MEASUREMENT   

The airtightness of a building envelope can be measured using the following techniques:

• Fan pressurisation (blower door) technique
• Tracer gas technique

Fan pressurisation technique

This is a simple, quick and widely used technique that involves the use of the following items of equipment:

• Portable variable speed fan
• Adjustable door frame and panel
• Fan speed controller
• Pressure and flow gauge

The technique involves sealing the portable variable speed fan into an external doorway, using the adjustable door frame and panel. The fan speed controller is then used to pressurise and/or depressurise the building. The airflow rate that is required to maintain a number of particular pressure differences across the building envelope is measured and recorded. The leakier the building, the greater the air flow required to maintain a given pressure differential. In most cases a pressure differential of 50Pa is used. The test is undertaken with all external windows closed, all trickle ventilators closed, all internal doors within the heated envelope open, all mechanical extract fans sealed, all chimneys and flues sealed and all water traps and U-bends filled with water.

Fan pressurisation technique

The following video is a short clip taken from a fan pressurisation test that was performed on a dwelling. The tester can be seen altering the speed of the portable variable speed fan using the fan speed controller. The resulting flow and pressure readings from the digital pressure and flow gauge are then recorded.

Pressurisation test video

For large domestic and non-domestic buildings, a series of multiple fans or trailer mounted fans are used for pressure testing.

Multiple and trailer mounted fans [Source: S. Borland, BSL]

Pressurisation tests are the basis for guidelines and legislation in a number of countries. In the UK, tests should be undertaken in accordance with the Air Tightness Testing and Measurement Association (ATTMA) Standard 1 – Measuring Air Permeability of Building Envelopes (ATTMA, 2007). Further details about ATTMA and a downloadable copy of the standard can be obtained from http://www.attma.org/ Further information and guidance on pressure testing can be found in CIBSE TM 23 (CIBSE, 2000) and BS EN 13829:2001 (British Standard Institution, 2001).

When pressure testing a building, it is possible to obtain test results by undertaking only one set of measurements during either pressurisation or depressurisation. However, there are valid reasons why two sets of measurements should be undertaken, one for pressurisation and one for depressurisation, and the results averaged. All of the air leakage paths occurring within a dwelling will have particular aerodynamic characteristics that are dependent upon the direction in which the air is flowing. In addition, various elements of the building fabric, such as windows and doors, can either be pushed tight against their seals or pushed off, depending upon whether the dwelling is being pressurised or depressurised.

Therefore, it is likely that more favourable results could be achieved by depressurising rather than pressurising the dwelling. By undertaking both sets of measurements and averaging the results, any aerodynamic effects cancel one another out and no preference is given to one set of results as opposed to the other.

Significant differences in the two sets of air permeability measurements can occur. For dwellings, CIBSE claim that it is common for the difference between the pressurisation and the depressurisation results to be more than 10% (CIBSE, 2000). Stephen (1998), on the other hand, suggests that the results can differ by as much as 20%. These sorts of differences in air permeability could make the difference between a dwelling complying with a particular airtightness standard or not. 

Tracer gas technique

Tracer gas techniques are normally used to measure whole building ventilation rates, but can also be used to measure air leakage. The technique involves introducing an inert gas into the building and then observing how the gas behaves as air leaks both into and out of the building. The air leakage rate can then be determined by either measuring the concentration of the gas inside the building over time, or measuring the rate at which the tracer gas needs to be introduced into the building to maintain a specific concentration. Typical tracer gases include nitrous oxide, sulphur hexafluoride, various perfluoro tracers (PFTs) and carbon dioxide. If appropriate tracer gases are chosen, measurements can be undertaken in occupied dwellings.

Tracer gas technique