Water Stains Under a Window? Leak or Condensation

Short answer

First decide whether it is a leak or condensation, because the timing gives it away. Condensation shows up in cold weather, with fogged or wet glass on the inside, water pooling on the inside sill, and it has nothing to do with rain. A leak shows up after rain, especially wind-driven rain on one side of the building, and often stains the wall below the window rather than the sill. Leaks usually come in around the window, not through it: failed perimeter caulking, a sill with no proper flashing or drainage, or water entering above the window (a head flashing, a crack in the wall, a roof or gutter) and running down inside the wall to show up at the window. A leak has to be traced to its entry point before anything is sealed. Condensation is fixed by lowering indoor humidity and improving air movement, not by caulking.

A window sill being re-caulked by FAIME during window repairs on a detached home in Willowdale, North York — Re-caulking a window sill during the window repairs on FAIME’s Willowdale project in North York.

Leak or condensation: read the timing

Condensation

Winter, on the coldest days
Fogged, wet or frosted glass on the inside
Water on the inside sill and the bottom of the frame
Worst in bedrooms, bathrooms and kitchens, often first thing in the morning
No link to rain at all

A leak

Appears during or after rain, especially wind-driven rain
On one side of the building, the side the weather hits
Stains on the wall below the window, or at the corners of the frame
Bubbling paint, soft drywall or swollen trim below the sill
Can appear in any season

Keep a note for a few weeks: the date, the weather, which window, and what you saw. It is the most useful thing you can hand a contractor, and it often settles the question on its own.

If it is condensation

Condensation forms when warm, moist indoor air meets a surface colder than its dew point, and in winter the glass is the coldest thing in the room. It is not a building-envelope failure, and caulking the outside of the window will not change it. What helps is lowering indoor humidity (bathroom and kitchen fans that vent outside and actually get used, not drying laundry indoors, a humidifier set lower), keeping air moving across the glass (open blinds and curtains, clear heating vents below windows), and, where the glass or frame is failing, better-performing windows. Fogging between the panes of a sealed unit is a different thing again: the unit's seal has failed and the glass unit needs replacing.

If it is a leak: where the water actually gets in

A window leak very often has nothing to do with the glass. The usual entry points are:

The perimeter joint between the frame and the wall. Caulking that has cracked, shrunk or pulled away lets rain straight into the rough opening.
The sill. Water collects at the bottom of every opening. A sill with no slope, no flashing, a blocked drainage path, or a failed sill joint lets it in.
Above the window. A missing or failed head flashing, a crack in the stucco or brick above, an open joint in the cladding, or a roof or gutter problem higher up. Water gets into the wall above and runs down inside it until the window frame stops it, so the stain shows up at the window even though the window is not the cause.
The window unit itself. Failed corner seals in the frame, worn glazing seals, or blocked weep slots in the frame.

Why sealing everything shut is the wrong first move

Caulking every joint you can see around a leaking window can seal the water in: many windows and walls are designed to let water that gets behind them drain back out at the bottom, and sealing that path makes things worse. It also hides the evidence of where the water was actually coming in. Trace first, seal second.

How a leak is traced properly

Look above as well as around

Check the wall, flashings, gutters and roof line above the window, not just the window itself. Water travels.

Water-test from the bottom up

With someone watching inside, run a hose on one area at a time, starting low at the sill and working upward: sill, then each side, then the head, then the wall above. The area that makes the leak appear is where it gets in.

Open up where the evidence points

Remove the failed caulking or cladding at the entry point and look at what is behind. Wet, soft or stained material shows how far the water has travelled.

Fix the detail, then test again

Repair the flashing, sill or perimeter joint that failed, dry out and replace damaged material, and hose-test again before the wall is closed and finished.

What to do about the stain inside

Wait until the leak is fixed and the wall has dried out before repairing the drywall and repainting. A stain painted over while the wall is still wet comes straight back through. Stained drywall usually needs a stain-blocking primer before the finish coat. Drywall that is soft, crumbling or smells musty should be cut out and replaced, not painted.

Where FAIME fits

Window perimeter caulking, sealant joints, flashing details and the stucco and brick around openings are self-performed FAIME scopes across Toronto, North York, Scarborough, Vaughan, Richmond Hill, Markham, Mississauga, Brampton and the wider GTA, as is the interior drywall and paint repair afterwards.

On our Willowdale project, daylight was visible through the roof at one spot, but once the shingles came off the water track ran well past it. The leak was traced to its real source and hose-tested before the access came down, and all 12 upstairs windows were repaired, caulked and repainted. On our Vaughan commercial project, every window caulking line on the building was cut out and redone.

$2M general liability · Active WSIB (Class G5) · 2-year workmanship warranty
Leaks traced to the entry point before anything is sealed

Frequently Asked Questions

How do I know if water under my window is a leak or condensation?

Look at when it happens. Condensation appears in cold weather with fogged or wet glass on the inside and water on the inside sill, and has nothing to do with rain. A leak appears during or after rain, especially wind-driven rain on one side of the building, and often stains the wall below the window rather than the sill.

Why is water coming in under my window only when it rains hard?

Wind-driven rain pushes water into joints that stay dry in normal rain: a perimeter caulking joint that has opened, a sill with no proper flashing, or a crack or open joint in the wall above the window. Water entering above the window can run down inside the wall and show up at the window frame.

Can a window leak actually be coming from the roof?

Yes. Water that gets in at a roof, gutter, flashing or wall crack above a window runs down inside the wall until something stops it, and the window frame often does. The stain appears at the window even though the window is fine, which is why leaks have to be traced from the outside, working upward.

Should I caulk around my window to stop the leak?

Only once you know where the water is getting in. Many windows and walls are designed to drain water out at the bottom, and caulking that path shut traps water inside the wall. Trace the leak first, with a hose test if needed, then repair the joint or flashing that actually failed.

How do I stop condensation on my windows in winter?

Lower indoor humidity and keep air moving across the glass: use bathroom and kitchen exhaust fans that vent outside, avoid drying laundry indoors, turn humidifiers down, open blinds and curtains, and keep heating vents below windows clear. Fogging between the panes of a sealed unit means its seal has failed and the glass unit needs replacing.

Does FAIME fix window leaks in Toronto and the GTA?

Yes. Window perimeter caulking, sealant joints, flashing details and the stucco and brick around openings are self-performed FAIME scopes across Toronto, Vaughan, Richmond Hill, Markham, Mississauga, Brampton and the wider GTA, along with the drywall and paint repair inside. FAIME is WSIB Active under Class G5, carries $2M general liability and backs its workmanship with a 2-year warranty. Site visits are free.

Water Stains Under a Window Inside? Leak or Condensation