Building Shell Air Leakage

Q – Why is stopping air leakage so important?

·         Air leakage can drastically reduce insulation R-value

·         Air leakage can account for a major percentage of a building’s heat loss

·         Air leakage moves moisture into and out of the building, and so exerts a wetting and/or drying effect

·         Air leakage can cause building pressures that can interfere with the venting of combustion appliances

Q – What affects building air leakage?

·         Building height, location, weather and mechanical equipment

Q - What is an air barrier?

·         Air barriers control air leakage into and out of the building envelope.

Q – How does air typically move through a building?

·         Often air moves through a building as if it was a chimney; air enters low in the building through infiltration and exits at the top of the building as exfiltration, otherwise known as the stack effect.

Q – Why is air-leakage testing recommended?

·         There is too much uncertainly about air leakage. Testing is required because there is no exact prescriptive method for determining the severity and location of leaks.

How much is your air leakage costing you?

THE SCIENCE BEHIND AIR LEAKAGE

Two conditions are needed for air to leak. First there must be a hole, gap or crack from one side of the envelope to the other. Second, there must be an air-pressure differential, for which there are three causes: wind, stack effect and the HVAC system. Wind pressurizes the windward side of the building and depressurizes the back, sides and roof.

It cannot be controlled, only reduced by plugging the holes in the envelope. Stack or chimney effect is a buoyancy phenomenon where warm inside air rises through the building and exerts continuous pressure against the roof and upper parts of the exterior walls. The resulting lower pressure at the bottom of the building actually sucks in air.

This phenomenon reverses itself in air-conditioned structures during hot weather. The effects are stronger in winter climates due to larger differences between indoor and outdoor temperatures. The third source of pressure differential is the mechanical system.

Makeup air is often brought into a building to replenish exhaust air but unfortunately, this increases the air pressure at the top of the building and the result is even greater exfiltration in that area.