Healthy Housing Insight

Airtightness and Building Envelope Integrity

There is no such thing as an airtight building; however, achieving a high degree of airtightness is important to the integrity of the building envelope. The durability of buildings is a key aspect of resource efficient and environ mentally responsible healthy housing.

Research has shown that moisture can migrate into the building envelope through two mechanisms, vapour diffusion and air leakage. During the heating season, vapour diffusion is a relatively slow process which involves a high concentration of water molecules, from the air inside a building, diffusing through building materials toward a lower concentration of water molecules outdoors. A vapour barrier is required to adequately control vapour dif-fusion.

jy \ Air leakage from the inside of the building to the outside i.exfiltration) J will transport the water vapour it contains. Air leakage can transport (f[lHy upwards of 30 times as much moisture as vapour diffusion, depositing water within the building assembly in areas concentrated around air leakage points. This must be avoided since most building materials are highly susceptible to moisture damage. Only a continuous air barrier can prevent airborne moisture migration.

Key Points

-» The higher the level of airtightness the better. In addition to protecting the building envelope, airtightness promotes energy efficiency, allows for better control of natural and mechanical ventilation, and reduces the transmission of outdoor noise.

Properly located and sealed, polyethylene can serve as an effective air/vapour barrier — keep it on the warm side of the insulation.

An exterior air barrier membrane is also recommended, particularly if the insulation being used in the walls is air permeable. Ensure that the air barrier membrane is also not a vapour barrier.

-+ Airtightness is cumulative — good workmanship in the construction of the entire building envelope is essential to its proper performance. Properly installed air barriers are not a cure-all for poor construction practices.

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