Most buildings are not perfectly sealed nor do most have adequate, quality home insulation. Thus, air can infiltrate at the lower levels of a house and escape at the upper levels. This is known as the “stack effect.” During the heating season, colder air enters the building around basement windows or gaps in the area of the joist headers. As the air is warmed it rises up through the building and escapes at the top – either through insufficient home insulations, gaps around windows, ventilation openings, or gaps and holes in the attic floor. There is a pressure difference between the cold outside air and the warm air inside the building caused by the difference in temperatures. The rising warm air reduces the pressure in the base of the building, drawing cold air in and causing a draft. The taller the building, the more pronounced the stack effect.
Vapour sealing of the attic floor will help prevent humid, warm air from leaking into the attic. However, it is just important to cut off the incoming air at the source. We have found that by installing polyurethane spray foam in the joist header areas (the area immediately above the foundation and just below the main floor), not only do we insulate this area, but we also prevent cold air from entering the house. The lower-level drafts will be reduced, resulting in less leakage of air into the attic.