Last month's post generated some great discussions about Applied Building Science in New Zealand, and it can be concluded that airtightness is an absolute necessity. Therefore, it is advisable to measure the quality of the airtightness by means of a test – the Blower Door test.

From practical experience it is a known fact that absolute 100% airtightness is impossible to achieve. Even high quality constructions need capacity for drying in order to avoid the formation of mould and prevent structural damage in the long term. But let’s focus on the actual Blower Door test for now.

Inspection and measurement of the airtightness of a building envelope is common practice almost worldwide, and is a state of the art measurement in many countries. This measurement aims to identify weak points in the airtightness layer and assist in repairing defects in order to prevent structural damage due to vapour convection.

The Blower Door Test

A fan is fitted to an opening (exterior door or window) and a pressure difference between inside and outside of 50 Pa is created in the building by sucking air out or pushing it in. It is then possible to find leaks in the building envelope either simply using your senses (e.g. by feeling the draught on the back of your hand), or to make them visible using smoke (smoke tubes or a smoke generator). If leaks are found, they can then be put right using suitable remedies. The volume of air required to create the pressure difference between inside and outside varies, depending on the number and size of the leaks. By comparing the volume of air required with the volume of the building, it is possible to determine how airtight the building is.

The parameter used to describe the airtightness is called the n₅₀ value. This value describes the ratio between the volume of air moved by the fan and the volume of the building at a test pressure of 50 Pa.

The n₅₀ value is calculated by dividing the volume flow by the total volume of air in the building:

n₅₀ = Air flow volume of the fan / Total air volume in the building

The required n₅₀ values for most advanced building regulations are:

• Building without a ventilation system: ≤ 3 air changes per hour
• Building with a ventilation system: ≤ 1.5 air changes per hour
• Passive House (voluntary standard): ≤ 0.6 air changes per hour.

The only reference in the NZBC is in H1.3.3 (c) whereby "account must be taken of physical conditions likely to affect energy performance of buildings, including – (c) the airtightness of the building envelope." In future we will see mandatory requirements similar to the regulations in other countries above for a Blower Door test result.

One other interesting point is that often airtightness is only tested after construction work has been completed. If the legal requirements are not met, a new building may then need to be renovated at great expense, as this means that the entire inner lining first needs to be removed before the airtightness can be put right and then re-tested. After the re-test is successful, the inner lining can be replaced.

For this reason it is increasingly common in Europe to perform a simple and practical airtightness test before final assembly of the inner lining. In contrast to the measurement of the n₅₀ value, no actual value is measured. Instead, the effective airtightness of the building envelope is tested. This can either be done using a Blower Door or the cheaper WINCON, a practical unit with a higher volume flow rate but without any complex measuring equipment. Using the WINCON, it is possible to test the airtightness layer once the work has been completed to check whether it really is airtight, thus helping to avoid any nasty surprises when a Blower Door test is subsequently performed.

If you have not attended a Blower Door test yet please get in touch and we will try to link you with one of the Blower Door testers in the country to experience it. Alternatively keep watching Grand Designs to see the Blower Door in action.