Exfiltration is the uncontrolled escape of indoor air from a building through gaps, cracks, and openings in the envelope.

Exfiltration refers to the movement of conditioned indoor air out of a building through unsealed gaps, cracks, or penetrations in the building envelope. It is the opposite of infiltration, which occurs when outside air enters the building.

In construction and building science, exfiltration commonly occurs around windows, doors, wall penetrations, attics, and poorly sealed assemblies. Warm air escaping in cold climates can carry moisture into wall or roof cavities, where it may condense and cause insulation degradation, mold growth, or structural damage.

Controlling exfiltration is essential for energy efficiency, moisture management, and occupant comfort. Proper air sealing, continuous air barriers, and quality construction detailing help limit unwanted air leakage. Many energy codes and performance standards set maximum allowable air leakage rates to reduce exfiltration and improve building performance.

In cold climates, exfiltration is often a bigger moisture risk than rain because warm indoor air can carry large amounts of water vapor into wall assemblies.