Reverse flow of combustion gases down the chimney of a vented combustion appliance, which is often caused by depressurization of the room where the appliance is located.


Process of adjusting the flow of air in duct systems, or water flow in hot-water heating systems.

Blower Door

A blower door is a calibrated fan (or blower) in a portable expanding frame, typically installed in an exterior door opening. Blower doors are used to determine the leakiness of a building envelope. A blower door measures the total air flow rate through all of the openings in a house by extracting air from (or blowing it into) the house and measuring the air pressure drop across the house. From this information, the area of al house cracks and openings can be calculated.


Amount of energy required to heat one pound of water one Fahrenheit degree.


British thermal units per hour. A BTU is the quantity of heat required to raise 1 pond of water by 1 degree Fahrenheit.

Building Enclosure

The system or assembly of components that provides environmental separation between the conditioned space and the exterior environment.

Building Science

The relationship of building materials and the building enclosure to heat, air, radiation, and moisture; controlling damage caused by these elements.