The Problem of Air Movement 

Air sealing addresses problems related to the natural movement of air.  One cause of this movement, called convection, is that air moves from high pressure zones to low pressure zones.  A second cause is the way that air behaves as it becomes warmer or cooler:  As air gets warmer it expands and becomes less dense; as air gets cooler it compresses and its density increases. The warm, less dense air begins to rise while the cold, dense air falls. Taken in combination, these factors can result in air moving into and out of a home.

If the temperature inside a home is warmer than outside, the warm, inside air will rise to the top of the house and into the attic; air will pass through whatever path it can take to escape from the house and continue rising. As the warm air rises and escapes from the top of the building the pressure in the lower areas decreases which causes cold air to come in from outside. Typical places where this air movement can occur are within walls, through hidden cavities left by the original builders, through cracks in the foundation and through poorly sealed windows and doors.

Air movement can also be effected by mechanical equipment within the house and by the wind outside the house. As the wind drives against the walls of the house, it creates a high pressure zone on the outside and a low pressure zone on the inside, causing air to move through the wall and into the house. Mechanical equipment often will draw air from the inside of the house for combustion, resulting in replacement air that enters the house from the outside through leaks.

Identifying the locations where air is leaking and sealing these openings in order to stop air movement are high priority tasks for anyone interested in energy efficiency.