboratory may be appropriate as well as on site element
8.1 Climatic conditions
As mentioned in Chapter 1, the raised pressure
differential of 50 Pascals created during an air test /air leakage test
Air tests uk/ air tightness tests uk require calm days
– i.e. a reading on the Beaufort Scale of 3 or less (3.4 to 5.4 metres per
second wind speed at 10 m above ground) This corresponds to a gentle breeze
with leaves and small twigs in constant motion. In winter conditions and on
exposed sites therefore testing may not be possible, although it is often possible
to make allowances, so long as these are carefully recorded, during the air test .
8.3 The Air Test itself
Essentially the process is one of pressurising the
inside of the whole building (during the air test ) and of measuring the rate at which air needs
to be blown or sucked to maintain that pressure differential; a building
suffering large amounts of air leakage will equalise readily and require a
greater measurable effort to maintain the 50 Pascal differential, while a air
tight building will easily contain the enforced differential and require little
additional input during the air test , this will be easily recognisable
within the first couple of minutes during the air test / air tightness test
uk
The pressure difference is induced by one or more
calibrated fans that are normally mounted within a suitable doorway. An
adjustable door panel system, sealed around the edges is used which can also be
connected to large external fans via collapsible ductwork if required. The
rate of the fan, or the volume flow of air through the fan can be understood as
the rate of air entering / escaping throughout the remainder of the building
envelope, this is recorded throughout the
Buildings are air tested in such a way as to recreate
‘normal’ conditions. Doors and windows are closed,
trickle ventilators closed, extract fans and such like are closed but not
sealed. Internal doors are wedged open.
If the building is under construction the air test /air leakage testing uk is ideally
undertaken out of working hours, but sometimes this is not practical so some
scheduling of work needs to be thought through in advance. With all external
doors and windows sealed shut, some work becomes impossible (such as work with
solvents requiring ventilation) and internal trades are normally ‘sealed in’
for a short time, where they can carry on undisturbed during air test .
In existing buildings, air tests uk/air tightness
tests are normally carried out when the building is unoccupied if possible
because of the disruption.
8.3 Air Leakage Audits
The air test /air tightness test
In many cases a simple visual inspection may be
sufficient – especially if undertaken by someone with experience of the likely
locations of leakage, this is a good way of lowering the risk of a air test /
air tightness test
However, most leakage routes are difficult or
impossible to spot without visual aids. One common technique is to use smoke
tracers – smoke pencils or smoke machines. These render the air leakage paths
visible in certain situations during the
Another technique, which has certain advantages and
disadvantages compared to smoke tracing, is the use of an infrared camera by
undertaking a thermographic survey, Used either externally or internally, these
thermographic cameras register the radiant heat levels of surfaces and so are
able to ‘see’ for example, where cold air is cooling the fabric around a gap
internally, or conversely where warm air is escaping and heating the colder
materials on the external face.
To work effectively, there needs to be a recognisable
difference between the internal and external ambient temperature, so before
any heating has been installed and on a warm summer’s day thermography /
thermographic surveys may not be effective. Similarly on warm and sunny days,
sunshine on external surfaces can distort the true situation so it is better on
such days to wait until early evening. Conversely, rain on external surfaces
can be equally distorting of the true thermal situation. However, these cameras
are useful in identifying problems at high level or difficult to reach areas,
and are also very helpful in identifying other construction defects such as
poorly installed (or non-existent!) insulation within the fabric.
On larger commercial buildings, air tests
8.4 Component Testing
A
distinct aspect of overall air tightness testing is the individual component
Insitu
element
9. The
Details
Caveat
It
is important to emphasise the scope and purpose of the following drawings and
specifications.
They
are included solely to show practitioners the sort of alterations that can be
made in order to enable buildings to be much more airtight in general.
Their
purpose is not to offer approved details in any sense, but to illustrate the difference between details and specifications which do not
address airtightness issues, and those that do. It is the differences
between the originals and alternatives which is intended to be
illustrative, not necessarily the alternatives themselves.
The
original details have been taken from conventional details and specifications
we believe to be broadly representative of their construction types. We hope
the principles shown, and the specific references made will assist designers in
making similar changes in their own work, but it goes without saying that air test /air testing cannot take responsibility for any work undertaken as a
result of the use of these details.
Specifically,
these details are not intended to show best practice in any sense, nor are they
even intended to be up to date. We have striven in the preparation of these
details and specifications to keep as close to the original as possible. We
have done this in order to show that some quite fundamental alterations – in
terms of airtightness - may be made with the minimum of visual or functional
impact on the original. Where these original details and specifications do not
meet current standards or aspirations, the alternatives given are likely to be
similarly wanting. To re-iterate, the purpose is not to produce approved
details, but to illustrate the process of improvement – in terms of
airtightness only – that may be made.
