Like other comfort-based considerations, air quality is an essential design consideration that aims to identify unacceptable air quality within the building envelope.
We determine air quality via the presence of microbial contaminants (such as mould and bacteria), gases (including carbon monoxide, radon, and volatile organic compounds), and other air pollutants, all of which can affect our health.
Therefore, we defined the acceptability of indoor air quality (IAQ) within the building envelope as that which has no contaminants at harmful concentrations, and that satisfies at least 80% of the occupants.
Acceptable IAQ is dependent on many building systems, such as HVAC systems, air-mixing techniques, interior finish materials, and building operations. In the context of the building envelope, air infiltration or leakage through the building envelope can affect IAQ, therefore must be considered.
Air infiltration occurs when the exterior air enters through cracks in the building envelope. The amount of infiltration will depend on the air pressures across the building envelope and can be mitigated by sealing openings and providing air barriers within the assembly.
Well-designed and correctly installed air control layers will prevent the movement of air through the exterior wall assembly. Air control layers limit the airflow between conditioned spaces and unconditioned spaces to resist air-pressure differences, stack effect, and wind loads.
No exterior wall can ever be designed or constructed to be completely airtight, although passive house construction pushes the concept of airtightness to its limit. Therefore, building envelope leakage is expected and specified for a maximum allowable air infiltration rate consistent with industry testing standards.