Cut your heating bill · a fall afternoon's work
How to find and seal drafts before winter
Air sealing is the cheapest efficiency upgrade a house can get. Every gap where outside air sneaks in is heat you're paying for, escaping — and most of it is fixable in an afternoon with a few dollars of caulk and weatherstripping. The trick is that the leaks you can feel (a drafty window) are rarely the leaks that cost you the most (hidden gaps in the attic and basement). This guide shows you how to find both, and which material seals which gap.
ENERGY STAR estimates that by air-sealing their home and adding insulation, a typical homeowner can save an average of about 15% on heating and cooling costs. Caulk runs a few dollars a tube and weatherstripping a few dollars a door, so the payback on the small stuff is fast. Do this in the fall while it's still comfortable to work, and you'll feel the difference the first cold week.
Step 1 — Find the drafts (the detective part)
Pick a cool, windy day — the bigger the temperature difference between inside and out, the easier leaks are to spot. Then use one of these:
- The smoke test. Light an incense stick and move it slowly along the edges of windows, doors, outlets, switch plates, and where pipes or wires enter. Where the smoke stream wavers or gets pulled sideways, air is moving. (A candle flame works too — just keep any flame away from curtains and drafts strong enough to be a fire risk.)
- The hand test. A slightly damp hand is a surprisingly good draft detector — moving air feels cool on wet skin. Run it around window sashes, door frames, and outlet covers.
- The flashlight test. At night, have someone shine a bright flashlight along the edges of an exterior door while you look from inside (or vice versa). Light coming through is a gap you can seal.
Write down what you find as you go — you'll do the fixing in one trip, and it's easy to forget the outlet in the back bedroom.
Step 2 — Check the places that leak most (not the obvious ones)
Here's the counterintuitive part the Department of Energy stresses: the largest air leaks usually aren't the windows you can feel. Because warm air rises and escapes high while cold air is drawn in low (the "stack effect"), the big leaks hide at the top and bottom of the house:
- In the attic: the access hatch or pull-down stairs, around recessed lights, and where plumbing vents, wiring, and the chimney pass through.
- In the basement or crawlspace: the rim joist (where the framing sits on the foundation), and around every pipe, wire, and duct entering the house.
- Everywhere: exterior door and window frames, the sill plate, mail slots, dryer vents, and where any utility line penetrates an exterior wall.
Sealing the big hidden leaks in the attic and basement typically buys you more than sealing every window — though doing both is the goal.
Step 3 — Seal each gap with the right material
Matching the material to the gap is what makes a seal last:
- Caulk — for stationary gaps between materials that don't move, generally under about a quarter-inch: the seam where a window frame meets siding, cracks around trim, small penetrations. Use exterior-grade caulk outdoors.
- Weatherstripping — for gaps around things that open and close: the sides and top of a door, an operable window sash, an attic hatch. Adhesive-backed foam, V-strip, and door sweeps each suit different spots.
- Expanding spray foam — for larger gaps and holes, like around a pipe entering the basement. Use minimally-expanding foam around windows and doors so it doesn't bow the frame.
- Foam outlet/switch gaskets — cheap pre-cut pads that go behind the cover plates on exterior walls, sealing a leak most people never think of.
- Rope caulk or a door draft stopper — renter-friendly, removable options for windows and the bottom of a door.
Don't seal an air leak that's actually a combustion vent. The area around a gas water heater, furnace flue, chimney, or any fuel-burning appliance needs proper clearance and specific high-temperature materials — never stuff ordinary foam or caulk near a flue. That's a licensed-pro question, not a DIY seal.
A house can be sealed too tight. Combustion appliances and bath/kitchen fans need makeup air to work safely. If you're doing aggressive sealing, or you have older combustion appliances, that's worth a conversation with an HVAC pro or an energy auditor — and a working carbon-monoxide alarm on every level is non-negotiable either way.
Find the leaks on a cold windy day with incense, a wet hand, or a flashlight; give extra attention to the hidden attic and basement gaps that cost the most; then seal each with the right material — caulk for still gaps, weatherstripping for moving ones, foam for big holes. It's a few dollars and an afternoon for one of the best returns in home upkeep — just keep clear of combustion vents and keep a CO alarm working.
Turn your draft hunt into next year's short list
Jot the gaps you sealed — and the ones you're saving for a pro — into HouseHum's free, on-device record, and the app rolls them into a seasonal list tuned to your home. No account, nothing to install.
Open the free appFrequently asked
How do I find drafts in my house?
On a cool, windy day, move a lit incense stick or a candle flame slowly around the edges of windows, doors, outlets, and other suspects — where the smoke or flame wavers, air is moving. You can also just feel with a wet hand, or shine a flashlight along a door's edge at night while someone outside looks for the light escaping. The biggest leaks are usually not the windows you can feel; they're hidden in the attic and basement.
What's the difference between caulk and weatherstripping?
Use caulk for gaps between materials that don't move — the seam where a window frame meets the wall, or around where a pipe or wire enters. Use weatherstripping for gaps around things that open and close, like a door or an operable window sash. A rule of thumb from the Department of Energy: caulk for stationary cracks generally under about a quarter-inch; weatherstripping for moving parts.
Where do houses lose the most heat?
Often not where you'd guess. The largest air leaks are usually hidden high and low — around attic hatches, recessed lights, and plumbing and wiring penetrations in the attic, and around the rim joist and utility entries in the basement or crawlspace. Sealing those big hidden leaks typically does more than sealing every visible window gap, though doing both is ideal.
Is sealing drafts actually worth it?
It's one of the cheapest efficiency improvements a homeowner can make. ENERGY STAR estimates that by air-sealing their home and adding insulation, a typical homeowner can save an average of about 15% on heating and cooling costs. Caulk and weatherstripping cost a few dollars per gap, making the payback on the small stuff quick.
Sources for this page
| Claim | Named authority |
|---|---|
| Air-sealing plus insulation saves a typical homeowner an average of ~15% on heating and cooling costs | ENERGY STAR — "Seal and Insulate" / Rule of Thumb methodology |
| Largest air leaks are often hidden in the attic and basement (hatch, recessed lights, penetrations, rim joist), not the visible windows; stack effect drives loss high and low | U.S. Department of Energy (energy.gov) — Air Sealing Your Home |
| Use caulk for stationary gaps (generally under ~¼ inch) and weatherstripping for moving components; smoke/incense draft-detection method | U.S. Department of Energy (energy.gov) — Air Leaks / Weatherstripping / Caulking |
| Don't seal combustion venting; a too-tight house needs makeup air and a working CO alarm | U.S. Department of Energy combustion-safety guidance; CDC — carbon monoxide alarms |
| Room-by-room detection tips (wet hand, flashlight), material selection, sequencing | Home-upkeep craft — labeled as practice, no evidence claim made |