What causes drywall to develop mould around windows during Toronto winters and how can I prevent it?

Mould around windows during Toronto winters is caused by condensation — warm, moist indoor air hitting the cold surface of the window frame and surrounding drywall, creating liquid water that feeds mould growth. This is one of the most common drywall moisture issues in GTA homes, particularly in older houses with single-pane or early double-pane windows, and it is almost entirely preventable.

The physics are simple. Toronto winters regularly bring temperatures of -10 to -20 degrees Celsius, making window glass and frames the coldest surfaces in your home. Meanwhile, indoor activities — cooking, showering, breathing, drying laundry — add moisture to the air. When that warm, humid air contacts the cold window area, it drops below its dew point and condenses into water droplets on the glass, frame, and the surrounding drywall. Over the course of a GTA winter, this daily condensation cycle saturates the drywall around the window, softening the paper face and gypsum core and creating ideal conditions for mould. You will typically see it first as dark spots on the drywall at the bottom corners of the window frame, on the window sill, or along the drywall return where it meets the window frame.

Several factors make this worse in GTA homes. Older Toronto housing stock — particularly the post-war bungalows and split-levels across Scarborough, North York, and Etobicoke — often has original aluminum-frame windows with poor thermal breaks, meaning the entire frame conducts cold directly to the interior. Even in homes with newer vinyl windows, insufficient insulation around the window rough opening allows cold air to reach the drywall. Homes that have been extensively air-sealed and weatherstripped without upgrading ventilation trap more moisture inside, raising indoor humidity to levels that guarantee condensation on windows.

Prevention Strategies

Control indoor humidity — this is the single most effective measure. During Toronto winters, keep indoor relative humidity between 30% and 40%. When outdoor temperatures drop below -20 degrees, you may need to reduce indoor humidity to 25-30% to prevent window condensation. Use exhaust fans in bathrooms (run for 20 minutes after every shower) and a range hood while cooking. If you dry laundry indoors, ensure the room is well ventilated. A hygrometer ($15 to $30 at any GTA hardware store) is essential for monitoring.

Improve air circulation around windows. Mould thrives in stagnant air pockets. Keep window coverings slightly open at the bottom to allow warm room air to circulate against the glass. If you have forced-air heating, ensure floor registers near windows are not blocked by furniture or curtains — that warm air rising past the window is your first line of defence against condensation.

Upgrade windows if budget allows. Modern triple-pane windows with warm-edge spacers and insulated vinyl or fibreglass frames virtually eliminate condensation because the interior glass surface stays warm enough to prevent moisture from condensing. This is a significant investment ($600 to $1,200 per window installed in the GTA) but it permanently solves the root cause.

Insulate around the window frame. When drywall around a window is being replaced or during renovation, ensure the gap between the window frame and the rough opening is properly insulated with low-expansion spray foam (not fibreglass, which allows air movement). The drywall return should be tight to the window frame with a bead of paintable caulk to prevent air leakage.

If mould has already developed on the drywall around your windows, do not simply paint over it. Surface mould on a small area of drywall can be cleaned with a solution of water and detergent, dried thoroughly, primed with a mould-killing primer, and painted. However, if the drywall is soft, swollen, or the mould has penetrated beyond the surface, the affected drywall must be cut out and replaced. For replacement, consider using mould-resistant (purple board) drywall around windows — it costs $24 to $32 per sheet compared to $14 to $20 for standard board, but the fibreglass facing eliminates the paper food source that mould needs to grow. A small patch repair around a window typically costs $150 to $400 from a GTA drywall contractor.

Preventing the condensation in the first place is always cheaper than repairing mould damage afterward. If you are seeing recurring mould around multiple windows, that is a sign of a whole-house humidity and ventilation problem worth addressing with an HRV (heat recovery ventilator) — a common upgrade in GTA homes that provides fresh air ventilation without losing heat.