Window Air Leakage: How to Find It and Fix It Properly

Window air leakage is outside air getting into your home (or heated air getting out) through gaps around or inside a window. You feel it as a cold draft, hear it as a whistle on windy nights, and pay for it on the heating bill. Most leaks come from worn caulking, tired weatherstripping or a sash that no longer closes square, and most can be found in an afternoon with a few simple checks. The fix depends on where the air is getting through, so find the source before you buy anything.

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TL;DR

Air leaks need a gap and a pressure difference. In winter, warm air rises and leaves at the top of the house, pulling cold air in through gaps lower down.
Find leaks on a cold, windy day with a stick of incense or a hand held near the frame, trim and sash.
Caulk seals the stationary joints (trim to wall, trim to frame). Weatherstripping seals the parts that open. Hardware and alignment come first.
Seal from the inside before the outside, and do not caulk below 5°C.
Persistent leaks, frost, stains or rot are signs to have the window and wall assessed, not patched.

Why windows leak air
Air leak, water leak or condensation?
How to find window air leaks
Wind noise around windows
Caulking, weatherstripping or hardware: which fix, where
Doing the caulking properly
What window air leakage ratings mean
Our perspective: fix the cause, not the symptom
When to call a professional
Landlords and property managers: inspect on a schedule
FAQ
Sources

Why windows leak air

Air moves when two things line up: an opening in the building envelope and a pressure difference across it. Natural Resources Canada explains that in winter the house acts like a chimney, drawing air in at lower levels and pushing it out at upper levels and through the ceiling. Wind adds to this on the windward side of the house.

A window gives air several places to get through:

The joint between the frame and the wall. This is the rough opening that was filled with insulation, foam or nothing at all when the window went in. Over time the sealant at the exterior and the trim at the interior both age.
The trim and mitred corners. Interior casing shrinks and opens up with the dry winter air. Small gaps become visible lines.
The sash-to-frame seal. Weatherstripping compresses, hardens, cracks or falls out of its groove.
Hardware and alignment. Latches, cranks and locks that no longer pull the sash tight leave a gap even with perfect weatherstripping.
The glass unit itself. Failed seals in a double or triple pane let moisture in between the panes, which is a different problem from air leakage.

In 25 years of sealing windows around Oshawa and Durham Region, the pattern we see most is not a single dramatic gap. It is five or six small ones that add up, usually on the north and west sides where the wind hits hardest.

Air leak, water leak or condensation?

Three different problems get lumped together as “my window is leaking.” They have different causes and different fixes, so sort them out first.

Air leaks show up as drafts you can feel, whistling or wind noise, curtains that move on a still day, and cold spots on the wall or sill. There is usually no water.

Condensation is water beading on the glass or fogging it. It forms when warm, humid indoor air meets a cold surface. It is not a leak in the frame at all. CMHC notes that a home can release 10 to 50 litres of moisture a day, from cooking, showers, breathing and drying laundry, so a very tight house with no ventilation will fog its windows even when the windows are fine.

Water leaks leave stains, swollen wood, peeling paint, or drips that appear after rain. A forensic engineering primer from WJE notes that streaking down vertical window elements may mean water got in at the head because flashing was poorly installed or missing, while pooling on horizontal mullions can point to a breach in the gaskets or glazing above.

A quick way to separate them: if the dampness is on the glass and clears when the room dries out, think condensation. If it shows up only after rain, or on the frame and wall below the window, think water entry. If you can feel moving air on a dry day, you have air leakage. You can have more than one at the same time.

If you suspect water, our five minute hose test for window leaks after rain shows you how to isolate the entry point, and our piece on whether caulking can fix window leaks covers what caulk can and cannot do.

How to find window air leaks

Pick a cold day, ideally a windy one. The bigger the pressure difference, the easier the leaks are to feel.

The incense or smoke test

Natural Resources Canada describes using burning incense to locate leaks. Hold it near the frame, along the trim, at the corners and where the sash meets the frame. A strong leak scatters the smoke. A slower one makes it trail away or drift toward the gap.

The hand and paper check

Run the back of your hand slowly along every edge. Then close a strip of paper in the sash and try to pull it out. If it slides out with no resistance, the weatherstripping is not compressing at that spot.

A whole-house fan test

NRCan also describes a basic depressurization test: close all windows and doors, turn on the exhaust appliances, and use a leak detector to mark where air is pulled in. One safety point matters here. Turn off fuel-fired space and water heaters first, so the test does not cause backdrafting of combustion gases. If you are not sure how your furnace or water heater vents, leave this test to a professional.

