How to Find and Seal Air Leaks in Your Home
Most leaks aren't around windows — they're at the top and bottom of the house: attic hatches, recessed lights, plumbing and wiring penetrations, rim joists, and the sill plate. Seal those before you touch a window.
Air leakage is the loss people underestimate, partly because you can't see it and partly because the obvious suspect — the draughty window — is rarely the main culprit.
Find the leaks
The DOE-described approach costs almost nothing: on a cold, windy day, close the house up, turn off any combustion appliances, and run your kitchen and bathroom exhaust fans. That pulls the indoor pressure down and exaggerates incoming draughts. Then move a lit incense stick slowly past the usual suspects and watch where the smoke bends.
Where to look, in order of typical payoff
- Attic hatch or pull-down stairs — often a large, uninsulated hole in the ceiling.
- Recessed light fixtures and ceiling penetrations for wiring and plumbing.
- The rim joist band in the basement or crawl space.
- Where the sill plate meets the foundation.
- Around chimneys and flues (fire-rated sealant only).
- Then, and only then, doors and windows.
What to seal with
| Gap | Material |
|---|---|
| Under ¼ inch | Caulk |
| ¼ inch to ~3 inches | Expanding spray foam |
| Large or irregular openings | Rigid foam board cut to fit, sealed at the edges |
| Around flues and chimneys | Fire-rated sealant or sheet metal — never standard foam |
| Moving parts (doors, sashes) | Weatherstripping, not caulk |
When results will differ
- A 1920s balloon-framed house and a 2015 build start from completely different leakage levels.
- Windy, exposed sites lose more through the same gaps than sheltered ones.
- In humid climates, sealing changes indoor moisture behaviour — watch for condensation after a big job.
What to do next
Do one session in the attic and one in the basement before buying anything for the windows. Those two areas usually hold the largest, cheapest wins, and they make any insulation you add afterwards work as intended.
Sources
- Air Sealing Your Home — U.S. Department of Energy — Energy Saver. Consulted July 22, 2026.
- Insulation — U.S. Department of Energy — Energy Saver. Consulted July 22, 2026.
- Energy Saver: Save Energy, Save Money — U.S. Department of Energy. Consulted July 22, 2026.
Frequently asked questions
How do I find air leaks without special tools?
On a windy, cold day, close all windows and exterior doors, turn off combustion appliances, run bathroom and kitchen exhaust fans to depressurise the house, then move a lit incense stick slowly around suspect areas and watch the smoke move.
Are windows really the main source of leaks?
Usually not. The biggest losses tend to be at the top and bottom of the building — attic penetrations and the basement rim joist — because that's where the pressure difference is greatest.
Can a house be too airtight?
Yes. Tight homes need mechanical ventilation, and any home with gas or oil appliances needs sufficient combustion air. If you're sealing aggressively, have combustion safety checked.