Air Leakage

The control of air flow is a crucial and intrinsic part of heat and moisture control in building enclosures in all climates. A large fraction of a modern, well-insulated building's space conditioning energy load is due to uncontrolled air leakage. Wintertime condensation of water vapour in exfiltrating air (or summertime condensation of infiltrating air) within assemblies is one of the two major sources of moisture in the above-grade enclosure (driving rain being the other). Air flow through the enclosure can also carry the spores generated by mould within the wall into the air and thereby create health problems. Uncontrolled air leakage through the enclosure is therefore often a major cause of performance (e.g. comfort, health, energy, durability, etc.) problems. Water vapour diffusion, while amenable to simple analysis, is often an insignificant source of moisture in modern building envelopes. Wintertime exfiltration condensation is however acknowledged as a common building performance problem in cold climates. Warm weather infiltration condensation is often a problem in warm and humid climates (e.g. the south-eastern States) and in some cases in cool climates, especially when air conditioning is used.

Therefore, there are three primary reasons why the control of air flow is important to building performance:

Moisture control - water vapour in the air can be deposited within the envelope and cause serious problems
Energy savings -air leaking out of a building must be replaced with outdoor air which requires energy to condition it. Approximately 30% to 50% of space conditioning energy consumption in a well-insulated house is due to air leakage through the building envelope. Convective circulation and wind washing reduces the effectiveness of thermal insulation and thus increases energy transfer across the envelope.
Comfort and health - cold drafts, and the excessively dry air from large uncontrolled ventilation flows affect human comfort and wind-cooled portions of the interior of the envelope promotes condensation which supports biological growth which in turn affects indoor air quality. Off-gasing materials within the enclosure assembly may not be harmful to health if these gases are excluded from the interior by controlling air flow.

There are other circumstances that require the control of air flow; for example, smoke and fire spread control through air spaces and building voids and shafts and airborne sound transmission control requires good airflow control. The flow of undesirable or dangerous odours, gases, and fumes between zones within buildings (e.g., the parking garage and the office) and between the exterior and interior (e.g., between a busy street and a classroom) must also be controlled.

Click here to learn more from Dr. Straube's paper, Air Flow Control -- Beyond Just Air Barriers.