What is Air Tightness

In the first of a series of articles, Chris Moore looks at the meaning of air tightness and some frequently asked questions about this relatively new building discipline.

Q. What is meant by building airtightness?

Building airtightness is defined as the resistance to air leakage through unintentional points or areas in the building envelope. The leakage can be inward or outward. It is caused by differential pressure across the building envelope which is driven by wind, mechanical ventilation systems and the ‘stack effect’. Infiltration relates to uncontrolled inward leakage of outdoor air, whereas exfiltration is the uncontrolled outward leakage of indoor air.

Q. Why is there an emphasis on air tightness in new buildings?

Uncontrolled air leakage increases the ventilation air change rate. This is the rate that air enters or leaves a building, expressed as air changes per hour (ac/h). An increase in the ac/h leads to an increase in the ventilation heat loss rate (expressed in W/K) which in turn effects the overall heating or cooling requirement.

Q. What is an airtightness test?

This is a method of quantifying how much air leaks into or out of an enclosure. The European Standard I.S. EN ISO 9972:2015 is the test method used for buildings in Ireland. Building airtightness levels can be measured by using a fan, temporarily installed in the building envelope (a blower door), to pressurize and to depressurize the building. Air flow through the fan creates an internal, uniform, static pressure within the building. The aim of this type of measurement is to relate the pressure differential across the envelope to the air flow rate required to produce it. Generally, the higher the flow rate required to produce a given pressure difference, the less airtight the building

Q. Is an airtightness test mandatory for buildings?

In Ireland, it is mandatory to test new residential buildings for air tightness or (air permeability as it also referred to). It is not mandatory for non-residential buildings, although this is likely to change in the very near future. On large development sites, only a proportion of each dwelling type needs to be tested. The dwellings must achieve a certain minimum standard to comply with the Building Regulations.

Q. What standard must be achieved for new dwellings?

Technical Guidance Document (TGD) L 2011 of the Building Regulations stipulates an upper limit for air permeability of the envelope of 7m³/m²/hr, that is a maximum of 7 cubic metres of air leakage per square meter of envelope area every hour. The envelope area is calculated by adding the internal surface areas of the ground floor, the external walls and the roof.

Q. How is building airtightness quantified?

In Ireland, the airtightness of a building is expressed in terms of the leakage airflow rate through the building’s envelope at a given reference pressure, namely 50 Pa, divided by the envelope area. At 50 Pa, it is called the air change rate at 50 Pa or q50. The lower the q50 value is, the more airtight the building’s envelope is. Pa stands for Pascal, the SI unit of pressure. Atmospheric pressure at sea level is 101,325 Pa.

Q. What is a Blower Door?

A Blower Door is a device that fits into a doorway or building containing a fan, for supplying or extracting a measured rate of air flow. It is normally used for testing air leakage by pressurization or depressurization.

Q. Is good indoor air quality compatible with good building air tightness?

Yes, provided that the building is equipped with an appropriate ventilation system. This can be natural, mechanical or a hybrid system. Better building air tightness results in better indoor air quality because ventilation systems operate more efficiently. Uncontrolled airflow because of envelope leaks can cause individual rooms to be poorly ventilated even though the overall air exchange rate for the building may be sufficient.

Q. What are the most common air leakage paths?

Common leakage sites are listed in Figure 1 below. Figure 2 gives the classification of these sites in 4 categories [1]:

Figure 1: Vertical section of a typical building with identification of potential leakage junctions (Source: CEREMA – Pôle QERA)