Successful building leakage air tests and air tight building construction
stems from an attention to detail at both the design and construction
stages of the project.
An airtight barrier layer needs to be identified within the building,
and this layer extended to all facets of the building elevations. The
following information identifies the most common types of materials used
to construct building envelopes, and highlights the areas where particular
attention needs to be paid.
The inner skin of the external walls should ideally form the airtight
barrier in wall constructions. External walls should be extended from
floor level to the underside of intermediate floors and roofs. All gaps
between blockwork and intermediate floors, roof members, windows/doors,
steel columns, etc should be sealed.
Standard blockwork is not impermeable to air unless it is plastered. The
block density and standard of mortar joints have a significant impact
on the overall permeability of the construction. Full mortar joints are
preferable. Painting blockwork reduces the permeability and two coats
of emulsion can make a significant improvement.
Ensure that the external walls are completed and fully sealed prior to
springing internal stud partitions from them. Avoid/minimise service outlets
such as sockets, data outlets etc in the dry lining of external walls
as they are rarely well sealed. Make sure that blockwork behind dry lining
has full mortar bed joints and perps. The gaps at the top & bottom
of the plasterboard should be sealed.
Sheet Metal Cladding:
Insulation to the correct density needs to be used and compressed to the
correct thickness within the construction and mastic joints used to seal
the sheets. Profiled closer pieces of the correct size and profile must
be used and sealed where panels meet other constructions.
Composite panel systems are generally better than in-situ built up systems,
since the joints between panels have gaskets and the steel cladding layers
are completely filled with foam. Special attention needs to be paid at
the interface between wall cladding and other methods of construction.
The head of the wall/ underside of roof junction is particularly susceptible
Curtain Walling Systems
Curtain walling systems are normally well designed and constructed with
gasket joints between panels and frame construction. Generally it is the
joints with other types of construction such as Concrete edge beams, roof
parapets, brickwork etc. In addition care needs to be taken with multi
storey construction to avoid air leakage between floors.
Cast In situ Concrete Slab:
This type of floor provides the best type of airtight seal since the concrete
fills any voids, both horizontally and vertically.
Pre-cast Hollow Concrete Floor Planks:
The hollow core of this type of floor can allow air to track horizontally
and end up outside the airtight layer of a cavity wall. The problems are
made worse with drainage and service penetrations, which if poorly sealed
allows air to enter the hollow core and escape. Care should be taken to
ensure all penetrations are sealed properly. We would recommend that the
ends of the concrete planks are also sealed.
Beam and Block Floors:
This type of floor construction can lead to similar problems to those
described for the pre cast planks above.
Where roof/ceiling voids exist, lay in type false/suspended ceilings
do not form an airtight barrier layer. Plasterboard type ceilings if plastered
and edges sealed are significantly better than other types of lay in ceilings.
Profiled Metal Decking:
The inside surface of the decking if possible should form the airtight
barrier and needs to be sealed to all the adjoining structure and the
sheets sealed to each other during the construction of the roof. Use of
the vapour barrier as the airtight layer should be avoided since this
is rarely an effective seal. Steelwork supporting the roof can sometimes
prevent access to the roof decking above and prevent a satisfactory seal
being formed. Careful detail is needed to address this. Any Profiled decking
can cause difficulties in sealing walls where they run on the underside.
A profiled closer piece should be used to fill the void, and this in turn
should be sealed with mastic.
These types of roof generally provide a good airtight barrier over the
main horizontal area of the roof, since they are designed to prevent water
ingress. Particular care needs to be taken around parapet walls, flashings
and roof penetrations.
Doors and Windows:
Door and window frames need to be sealed to the internal air tight barrier
and not just sealed to the external façade. Window sills are often
an area, which performs particularly poorly and should be fitted after
the cavity has been sealed using an appropriate cavity closer.
Lift Shaft Doors:
Lift shafts have a permanent vent to outside at the head of the shaft
and should be fitted with effective doors seals at each landing. Any floor
or ceiling voids adjacent to the shaft should also be adequately sealed
to prevent leakage.
Loading Bay Doors:
Concertina type sliding doors are very difficult to seal. Where possible,
Roller Shutter or Sectional type doors, with good quality rubber seals
should be specified. The seal between the head of the door and the cladding
soffit should be checked to make sure it fully closes the gap.
Mechanical and Electrical Systems:
The airtight barrier should generally step inside the building separate
the plant rooms from the rest of the building. The plant room partition
walls, floors and ceiling as well as service risers require careful sealing
around all mechanical and electrical service penetrations. Particular
care is needed to fill voids INSIDE electrical service trunking.
All incoming service ducts should be sealed after the services have been
All drainage traps including floor gullies and air handling plant condensate
traps should be filled with water prior to the test.
Detailing and materials selection needs to be high on the list of priorities
for all members of the team, right from the start of the project to ensure
1) The airtight barrier layer is identified
2) The correct products are used where required
3) The relevant areas are monitored throughout the construction phase
The use of certain types of products such as adhesive tapes or expanding
foam should be avoided since these do not perform over the life of the
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