Windows and doors are some of the most common places for air to leak in or out of a house. Most of the leakage is not through the glass but around it: worn weatherstripping, gaps between the frame and the wall, and failed seals at the trim. Many of these leaks can be fixed without replacing the window.
Why air leakage matters
Air leaks cost more than just heat. In a Canadian winter they cause:
- Drafts and cold spots that make rooms uncomfortable.
- Higher heating and cooling bills, because conditioned air is replaced by outside air.
- Moisture problems. Warm, moist indoor air leaking outward through gaps around a window can condense inside the wall, leading to rot and mould around the opening.
- Frost and condensation on frames and sills.
- Noise and dust from outside.
Windows and doors are part of the building's air barrier. See building envelope basics for how the air barrier works as a whole.
Where the leaks are
Air can leak at several points in a window or door assembly:
- Between the moving parts. Worn, compressed or missing weatherstripping between the sash and frame, or between a door and its jamb.
- At the threshold. Door sweeps and thresholds wear out and leave gaps at the bottom of doors.
- Between the frame and the wall. The rough opening around a window or door frame is often only stuffed with loose insulation, which slows heat flow but does little to stop air.
- Behind trim. Interior casings and exterior trim hide the gap between frame and wall.
- Through hardware and mechanisms. Loose locks, misaligned sashes and worn hinges stop windows and doors from closing tightly.
Finding drafts
Simple ways to find leaks:
- On a cold, windy day, move the back of your hand slowly around the frame and trim.
- Hold an incense stick or smoke pencil near suspected gaps and watch the smoke.
- Close a window or door on a strip of paper. If it slides out easily, the weatherstripping is not sealing.
- Look for daylight around closed doors.
- Look for dirty streaks on carpet or trim, which can mark where air has been moving.
- Check for frost, condensation or water staining on sills and frames.
A blower door test depressurizes the house so leaks are easier to find and measure, and is often combined with thermal imaging to show cold air entering around frames.
Fixes that work
Weatherstripping and sweeps
Replacing worn weatherstripping is usually the cheapest and quickest fix. Match the type to the gap and the window or door; many manufacturers sell replacement parts for their units. Add or replace door sweeps and adjust thresholds so the door closes against them.
Adjusting and repairing
Tighten loose hinges and strike plates, adjust latches so doors close tightly, and lubricate window hardware. Sashes that do not lock firmly against the frame will leak no matter how good the weatherstripping is.
Sealing the rough opening
The gap between the frame and the wall framing is a major leakage path. With interior trim removed, it can be sealed with a low-expansion foam made for windows and doors, or backer rod and sealant. Avoid high-expansion foam, which can bow frames and make windows hard to operate. Tie the seal to the wall's air barrier where possible.
Interior caulking
Caulk gaps between the interior trim and the wall and between trim and the frame. This is a quick improvement but does not replace sealing the rough opening.
Do not seal exterior drainage paths. Window frames and some cladding systems have weep holes and drainage gaps meant to let water out. Sealing them shut can trap water in the wall. Keep exterior sealant to the joints that are meant to be sealed.
A room-by-room draft survey
A typical case: a family in an older two-storey house finds the front rooms drafty and the windows frosting up each winter. Before buying new windows, they work through the house on a cold, windy day with the exhaust fans and dryer running, which pulls outside air in through any gaps and makes leaks easier to feel. For each window and door they record:
| What to note | Why it matters |
|---|---|
| Where the draft is felt: sash, frame edge, trim or threshold | Points to weatherstripping, the rough opening or the door bottom |
| Whether the unit closes and locks tightly | Hardware problems defeat new weatherstripping |
| Frost, condensation or staining, and where | Shows cold spots and possible moisture inside the wall |
| Fogging between panes | A failed sealed unit, which can often be replaced without a new frame |
| Soft or stained wood at sills and frames | Possible water leakage that needs fixing first |
In this case, most of the drafts turn out to be at the frame edges behind the trim, plus two exterior doors with worn sweeps. Removing the casings, sealing the rough openings and replacing the sweeps and weatherstripping fixes most of the problem at a fraction of the cost of replacement. One window with a rotten sill is set aside for replacement.
Common mistakes
- Caulking only the interior trim and assuming the job is done. The leak path behind the trim is still open.
- Filling the rough opening with high-expansion foam, which can bow the frame and stop the window working.
- Sealing exterior weep holes, sill drainage or the bottom of the cladding.
- Adding weatherstripping that is too thick, so the window no longer locks and leaks anyway.
- Tightening the house without thinking about ventilation. A tighter house still needs exhaust fans and fresh air to control humidity.
Repair or replace?
Replacing windows and doors is expensive and should be weighed against repair. Replacement may make sense when frames are rotten, glazing units have failed (fogging between panes), hardware cannot be repaired, or the units are single-glazed and drafty even after repair. If you replace, the installation matters as much as the product: the new unit needs proper flashing for water, a sealed connection to the air barrier and insulation in the gap.
How ehZee can help
An ehZee home inspection looks at windows and doors for operation, seals, condensation and signs of water and air leakage. You can walk through with the inspector and hear findings on the day, with a written report within two days.
Common questions
Why is there condensation on the inside of my windows?
Indoor humidity is condensing on a cold surface. It can mean humidity is high, the window is cold because of poor glazing or frames, or warm air is not reaching the glass. Lowering humidity and improving air circulation often help.
Is spray foam around windows a good idea?
Low-expansion foam designed for windows and doors is common and effective. Regular high-expansion foam can distort frames. Check the window manufacturer's installation instructions.
When is the best time to look for drafts?
A cold, windy day in the heating season, when the difference between inside and outside is greatest. Running exhaust fans at the same time pulls outside air in through gaps and makes leaks easier to feel or see with smoke.