Air infiltration occurs when outdoor air leaks into the home through openings in the building shell, or envelope. This phenomenon often occurs due to wind, but is also caused by air pressure differences between the interior and exterior of a structure. The shell of the building consists of the basic structures such as the wall and roof, as well as doors, windows, and insulation. Most homes are loosely constructed, and thus contain hundreds of small cracks or openings where air infiltration can occur. The opposite of this effect is known as "air exfiltration," and occurs when conditioned air from inside leaks outside through these same openings.
While some air infiltration is helpful to maintain adequate ventilation, too much home air infiltration can cause a number of problems for homeowners. This air is unconditioned, which means that it causes unwanted heating or cooling. This means air conditioners or furnaces have to work harder to maintain a comfortable temperature range. As these systems work harder, utility bills often increase. Uncontrolled air infiltration also brings dust and dirt in from the outdoors, making it harder to keep the home clean.
To minimize air infiltration, homeowners should seal up cracks and other openings within the building envelope. Caulk can be used inside and out to fill holes in siding or masonry, or to seal the transition gap between two different types of finishes. Caulk or expandable foam is also helpful for filling large wall penetrations, which typically occur where pipes or cables enter the home. Insulation can also reduce air filtration, particularly in attics and crawl spaces, which are often poorly insulated.
A great deal of home air infiltration occurs at windows and doors. Homeowners can use gaskets or weatherstripping to seal between the door and frame to minimize drafts. A door bottom or sweep can be installed along the base of the door. Combined with a threshold, this eliminates many drafts. It is also helpful to caulk the gap between door or window frames and the surrounding wall, both on the interior and exterior sides of the home.
Homeowners planning a major remodel can add an air infiltration barrier system to keep unwanted air out. This system includes building paper or moisture barriers combined with insulation and carefully sealed exterior finishes. Those who want to reduce infiltration through existing walls can use spray foam insulation. This material is installed through tiny holes drilled in the wall, and can block air infiltration without damage to the existing structure.