Mold on window sills and frames

Why windows are the first place it shows

Glass is the coldest surface in a heated room. In an Ontario winter, indoor air touching it gives up moisture as fog, droplets or frost. That water runs down and collects on the sill, in the tracks and at the bottom corners of the frame, and it does so every cold day for months.

Dust, paint and wood provide the food. The result is black spotting along the bottom of the frame, in the corners and on the sill.

Condensation or a leak?

Condensation

• Appears in cold weather and is worst on the coldest days
• Water forms on the room side of the glass
• Affects several windows
• Worse in bedrooms overnight, in bathrooms and in kitchens

A leak

• Appears during or after rain, at any time of year
• Water shows at the top of the frame, at one side, or on the wall below
• Affects one window
• The drywall beneath is stained or soft

A leak points to failed caulking, flashing or the window itself and needs repair from outside.

Cleaning

1. Wear gloves and eye protection.
2. Wipe up standing water.
3. Wash the glass edge, frame, sill and tracks with water and dish detergent. An old toothbrush reaches corners and tracks.
4. Rinse with a clean damp cloth.
5. Dry thoroughly.

Clear the small drainage holes at the bottom of the window frame. Blocked weep holes hold water in the track.

When cleaning is not enough

• Paint is peeling and the wood is soft. The sill has absorbed water repeatedly and may be rotting. It needs repair or replacement.
• The drywall under the window is stained, soft or bubbling. Water has run down inside or behind it. The section may need to be cut out.
• Mold is in the caulk. Cut it out and recaulk once dry.
• Fog is between the panes of a double-glazed unit. The seal has failed. That is a window repair, not a humidity problem.

Reducing condensation

Lower indoor humidity

• Run bathroom and kitchen exhaust fans that vent outdoors
• Vent the dryer outdoors and do not air-dry laundry indoors in winter
• Turn down a furnace humidifier as the weather gets colder. The colder it is outside, the lower the indoor humidity needs to be to avoid condensation.
• Cover aquariums and reduce the number of plants on sills

Let warm air reach the glass

• Open curtains and blinds during the day
• Do not let heavy curtains seal the window off from the room
• Keep heat registers below windows clear
• Open bedroom doors at night where possible

Improve the window

• Storm windows or interior window film raise the temperature of the inner pane
• Replace failed weatherstripping
• Older single-pane or aluminium-framed windows will always sweat in deep cold. Replacement helps, but humidity control still matters.

A warning in newly tightened houses

After new windows, added insulation or air sealing, a house leaks less air and holds more moisture. Condensation that never appeared before can start. Mechanical ventilation, such as a heat recovery ventilator or regular use of exhaust fans, is the answer.

Bedrooms

People breathe out moisture all night. A closed bedroom with heavy curtains commonly has wet windows by morning. Leaving the door open, opening the blinds a little at the bottom and keeping the room warm all help.

By window type

Vinyl. The frame does not rot, but mold grows on the dust and dirt that collect in tracks and on seals. It cleans off fully.

Wood. Repeated wetting lifts the finish, stains the wood grey or black and eventually rots it. Clean it, let it dry completely, sand and refinish. Wood that is soft when pressed with a screwdriver needs repair or replacement.

Aluminium. Older aluminium frames without a thermal break are very cold on the inside in winter and collect heavy condensation, sometimes frost. The frame survives, but the sill and the wall beneath take the water.

Painted drywall returns. Many windows have drywall, not wood, around the opening. It is the first thing to be damaged. Soft or swollen drywall at the bottom corners needs replacing.

The track and the weep holes

Sliding windows and doors have a bottom track that collects water, dead insects and dust, which is everything mold needs. The track drains to the outside through small weep holes. When those are blocked, water sits in the track and can overflow to the inside.

• Vacuum the track
• Scrub it with a small brush and detergent
• Clear the weep holes with a toothpick or a pipe cleaner
• Pour a little water in to check that it drains outside

Curtains and blinds

Fabric that touches wet glass grows mold along the edge. Wash or dry-clean affected curtains, and wipe blinds slat by slat. Keep window coverings from resting against the glass, and open them every day in winter so that warm room air can reach the window.

Does it mean the windows need replacing?

Not necessarily. Condensation on the room side of the glass says more about the humidity in the house than about the window. Consider replacement when the sealed unit has failed, shown by fogging between the panes, when frames are rotted, or when the windows are old single-pane or unbroken aluminium units. Better windows have warmer inside surfaces and tolerate higher humidity, but a very humid house will still wet them.

Basement windows

Small basement windows are often the coldest and least looked-at in the house. Mold on the frame and on the wall below can come from condensation, from a leaking window well, or from both. Check that the well drains and is clear of leaves.

More: mold removal and remediation.