Principles of Heat Transfer

Knowledge Center: Princles of Heat Transfer

Air Infiltration

Air Infiltration, in essence, bypasses insulation. It transfers heat by the gross flow of air between the exterior and the interior. The underlying force behind air infiltration is the air pressure difference between the exterior and the interior. Air pressure differences can be caused by wind or stack effect.

Air infiltration may be eliminated by air sealing the building envelope. Conventional insulations traditionally use “house wraps” in an attempt to air seal the building envelope. Vapor retards may also serve to reduce air infiltration.

ASTM E 779 – Standard Test Method for Determining Air Leakage Rate by Fan Pressurization

Air infiltration can be measured using the “Blower Door Test.” A fan is installed in an exterior door of the house to be tested (and all other doors, windows, etc. are closed or sealed). The fan speed is adjusted to create a calibrated pressure differential between the outside air and the inside air. The air flow through the fan needed to maintain this pressure difference is measured and used to determine the air leakage rate.

Air Intrusion

Air intrusion is also called “wind wash.” Unlike air infiltration, wherein air moves from the exterior to the interior, air intrusion occurs when air enters the insulation from the exterior and exits back to the exterior. There is no drafting of air to the interior of the building but the thermal gradient of the insulation is disrupted. In effect, air intrusion introduces forced convection into the building envelope (wall, ceiling, etc.). Air intrusion is caused by wind.

Spray polyurethane foam

For the same reasons spray foam stops air infiltration, it will stop air intrusion.


spray polyurethane foam effectively blocks the three primary and the three secondary mechanisms of heat transfer, offering insulation efficiencies well beyond that suggested by its high R-value alone. Spray foam’s insulating efficiency remains high when other insulations’ efficiencies drop due to wind, high temperature differences, and moisture. Read our article about R-value to learn more.