Mark what you find

Use painter’s tape to mark each leak and note the window, the side and whether it is trim, sash or corner. That list becomes your repair plan, and it is exactly what a contractor will want to see. A printed building inspection checklist is a useful model for structuring this kind of walk-through room by room.

Wind noise around windows

Whistling or rushing sounds are air leakage you can hear. The sound comes from air squeezing through a narrow gap at speed, so the source is usually a thin, consistent gap rather than a large hole. Check these in order:

Worn or flattened weatherstripping on the sash, especially at the top and sides of sliders and casements.
A sash that has dropped or gone out of square. Look for uneven gaps between the sash and frame.
Loose or worn latches and hinges that no longer pull the sash tight.
Open joints in the exterior caulking or interior trim.

When the noise changes with wind direction, it is almost always at the envelope joint rather than in the sash. That points to caulking, not weatherstripping.

Caulking, weatherstripping or hardware: which fix, where

NRCan’s guidance on upgrading windows is clear about the division of labour: caulk goes around window trim where it meets the wall, at mitred trim joints and between the trim and frame, while weatherstripping seals the frame to a movable sash. It also advises adjusting and squaring misaligned windows before you install weatherstripping.

Where the air is getting in	What fixes it	Notes
Trim to wall, trim to frame, mitred corners	Caulk	Stationary joints that should not move much
Sash to frame on a window that opens	Weatherstripping	Replace, do not layer new over old
Sash that does not close tight	Adjust or repair hardware, square the sash	Do this before weatherstripping
Window you never open	Removable strippable caulk	NRCan suggests it for windows not needed for ventilation or emergency escape
Gap larger than 6 mm (1/4 in.)	Backer rod plus caulk, or low-expansion foam	Small gaps may only need insulation and caulk
Fogging between the panes	Glass unit replacement	Not an air-sealing job

The gap sizes come from NRCan: for a gap of 6 mm or less, insulating then caulking may suffice, while larger gaps may need a backer rod with caulking or low-expansion foam.

The strippable caulk advice, for windows that are not needed for ventilation or as an emergency exit, appears in NRCan’s air leakage guide. Never seal shut a window that a bedroom needs as an escape route.

If you are weighing caulk against foam, we compare them in is it better to use caulk or foam around windows. And for the six most common draft sources, our guide to fixing drafty windows walks through each one.

Do the inside first

This is the step people skip. NRCan calls exterior caulking the last and weakest defence against rain entering a wall, and says to do it only after interior sealing is complete. If you seal the outside tight while warm, moist indoor air is still getting into the wall cavity, that air can be trapped and damage the wall over time.

In practice: stop the indoor air first (interior trim joints, the gap behind the casing), then renew the exterior sealant so rain stays out. The two jobs work together.

Doing the caulking properly

Caulking is easy to do badly. A bead laid over old, dirty sealant looks fine in October and peels by February. Here is what the job involves when it is done right.

Remove the old material. Hydro-Québec’s guidance is to clear dirt, loose paint and old caulking, removing any split or dried compound and brushing the area clean. New sealant bonds to a clean, dry surface, not to old sealant.
Match the sealant to the surface. Brick, vinyl, aluminum, wood and stucco do not all behave the same. Our guide to the best caulk for exterior windows and the one on window caulking for brick homes go through the options.
Size the gap. Wide joints need a backer rod so the sealant bonds on two sides only and can stretch. Our note on why joint width affects sealant performance explains the reasoning.
Mind the temperature. Hydro-Québec advises not caulking below 5°C, since the compound stiffens and becomes hard to work. In Ontario, that makes late spring through early fall the natural window for exterior work. Interior work can be done in winter if the product allows it.
Tool the bead. Press the sealant into the joint so it wets both surfaces, and smooth it. A bead that just sits on top will not hold.

For the prep side in detail, see preparing your windows for caulking and our joint surface preparation guide. If you are working at height, read our window caulking safety tips first.

Weatherstripping: measure before you buy

To estimate how much you need, add up the perimeters of the windows and doors, then add 5 to 10 percent for waste. Remove the old strip fully, clean the surface and make sure the new material meets tightly in the corners. If the sash still leaves a visible gap after new weatherstripping, the problem is alignment or hardware, not the strip.

What window air leakage ratings mean

If you are replacing windows, the label matters. Under the ENERGY STAR Canada specification, Version 5.0, air infiltration and exfiltration rates of 1.5 L/s/m² or less are required, and models must be registered with NRCan to carry the ENERGY STAR certified label in Canada. The specification took effect January 1, 2020.

A lower number means less air passes through the window under test conditions. That is a measurement of the window itself. It says nothing about how well it was installed, and a good window in a poorly sealed rough opening will still leak at the edges. This is why the perimeter sealing matters as much as the product.