Consideration
of priorities in airtightness design and specification is potentially
misleading since, in effect, all gaps, cracks or tears let in air and the
sealing of one simply redirects infiltration to somewhere else, this becomes
all the more apparent during the air test . Like thermal insulation, what is
important is the level of continuity generally, not any particular detail on its
own. Nonetheless some prioritisation has been attempted in order to help
Designers to prioritise their own efforts since not all measures may be
necessary.
9.1 Steel
Frame + Concrete Block Cavity Wall
Original Specification
Discussion
Because
of the largely wet trades involved, one might imagine a masonry construction is inherently more
airtight than the dry fixed timber frame and curtain walling construction
types. However, insofar as concrete inevitably shrinks as it dries, as mortar
beds and perpends are often poorly filled, and due to the differential movement
between masonry and the steel frame, the myriad pathways that open up can make
masonry buildings extremely susceptible to infiltration, this can lead to an air
test failure.
To
make things worse, construction such as this does not easily lend itself to a
simple, single airtight layer which can be applied separately and therefore the
need for vigilance, and some care and attention to a number of small but
potentially time consuming sealing jobs is high, however these must be
undertaken if you are to pass an air test on the first attempt.
It
would be possible to form an airtight layer internally through the use of an
applied membrane and the adoption of a service void. This would have the
advantage of allowing for changes in the service or fit-out provision without
the risk of damage of compromise of the airtight membrane,(this will lead to a
air test failure) and for those inclined to this solution.
A
parge coat and service void could have a similar effect, but the use of plaster
internally is a common and effective technique for creating an airtight layer
and is preferable in this instance as it is closer to the original detail and
will improve the overall air test results
HIGH PRIORITY
·
Wet Plaster Finish or Wet plaster coat costs more but
provides a better finish overall, as well as significantly improved
airtightness across the masonry leaf. Plaster should be extended to all wall
areas and not left off in areas which will not be seen,(
such as suspended ceilings.
·
Membranes Lapped & Sealed2 lines of tape and a
positive mechanical fixing by batten ensure laps are sealed for the long term
·
Mastic to Skirtings, Linings etc.
·
Critical in this detail since the plaster cannot form a
continuous layer at these junctions
·
Sealed Cavity Closer: Gaps around openings are common so
care is needed here to prevent infiltration around the frame and into the
cavity
·
Vapour Barrier Seal at Eaves: Important here since no
effective seal is noted on the original which could lead to excessive airflow
at this vulnerable point.
Costs
The
most significant cost implication is associated with the addition of the wet
plaster coat to the inner leaf of blockwork. This results in approximately a
60% increase in cost, although the quality of the blockwork is not as critical.
This item is also significant in that is changes the ‘look’ of the detail but
is probably the highest priority.
Otherwise,
most of the costs are associated with the additional time, effort and care
implicated within the specification and details. Of these, the most significant
is the additional labour and materials required for the joining of the vapour
barrier in the roof, and sealing it around the perimeter. This work almost
certainly more than doubles the cost of the vapour barrier in the original
detail, but again, represents a critical factor in reducing air leakage and
saves the cost of multiple air test failures.
A
number of the measures described represent no more than a re-iteration of good
practice, such as the sealing of perpends, lapping and sealing of membranes,
draught stripping of windows and so on. These may assumed to incur no cost
implication, but perhaps one of attention to details (this usually results in
first time air test passes) on site.
The
mastic sealant to skirtings, cills and the like would add about 50% to the
costs of these items, though these items represent only a small fraction of the
overall costs.
Taping
of the insulation boards would depend largely on the board type, but might
realistically attract only a marginal cost increase, as would the use of compressible
foam around the steelwork.
MEDIUM PRIORITY
·
Concrete Slab Floors:Concrete
slabs form an airtight layer but joints with penetrations such as perimeter
blockwork, insulation or structural columns must be sealed.
·
Cill to Window Sealing: Double sealed detail which increases
the chance of securing an airtight seal at this often overlooked junction
·
Compressible Foam between Steel and Blockwork: Potential solution to the
inevitable gap which will form here, also sealable with mastic oninside face
only.
·
Perpends Fully Filled: Not critical if a wet plaster finish
is applied internally, but high priority if they are not.
· T&G and Taped Insulation:Not technically part of the airtight layer, but gaps here simply increas