The Building Envelope is the shell of the house, comprising the basement or crawl space walls and floor (i.e. the foundation), the above-grade walls, the roof, and the windows and doors. The envelope separates our indoor environment from the weather outside. To maintain our indoor environment, the envelope must control the flow of heat, air and moisture from the inside to the outdoors.
Uncontrolled airflow (i.e. leakage) through the envelope can be the major source of heat loss and can lead to other problems. Since warm air can carry large amounts of water vapour, airflow is also the main means by which moisture is carried into the envelope.
Under winter conditions, inside air is forced out through the building envelope, carrying heat and moisture, while incoming replacement air brings drafts and dry winter air. For air to move from one side of the building envelope to the other, there must be holes in the envelope and a difference in air pressure between the inside and outside. The difference in air pressure can result from any combination of wind, a temperature difference creating a stack effect in the home, and combustion appliances or exhaust fans.
Controlling airflow protects building materials from moisture damage, improves comfort and makes for a cleaner, healthier, safer and quieter home. Controlling airflow prevents uncontrolled air leakage through the building envelope, provides for fresh air supply and the exhaust of stale air, and provides draft and combustion air for fuel-burning appliances.