For larger buildings the requirements get tighter. A University of Waterloo document on curtain walls notes the Canadian limit of 0.1 L/s/m² at 75 Pa, against 0.3 in the United States, and ties that stricter limit to ice buildup, condensation in glazing cavities and rain penetration in wall cavities.

Our perspective: fix the cause, not the symptom

We get calls every fall from people who have already tried a tube of caulk from the hardware store and a roll of foam tape. Sometimes it helped. More often, the new bead is sitting on old, failing sealant, or it was applied in cold weather and never cured properly.

Our view after years of working through Ontario’s freeze-thaw cycles is simple. A window is a moving joint. It expands and contracts with every temperature swing, and sealant has to flex with it for years. That means three things: a clean joint, the right sealant for the material, and the right joint depth. Skip any one and the repair fails the first hard winter.

We also tell people when caulking is not the answer. If a sash is out of square, if the glass unit has failed, or if there are signs of water getting into the wall, sealing the outside edge only hides the problem. We would rather tell you that on the first visit than come back in two years.

And for the draft you can feel right now, a small, targeted interior repair often does more than people expect. Our overview of caulking and energy efficiency explains what to expect, and the answer is honest: it helps, and it is one part of a sealed envelope, not the whole thing.

— Felix

When to call a professional

Natural Resources Canada recommends hiring an air-sealing contractor or a professional energy advisor for the best results in locating and sealing air leaks. In our experience, these are the situations where that call makes sense:

Drafts remain after you have renewed caulking and weatherstripping.
You see staining, soft wood, peeling paint or mould on or below the window.
Several windows leak on the same side of the house, which suggests a wall or envelope issue.
Brick or mortar around the window is cracked, or the sill is damaged.
Leaks appear only in certain wind-driven rain, which is harder to trace.

That last point is a diagnostic clue in its own right. The WJE primer suggests tracking whether a leak happens every day or only during heavy directional rain, and recording the conditions. Write down the date, the weather and the wind direction. It saves time on site.

If you want us to take a look, we seal windows, doors, expansion joints and full building envelopes across the Greater Toronto Area, including Oshawa, Whitby, Ajax, Pickering and Bowmanville. See our residential window caulking service, or the door caulking and weather sealing page if the draft is at an entry. If the leaks run across the whole wall, building envelope sealing is the better starting point. You can reach us through our main site.

Landlords and property managers: inspect on a schedule

In a multi-unit building, one drafty window is a complaint. Forty of them is a pattern, and a pattern is cheaper to fix as a program than as repeat service calls.

A practical routine looks like this:

Walk the building once a year, ideally in early fall, before heating season and while exterior work is still possible.
Check that frame seals have not been dislodged, and press them back into place if they have. A window manufacturer’s maintenance guide also advises keeping drainage slots and the gaps between frame and sill clear of debris and checking that hardware works correctly.
Log every complaint with the unit, the window, the weather and the time of day. Patterns by elevation or by floor tell you whether you have a sealant problem or a wall problem.
Reseal in batches. Sealing one elevation at a time keeps costs predictable and the sealant ages evenly.
Look at movement joints too. Window perimeters are not the only joints on a building. See our guidance on expansion joint sealing and commercial window caulking.

Heating equipment is part of the same picture. Drafts make a furnace work harder, so it makes sense to pair your envelope check with an annual HVAC inspection so you are not heating the street.

FAQ

Can caulking stop drafts around windows?

Yes, when the draft is coming through a stationary joint such as the trim, the corners or the gap between the frame and the wall. It will not fix a sash that does not close tight or worn weatherstripping on a moving part. We cover this in more detail in can window caulking stop drafts.

Should I caulk windows inside or outside?

Both often need attention, but start inside. Sealing the interior stops warm, moist indoor air from reaching the wall cavity. Exterior caulking then keeps rain out. If you only do the outside, you can trap moisture in the wall. You can read more about interior work in using window caulking indoors.

Why is my window whistling when it is windy?

Air is forcing through a narrow gap, usually worn weatherstripping, a sash out of square, or an open joint around the frame. Find it with incense or the paper test on a windy day, then fix that specific spot. If the noise changes with wind direction, check the perimeter joints first.

Is window condensation the same as an air leak?

No. Condensation forms on glass when humid indoor air meets a cold surface, and it points to indoor humidity and ventilation. Air leaks let outdoor air in through gaps. They can occur together, but the fixes differ.

How long does window caulking last?

It depends on the sealant, the joint and the exposure. South and west faces take more sun and movement, so they tend to wear first. Check your caulking every year for cracks, gaps or loss of adhesion, and reseal when you see them rather than waiting for a leak.