15 July

Buildings under pressure: part two

The infiltration and ex-filtration of air in buildings is the primary reason to install an air barrier system.  When installing an air barrier system it is critical to understand the building pressures that cause infiltration and ex-filtration.  Last post, we discussed wind pressure as one of three major causes of building air leaks. Today, we address a second major cause – stack pressure.

Stack pressure, or the “chimney effect,” moves large volumes of air through a building envelope. It occurs when temperature changes create an unequal distribution of atmospheric pressure at the top and bottom of a building.  Indoor and outdoor temperature variations affect the weight of air columns.  In the winter, warm air in a heated building is lighter (less dense) than the cold air outside the building.  That warm air bubble rises up and out. When it does, the flow of air leaving the top of the building draws cold air into cracks at the bottom of the building. The reverse occurs in warm climates with air-conditioning. 

Over the years, building professionals have learned that the best solution to stopping major sources of air leakage is to properly specify and install an air barrier system.

Next post, we’ll talk about how HVAC fan pressure contributes to air leakage.

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