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