Air Tightness Testing - what is it?

Many of you - our customers, have been asking just what exactly is involved in conducting an air tightness test in your new minimum carbon building. The following offers an explanation that is helpful to both you and your chosen third party specialist engaged in carrying out the test.  


An Air Tightness Test is carried out by connecting a fan, or a number of fans, in the doorway of a building and pressurising it over a range of pressure differences. The fan speed is increased in steps up to a maximum and then decreased in steps. Air volume flow rate through the fan (equal to the air leaking through the building envelope) and the pressure difference across the building envelope are recorded at each fan speed. In calculating air permeability, corrections are made for temperature and barometric pressure. Local wind speed should preferably be below 3m s-1.

Before the test is carried out, all mechanical ventilation systems must be switched off. All mechanical ventilation grilles and openings to the outside must be sealed. Sealing can be achieved with plywood sheeting or plastic sheeting and strong adhesive tape. Smoke vents should be closed but not sealed. Drainage traps should be filled. While the pressurisation test is being carried out all external doors and windows must be closed and secured to avoid them being blown open.

Internal doors must to be wedged open during the test to avoid them
slamming shut. The fan unit will create draughts in the building. Lightweight objects and paper near the fan unit should be removed or secured to avoid them being blown around. Fixtures and furniture will not be affected.

Access in and out of the building during the test will not be possible (unless there is an emergency and a need to evacuate the building). The test will take approximately 30 minutes. Site workers can remain in the building during the test, or will have to remain outside until the test is complete. There are no health risks to site workers who remain in the building during the pressure test, however there may be some discomfort due to cold draughts and some noise from the fan


Air permeability is expressed as volume flow per hour (m3 h-1) of air supplied to the space per square metre (m-2) of envelope area for an internal to external pressure difference of 50 Pa i.e. zz m3 h-1 m-2 at 50 Pa.