How do you find and seal air leaks in your house?

By Roof & Router Team. Updated October 11, 2026.

Brown wooden framed glass window — Photo: Unsplash

Short answer: Start in the attic and basement, where wiring holes, plumbing vents, flue chases, and rim joists leak the most. Find drafts by sight or with incense on a windy day. Seal gaps of 1/4 inch or less with caulk and gaps up to about 3 inches with spray foam. EPA estimates air sealing plus insulation saves an average 15% on heating and cooling.

Where are the biggest air leaks in a house?

The obvious drafts are at windows and doors. The bigger leaks are usually hidden in the attic and basement. ENERGY STAR’s DIY guide lists these common spots:

Location	What leaks
Attic hatch	Gap around an unsealed, uninsulated lid
Wiring holes	Holes drilled through top plates for cables
Plumbing vent	Gap where the vent pipe passes into the attic
Open soffit	Boxes that hide recessed lights or ductwork
Furnace flue or duct chase	Open shafts running from the basement to the attic
Behind kneewalls	Open framing behind short attic walls
Basement rim joist	Where the foundation meets the wood framing
Windows and doors	Gaps at frames, trim, and worn weatherstripping

Warm air rises and escapes through the attic, pulling cold air in low through the basement and rim joist. That is why those areas matter more than a drafty window.

How do you find air leaks yourself?

Look first. Walk the house and check where different materials meet. In the attic, dirty or dark insulation is a clue. ENERGY STAR notes it forms when insulation filters dusty air moving up from the house.

Use smoke on a windy day. NC Cooperative Extension suggests closing all doors and windows, then running every exhaust fan. That means the dryer, bath fans, and range hood. Hold a lit incense stick near suspect spots. Wherever the smoke wavers or gets pulled sideways, air is moving.

Fire caution: never leave lit incense unattended, and keep it away from curtains, insulation, and attic dust. Do not run this test while a gas furnace, water heater, or fireplace is burning. Strong exhaust fans can pull combustion gases back into the house. Turn the fans off when done.

Hire a blower door test. A professional energy auditor mounts a calibrated fan in an exterior door. It pulls air out of the house so leaks show up clearly. Ask for an auditor who uses a calibrated blower door.

What materials should you use?

Gap or location	Material
Cracks 1/4 inch or less	Caulk
Gaps 1/4 inch to about 3 inches	Expanding spray foam
Moving parts of doors and windows	Weatherstripping
Around metal flues and chimneys	Aluminum flashing plus high-temperature caulk
Large open chases and soffits	Rigid foam board or drywall, sealed at edges

The gap sizes come from ENERGY STAR’s DIY guide.

How do you seal around a chimney or furnace flue?

Never use spray foam or regular caulk against a flue. Flues get very hot. ENERGY STAR notes codes usually require 1 inch of clearance from metal flues and 2 inches from masonry chimneys. Do not fill that clearance with insulation. Bridge it with non-combustible material: cut aluminum flashing, sealed with high-temperature (fire-rated) caulk. Check your appliance and local code for exact clearances.

Around recessed “can” lights, keep insulation 3 inches away unless the fixture is IC-rated (insulation contact). ENERGY STAR recommends ICAT-rated fixtures, which are also airtight.

How much does air sealing save?

EPA estimates an average 15% savings on heating and cooling costs (11% on total energy costs) from air sealing plus insulating attics, floors over crawl spaces, and accessible rim joists. ENERGY STAR’s consumer page rounds this to “up to 10%” on annual energy bills. Both are about sealing and insulating together, not sealing alone.

These are modeled numbers for a typical 1970s-1980s home. Results vary by climate. EPA’s table shows total-house savings from 5% in Climate Zone 1 to 16% in Climate Zone 8.

Formula: annual savings = yearly heating and cooling spending x 0.15

Yearly heating + cooling spend	Estimated savings at 15%
$800	$120
$1,200	$180
$1,600	$240
$2,000	$300

Is it safe to make your house too tight?

Sealing changes how air moves through the house. ENERGY STAR warns CO can build up if gas-burning appliances are not venting properly. Ask your heating technician to check the furnace, water heater, and dryer venting after major sealing work. Install carbon monoxide alarms if you have any fuel-burning appliance. A tighter house can also hold more moisture, so watch indoor humidity in winter.

For other cold-weather prep, see the winterize house checklist.

Quick answers

Should I use caulk or spray foam? Caulk for cracks 1/4 inch or less. Spray foam for gaps from 1/4 inch to about 3 inches.

Can I use spray foam around a chimney? No. Use aluminum flashing and high-temperature caulk, and keep code-required clearances.

How much can sealing air leaks save? EPA estimates an average 15% on heating and cooling costs when combined with insulation. Colder climates tend to save more.