Air Leakage & Thermal Performance

On a project currently under construction we discovered the improper installation of the vapour barrier at electrical outlets done during a previous renovation. The vapour barrier is not sealed to the outlet using a pre-manufactured outlet box. Instead there are numerous holes for air and moisture to enter and exit the home through the exterior wall. This can lead to mould and rot issues.

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When on site reviewing projects under construction we review to ensure that contractors conform to the requirements of the contract documents (a.k.a. drawings and specifications). Two of the most important items to review for proper installation are the air and vapour barriers. These two membranes work hand-in-hand with properly balanced heating and cooling systems and their continuity are critical for providing an air-tight home, reducing risk of mould, and decreasing energy consumption.

According to John Straube, principal of Building Science Corporation and professor at the University of Waterloo in Ontario, air leaks can be responsible for up to one third or more of the energy loss of a home.[1] Air leaks are caused by pressure differentials between the interior and exterior of the home. These differentials are caused by three factors: wind, fans and the stack effect. For example, when wind pushes on the house the leeward side becomes negatively pressurized, drawing warm, damp air from inside the home out through holes in the walls. When this warm, damp air is drawn through the exterior walls of the home, if it is colder outside than it is inside the water vapour in the air will condense inside the wall. Over time this can cause mould and rot. Similarly, the stack effect works on the principal that warm air rises. Like a large chimney, air and heat will escape through holes and cracks of the roof of a house based on the upward air movement. If not sealed properly, moisture can build up in attic and ceiling spaces.

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