Window Condensation in Winter: When It Leads to Mold

Window condensation leads to mold in winter when the water shows up daily, runs down the glass and keeps the sill, frame or surrounding drywall wet. A light fog along the bottom of the glass on a very cold morning that clears by midday is normal and harmless. Water that pools, soaks into wood or leaves black spotting along the frame means indoor moisture is too high for the windows you have, and it needs attention.

Why windows sweat in an Ontario winter

Glass is the coldest surface in a heated room. Warm indoor air holds moisture from cooking, showers, laundry, plants and breathing. When that air touches cold glass, it cools and drops its moisture as water or frost. The colder it is outside and the damper the air inside, the more water forms. Older single-pane windows and aluminum frames in many Old North, Wortley Village and Old East homes run colder than modern units, so they sweat sooner.

The edges of the glass and the bottom rail are colder than the centre, which is why condensation starts there. Anything that keeps room air from reaching the window makes it worse, including closed blinds, heavy curtains and furniture pushed against the wall.

Normal condensation and the kind that causes trouble

• Normal: a band of fog along the bottom few centimetres of glass on the coldest mornings, gone within a few hours.
• Normal: brief fogging in a bathroom or kitchen after a shower or while cooking.
• A problem: water beading and running down to the sill on most winter days.
• A problem: ice building up on the inside of the glass or frame.
• A problem: sills that stay damp, with peeling paint, swollen wood or soft drywall at the corners.
• A problem: condensation on windows throughout the house at once, which points to whole-house humidity.

Where window condensation mold grows in winter

Glass and vinyl give mold little to eat, but the dust and dirt sitting on them do. The usual pattern is black or grey spotting along the bottom gasket, in the track of a sliding window, and in the lower corners of the frame. From there, water wicks into materials that mold feeds on readily: the wood sill, the painted trim, the paper face of the drywall return, and the wall below the window.

The part you cannot see matters more. Water that runs off the sill day after day can get behind the trim and into the wall cavity, wetting the framing and insulation below the window. Signs of that include a stain or bubbling paint under the window, a musty smell near it, or trim that feels soft when pressed.

How to stop condensation at the window

1. Open blinds and curtains during the day, and leave a gap at the bottom overnight so warm air can reach the glass.
2. Keep heat registers and radiators under windows clear. They are placed there to wash the glass with warm air.
3. Run the bathroom fan during showers and for a while after, and use the range hood when cooking.
4. Lower the humidifier setting as the temperature drops. Many homes are comfortable at 30 to 40 percent relative humidity in cold weather, and windows often need the lower end of that in a deep freeze.
5. Wipe the water up each morning with a towel while you work on the cause. A sill that dries daily does far less damage.
6. Remove interior window screens for the winter where the design allows. They block airflow across the glass.
7. Check the weatherstripping and the caulking around the frame. Cold air leaking in around a window chills the surfaces beside it.

If you have done all of this and the same windows still drip, the windows may be the limit. Interior storm panels or a seasonal plastic film kit add an insulating layer and raise the temperature of the inner surface for very little money. Replacement with better-insulated units is the long-term fix.

What if the fog is between the panes?

Moisture or haze trapped between the two panes of a sealed unit is a different issue. It means the seal has failed and outside air is getting into the space. It will not respond to lower humidity or more airflow, and you cannot wipe it off. The glass unit can usually be replaced without changing the whole window. It is mainly a visibility and insulation problem and seldom a mold source by itself.

Cleaning mold off window frames and sills

Light surface spotting on glass, vinyl or painted trim can be cleaned by the homeowner. Wear gloves, wipe with a cloth and soapy water, rinse and dry the area completely. Clean the tracks with a small brush. Health Canada advises removing indoor mold whatever the type, and it can irritate the airways and aggravate asthma and allergies, so do not leave it for spring. Talk to your doctor about any health concerns.

If the drywall is soft, the stain keeps returning after cleaning, or the wall below the window smells musty, the moisture has gone past the surface. That calls for a moisture meter reading in the wall before anything is repainted.

For now, buy a hygrometer, set it on a shelf away from the window, and note the reading on the mornings your windows drip. That number tells you whether to work on the air or on the window.

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