If certain rooms always feel chilly no matter where you set the thermostat, or your heating bill jumps every winter, the problem may not be your furnace. It’s probably air leaking in and out through gaps you can’t see.
The Department of Energy describes air sealing as one of the most cost-effective ways to improve a home’s comfort and energy efficiency. And the best tool for finding leaks costs about a dollar.
The incense test
Smoke reveals air movement that you can’t feel. Here’s how to use it:
- Pick a cool, windy day. Wind and the temperature difference between inside and outside make leaks much easier to spot.
- Depressurize the house to exaggerate leaks: close all windows and exterior doors, turn off the furnace, and switch on exhaust fans like the bathroom fans and the range hood. This pulls outside air in through every gap.
- Light a stick of incense and hold it near likely leak points, moving slowly.
- Watch the smoke. If it streams sideways, gets sucked toward a gap, or blows away from it, you’ve found a leak.
- Mark each leak with painter’s tape so you can come back and seal it.
Keep a hand under the incense to catch ash, stay clear of curtains and anything flammable, and never leave it unattended. If you’d rather not use smoke, a damp hand also works — you’ll feel cool air on wet skin — but it’s less sensitive.
Where to look for leaks
Doors and windows
Check around the frames, where the window sash meets the frame, and along the bottom of exterior doors. If you can see daylight around a closed door, you’re definitely losing heat.
Electrical outlets and switch plates on exterior walls
This one surprises many people. Outlets and switches on exterior walls sit in holes cut through the drywall, and air can flow through them from inside the wall cavity.
Baseboards and where walls meet floors
Gaps between flooring and baseboards on exterior walls can let in cold air.
Attic hatches and pull-down stairs
An attic access panel is often just a loose piece of drywall sitting on a frame with no insulation and no seal — essentially a hole in your ceiling into an unheated space.
Fireplaces
An open fireplace damper is like an open window. The DOE recommends keeping the damper closed whenever the fireplace isn’t in use. Check that yours actually seals shut.
Pipes, vents and cable entry points
Look where plumbing, dryer vents, cable lines and air conditioning lines pass through walls, both inside and outside.
Recessed lighting
Older recessed ceiling lights below an attic can leak a lot of warm air upward. Some are rated for direct insulation contact and air-tightness; older ones may not be.
How to fix common leaks
Door sweeps and thresholds
If there’s a gap under an exterior door, add a door sweep — a strip of rubber, vinyl or brush that attaches to the bottom of the door. Many simply screw or stick on. If your threshold is adjustable, raising it slightly can close a gap too.
Weatherstripping
Weatherstripping seals the moving parts of doors and windows. Common types include:
- Adhesive foam tape: cheap and easy, good for irregular gaps, but wears out faster.
- V-strip (tension seal): durable vinyl or metal strip that springs open to seal sides of double-hung windows and doors.
- Rubber or silicone tubular gaskets: great around door frames.
Clean and dry the surface first so adhesives stick, and make sure doors and windows still close and latch properly afterward.
Caulk
Use caulk for gaps that don’t move — around window and door trim, baseboards, and where pipes enter walls. Choose a caulk rated for the location (indoor, outdoor, or bathroom). Cut the tube tip small, apply a steady bead, and smooth it with a wet finger or tool.
Foam outlet gaskets
Pre-cut foam gaskets fit behind outlet and switch covers on exterior walls. Turn off power at the breaker first, remove the cover plate, place the gasket, and screw the plate back on. Child safety plugs in unused outlets can block a little more air.
Expanding spray foam
For larger gaps around pipes and wires, use expanding foam sealant. Use a minimal-expansion product around window and door frames so it doesn’t bow them. Follow the safety directions on the can.
Attic hatch
Glue a piece of rigid foam insulation board to the top of the attic hatch, and add weatherstripping around the frame where the hatch sits. Insulated covers are also sold for pull-down attic stairs.
Quick fixes for windows you can’t replace
- Window insulation film kits: a clear plastic film that you tape around the window frame and shrink tight with a hair dryer. It creates an insulating air pocket and is nearly invisible when done well. Peel it off in spring.
- Thermal or insulated curtains: close them at night to reduce heat loss.
- Use the sun: open curtains on south-facing windows during sunny winter days to let in free heat, then close them at night.
- Rope caulk: a removable putty for temporary sealing around windows that won’t be opened all winter.
Don’t seal everything without thinking about ventilation
A tighter house is a more efficient one, but homes still need fresh air. Keep using bathroom and kitchen exhaust fans to remove moisture. And if you have fuel-burning appliances such as a gas furnace, water heater, or fireplace, make sure they vent properly. Install and test carbon monoxide alarms on every level of the home. For major air sealing projects, a professional energy audit can measure how airtight your home is and check combustion safety.
Is a professional energy audit worth it?
Many utilities offer free or discounted energy audits. A professional audit often includes a “blower door test,” which depressurizes your home with a powerful fan to measure exactly how leaky it is, and may include infrared camera scans to spot missing insulation. It’s a great way to prioritize bigger projects, like adding attic insulation.
A weekend draft-busting checklist
- Do the incense test and tape-mark leaks
- Add door sweeps to exterior doors
- Weatherstrip leaky doors and windows
- Install foam gaskets on exterior-wall outlets
- Caulk gaps around trim and pipes
- Insulate and seal the attic hatch
- Close the fireplace damper
- Apply window film to the worst windows
Frequently asked questions
When’s the best time to seal drafts?
Fall is ideal — before the cold sets in — but leak-hunting is easiest on cold, windy days, so doing a test in early winter and fixing as you go works well.
Will draft-sealing help in summer too?
Yes. Air leaks let hot, humid air in during summer, making your air conditioner work harder.
Are draft stoppers (door snakes) worth it?
They’re a simple temporary fix for gaps under interior or exterior doors, especially in rentals where you can’t install a sweep.
