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There
are three essential steps to achieving airtightness:
• design for airtightness
• build for airtightness
• test for airtightness.
With complex designs, or when the project team is unfamiliar with
designing and constructing tight buildings, it is advisable to seek
advice from an expert at each of these three stages.
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The test procedure
Air tightness tests should be carried out in accordance with CIBSE
Technical Memorandum 23, and are usually carried out by an outside
consultant appointed by the main contractor. The method used is 'pressure
testing'.
Pressure-testing uses a large fan system to pressurise the building.
The system is connected via flexible ducting to a wooden template
that is temporarily sealed into an existing doorway. All external
doors, windows and trickle ventilators are closed and all the internal
doors are wedged open. Mechanical ventilation openings are sealed
with polythene sheet and tape or other appropriate materials. Smoke
extract fans/vents are closed but not sealed, and other integral openings
(eg lift shafts) are left unsealed. Checks are made during the test
to spot and correct any extraneous effects, such as a window or door
blowing open, or any temporary sealing failing.
A test can take as little as two hours for a straightforward case,
and good preparation by the contractor makes all the difference. More
complicated or less well-prepared cases can take up to a full day.
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Air leakage audit
This procedure can be carried out prior to airtightness testing. It
will highlight any potential air leakage paths and enable remedial
work to be carried out whilst access is still relatively easy. Air
leakage audits are also undertaken when a building fails the test
and again, is used to point out problem areas. BRE uses three audit
tools and selects the most appropriate combination for the job in
hand.
• Small smoke tubes locate specific air leakage paths
from inside the building. Identification of air leakage is easier
if the building is under depressurisation using a fan pressurisation
system operating in reverse, or using the building's own extract fan.
• Infrared thermography detects the locally cooled
areas where external air enters the building. This method is particularly
useful for detecting air leakage paths quickly in high-roofed areas.
However, it is only effective when the external temperature is several
degrees below the internal temperature. Again, this is best carried
out with the building under depressurisation.
• Large-scale smoke tests can be used in some cases.
This involves filling the building with smoke, then using a fan pressurisation
system or the building's own supply fan to pressurise the building.
The smoke is forced out through the gaps in the building envelope
and observed from the outside. The results can be recorded on video
camera.
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