Air Tightness
In a typical pre-finished steel-clad building, the airtightness barrier is provided by the interior, or ‘liner’, side of the envelope. While the outer sheet of an insulated envelope will provide weather protection, and will be sealed in order to prevent water leaks, it is important that it is not relied upon for air-tightness.
The first step in constructing an air-tight building, is to identify the liner as the air-tightness barrier and ensure this is focused in the design and construction of the building envelope. This system can provide extremely good air-tightness, using good detailing and construction. Air-tightness, for a built-up system, is achieved by the liner sheet. Sealant tapes, or beads, are used to seal the overlap joints between adjacent sheets. The insulation cavity is ventilated, through the ribs of the external profiled sheet, to minimise condensation risk.
Side laps
Side laps are formed where the profiled edge of one sheet overlaps the edge of the adjacent sheet. Liner sheets are typically 0.4mm, or 0.7mm gauge. The joint is sealed by using an external tape, which is placed over the joint line of the cavity side of the liner sheet.
The joint can be reinforced with stitcher screws or rivets, which are a requirement for some firewall constructions. The use of a good quality sealant tape, such as Polyband, or T-foil, will produce a good quality air resistant joint. When using an external tape, it is important to note that stitchers will not provide an additional air-tightness, although they can add to the longevity of the joint.
End laps
End laps are formed where the profiled edge of one
sheet overlaps the edge the adjacent sheet. Liner sheets
are typically 0.4mm, or 0.7mm gauge. The joint is
sealed using a sealant bead, which is placed inside the
overlap joint.
Fixings should be installed in the trough of each profile
to provide compression of the sealant bead.
The use of a good quality sealant bead will produce a
good quality air resistant joint.
Approved document L (2006)
Air-tightness is measured in m3 of air per hour per m2
of building envelope at an applied pressure of 50 Pa.
The area of the building envelope includes the walls,
roof and floor slab. In practice, the floor slab can be
considered air-tight.
The concept of air-tightness was first introduced into
Building Regulations in England and Wales in the 2002
revision of: ‘Approved Document L’. In this document,
a maximum allowable air leakage rate of 10m3/h/m2
was introduced for all buildings other than dwellings
greater than 1000m2.
The 2006 update to Part L has extended this further.
All buildings must be pressure tested, unless the floor
area is less than 500m2, when a default value of
15m3/h/m2 can be used. For buildings over 500m2,
a maximum reasonable design limit for air tightness is
10m3/h/m2. The new requirements are less prescriptive
than 2002. However, it will be difficult to meet the
overall building CO2 emission rate without a reasonably
air-tight building envelope.
Air-tightness is tested using a number of large
mobile fan units, to pressurise the building to 50 Pa.
The volume of air leakage is divided by the cladding
(including floor slab) area.
Air permeability tests can be undertaken by the following companies:
1. BRE, Watford
Contact: Brian Webb
Tel: 01923 664300
2. BSRIA, West Bracknell
Contact: David Pickervance
Tel: 01344 426511
3. Building Sciences, Wantage
Contact: Nick Selves
Tel: 01235 835323
4. HRS Services, Sheffield
Contact: E Westgate
Tel: 0114 2723003
