Low Carbon Housing Learning Zone

About the Learning Zone

Airtightness

Introduction

Definitions

Measurement

Airtightness & Ventilation

Identifying Air Leakage

Factors Influencing Airtightness

Airtightness of UK Housing

Construction Observations

Improving Airtightness

Case Studies

Additional Resources

References

Thermal Bridging

Thermal Design

DEFINITIONS

AIR LEAKAGE

Air leakage is the uncontrolled exchange of air both into (infiltration) and out of (exfiltration) a building through cracks, gaps and other unintentional openings in the building envelope. It is driven by the same physical processes that drive natural ventilation, namely:

The wind effect
The stack effect

The wind effect Air moving around and over a building induces pressure differences across the building and generally results in a small negative pressure internally. Outdoor air will enter the building through cracks and gaps in the construction (infiltration) on the windward side of the building that is under positive pressure, and indoor air will exit the building (exfiltration) on the leeward side of the building that is under negative pressure.

The stack effect Air inside the building is heated by solar gains, heating appliances, people and equipment. As the internal air increases in temperature, it becomes less dense and rises due to convection. This results in higher pressure air at the top of the building, relative to the external air pressure, leading to exfiltration. As the warm air exits the dwelling, cooler denser air from outside infiltrates the building through cracks and gaps in the construction.

The rate of air leakage is therefore dependent upon the wind speed and direction, the temperature difference between the inside and outside of the building, as well as within the building, and the air permeability of the construction.

AIRTIGHTNESS

Airtightness is a term used to describe the ‘leakiness’ of a building. The airtightness of a building determines the uncontrolled background ventilation or leakage rate of a building which, together with purpose-provided ventilation, makes up the total ventilation rate for the building. In other words:

Uncontrolled Background Ventilation + Purpose Provided Ventilation = Total Ventilation Rate

It is frequently expressed in terms of a whole building leakage rate at an artificially induced pressure, in the UK 50Pa is used (n50), or in terms of an equivalent leakage area. Traditionally, airtightness was expressed in air changes per hour (with units conventionally used being ach or h-1). However, nowadays air permeability is more commonly used (with units m³/(h.m²) representing m³ of air flow per hour, per m² of envelope area) as it takes into consideration the effects of shape and size. The lower the air permeability of a building, the greater the airtightness.

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