How to Air Seal Before You Insulate

Most leakage is at the top and bottom of the house — the stack effect pulls cold air in low and pushes warm air out high. Walk the attic with a bright light looking for dirty insulation (a filter stain showing air movement) and daylight around penetrations.

An infrared thermometer or thermal camera on a cold day makes leaks obvious. A cheap smoke pencil or an incense stick works too.

Under 1/4 inch: acrylic latex or siliconized caulk. 1/4 inch to 3 inches: low-expansion canned spray foam. Over 3 inches: rigid foam board or drywall scrap cut to fit, foamed at the edges.

Around metal flues and masonry chimneys use sheet metal and high-temperature sealant only — never canned foam, which is combustible and must be held 3 inches back from B-vent.

Outlet and switch gaskets on exterior walls, weatherstripping on doors, sweeps under doors, foam covers over recessed lights, and shrink-film kits on drafty single-pane windows are all cheap fixes that add up.

A house can be sealed to the point where it needs mechanical ventilation. If you are doing a whole-house air sealing project and combustion appliances are inside the envelope, get a blower door test and consider a bath-fan-based or ERV ventilation strategy.

Choosing caulk, foam and gaskets? See the best air sealing products.