Why Mold Grows on Window Sills in Winter, and How to Stop It

Mold on window sills in winter almost always comes from condensation. Warm, humid indoor air hits glass and frames that are much colder than the room, water beads up and runs down, and the sill, the caulking and the bottom of the frame stay wet for days at a time. Small spots on a hard surface can be cleaned at home. The lasting fix is lowering indoor humidity and keeping air moving across the glass, and anything that has soaked into drywall or soft wood needs a closer look.

How condensation turns into mold

Every surface in a house has a dew point. When indoor air at 21°C carries a lot of moisture from cooking, showers, drying laundry and breathing, it only takes a cool surface for that moisture to drop out as water. In a cold snap, the glass of a double-pane window can sit well below room temperature, and the edge of the glass near the spacer bar is colder still. That is why the water usually forms first along the bottom few centimetres of the pane.

The water itself is not the problem. The problem is that it keeps coming back. It runs down to the sill, collects in the corners of the frame and soaks into dust, paint and the paper face of any nearby drywall. Mold spores are already present in every home. Give them a surface that is damp for 24 to 48 hours at a stretch and some organic material to feed on, and they start to grow. Window sills offer both all winter long.

Bedrooms are hit hardest because people breathe out moisture for eight hours with the door closed and the blinds down. Blinds and heavy curtains trap a pocket of cold, still air against the glass, which makes the condensation worse.

Surface spots or a deeper problem

Most window mold is a surface issue. A few signs tell you it has gone further:

• Black or grey spots that wipe off a vinyl or painted sill. Surface growth. Clean it and fix the humidity.
• Staining that comes back within a week of cleaning. The surface is staying wet. The humidity or air circulation in that room has not changed enough.
• Soft, darkened or swollen wood on the sill or the bottom rail of the sash. Water has worked into the wood. The mold is inside the grain, and the wood may be starting to rot.
• Bubbling paint, brown streaks or spots on the drywall below the window. Water is getting into the wall, either from condensation running off the sill or from a leak in the window seal or flashing.
• A musty smell near the window even when the sill looks clean. Growth may be hidden behind trim or inside the wall cavity.
• Water between the two panes of glass. The seal on the insulated unit has failed. That glass will keep fogging and needs to be replaced.

If you see the last four, treat it as more than condensation. A mold inspection with a moisture meter can tell whether the wall behind the window is wet and how far the damage goes.

Cleaning mold off a window sill

For small areas on hard, non-porous surfaces like glass, vinyl frames, aluminum and well-painted sills, cleaning at home is reasonable. The US Environmental Protection Agency suggests homeowners can usually handle a mold area smaller than about 10 square feet (roughly 1 square metre) themselves. A window sill falls well inside that.

1. Open the window a crack or run a fan pointed out of the room. Wear gloves, safety glasses and a disposable N95 mask.
2. Dry the surface first with a paper towel so you are not spreading wet growth around.
3. Scrub with warm water and dish detergent, using a soft brush in the corners and along the frame tracks.
4. Wipe clean with a damp cloth, then dry completely. A fan or a hair dryer on low helps in the corners.
5. Bag the paper towels and cloths and throw them out.

Skip the bleach on wood. It can lighten the stain on top while leaving the growth inside the grain, and the extra water soaks into the sill. Our post on whether bleach kills mold covers why. Black staining in old caulk or in the rubber gasket around the glass usually will not clean out, so cut the caulk out and replace it once the area is dry.

If the drywall under the window is stained or soft, do not scrub it. Scrubbing paper-faced drywall releases spores and rarely removes the growth. That material normally has to be cut out and replaced, ideally with the area contained so dust does not spread through the house. That is a job for professional mold removal.

Stopping it from coming back

Cleaning without changing the conditions means the spots will be back by January. The fixes are simple, and most cost little or nothing.

Bring the humidity down

The EPA recommends keeping indoor relative humidity below 60 percent, ideally between 30 and 50 percent. In a cold climate, the low end of that range matters in winter: the colder it gets outside, the lower the humidity your windows can tolerate without sweating. Many homes in Québec, Ontario and the northern US need to sit near 30 to 35 percent when outdoor temperatures drop below about minus 10°C. A digital hygrometer costs little and takes the guesswork out of it. Put one in the worst bedroom.

If the reading is high:

• Turn off or lower any humidifier, including one built into the furnace.
• Run bathroom fans during showers and for 20 to 30 minutes after.
• Use the range hood every time you cook, vented outside, not a recirculating model.
• Dry laundry in a dryer that vents outdoors rather than on racks inside.
• Check that the dryer vent and bath fans actually exhaust outside and not into the attic.

Keep air moving across the glass

Open blinds and curtains during the day, at least partway. Leave bedroom doors open when you can. Make sure furniture is not blocking the heat register under the window, since that warm air washing up the glass is what keeps it above the dew point.

Ventilate the house as a whole

Newer, tighter homes trap moisture. If the house has an HRV or ERV, run it continuously in winter and clean its filters and core on the schedule in the manual. A home with no mechanical ventilation and chronic condensation on every window is a sign that the house needs more fresh air, not just a dehumidifier.

Check the window itself

Old single-pane windows, aluminum frames without a thermal break, and units with failed seals run colder and sweat more. Interior storm panels or plastic window film can buy a season. If replacement is already on your list, newer windows with warm-edge spacers stay warmer at the bottom of the glass, which is exactly where the water collects.

Frequently asked questions

Is black mold on window sills dangerous?

Small amounts of surface mold on a sill are common and are not usually a serious health risk for healthy adults, but they can irritate people with asthma, allergies or weakened immune systems. The colour does not tell you the species, so treat any growth the same way: clean it promptly and fix the moisture. Growth that keeps spreading or reaches the drywall deserves a professional look.

Why does only one window in the house get mold?

That window is colder or its room is more humid than the rest of the house. Bedrooms with closed doors, rooms over a garage, windows on the north side and windows with a failed seal are common culprits. Blocked heat registers and heavy curtains also make one window stand out.

Will a dehumidifier stop window condensation?

A portable dehumidifier can help in a single damp room or a basement, but in winter it is rarely the full answer for the upper floors. Lowering humidifier settings, using exhaust fans and running the HRV usually do more. If the hygrometer stays above 40 percent in cold weather after those steps, a dehumidifier is worth adding.

Should windows that grow mold every winter be replaced?

Not automatically. Most condensation problems are solved by bringing humidity down and improving air flow. Replacement makes sense when the seal between panes has failed, the frames are rotting or the windows are single-pane, since those stay cold no matter what you do inside.

Get a written quote

If the mold has moved past the sill into the wall, the trim or the wood frame, send us photos and a short description through the quote form. We will match you with a mold specialist near you who inspects the area, traces the water and writes up the cleanup plan and price.