How do professionals repair drywall around a window that has condensation damage in a cold Toronto winter?

Window condensation damage is one of the most common drywall repair calls during Toronto winters, and professional repair involves not just replacing the damaged drywall but addressing the underlying moisture problem — otherwise the new drywall will be damaged again by the next heating season. The damage typically appears as bubbling paint, soft or crumbling drywall, dark mould staining, and peeling or flaking compound on the wall immediately below and beside the window frame, and on the window sill or returns (the drywall sections that wrap into the window opening).

The root cause is straightforward: during Toronto's cold winters, when outdoor temperatures drop to -10 to -20 degrees Celsius, the interior surface of older or poorly insulated windows falls below the dew point of the warm, humid indoor air. Moisture condenses on the glass and window frame, runs down onto the drywall sill and returns, and saturates the gypsum over weeks and months of continuous condensation. Homes in older Toronto neighbourhoods — the Danforth, Scarborough, East York, and North York — that still have original single-pane or early double-pane windows from the 1970s and 1980s are particularly affected. Even newer windows with failed seals (visible fog between panes) can cause this problem.

The Professional Repair Process

A professional drywall contractor will start by assessing the full extent of the damage, which is often worse than what is visible on the surface. They will probe the drywall around the entire window opening with a utility knife or moisture metre. Saturated drywall will be soft and crumbly, and it is common to find that the damage extends 6–12 inches beyond the visible staining, particularly below the window sill where water has been running for months.

All damaged drywall is removed completely — cutting back to dry, solid material on all sides. This means removing the window returns (the narrow drywall strips on the sides and top of the window opening), the sill, and any portion of the wall below the window that has been affected. Once the drywall is removed, the contractor inspects the underlying framing for mould and rot. It is very common to find mould growth on the rough window framing and the jack studs beside the window in cases where condensation has been occurring for multiple winters. Surface mould on wood is cleaned with a borax solution, and any rotted framing is sistered or replaced.

The contractor also checks the insulation around the window frame. Many Toronto homes have gaps between the window frame and the rough opening that are either uninsulated or stuffed with fibreglass batts that have become wet and compacted. The proper insulation for this space is low-expansion spray foam (use only low-expansion around windows — high-expansion foam can bow the frame and prevent the window from operating). This both insulates and air-seals the rough opening, reducing the temperature differential that causes condensation.

New drywall is then installed to match the existing thickness — typically 1/2-inch regular drywall for interior walls. For the window returns and sill, moisture-resistant drywall (green board or purple board) is an excellent upgrade at minimal additional cost ($20–$32 per sheet versus $14–$20 for regular) and provides significantly better performance in this chronically moisture-exposed location. The returns are cut to fit tightly against the window frame, with a small gap sealed with paintable caulk where the drywall meets the window frame. This caulk joint is critical — it prevents future condensation from wicking behind the drywall.

All joints are taped and finished with setting compound for the first coat (it resists moisture better than pre-mixed compound), followed by topping compound for finish coats. The entire repair area is primed with a mould-resistant primer and painted with a semi-gloss or satin finish paint, which resists moisture better than flat or eggshell finishes.

Preventing recurrence is equally important. If the windows are old and inefficient, replacement with modern double- or triple-pane windows is the definitive solution but costs $800–$1,500 per window installed. Short of window replacement, managing indoor humidity during winter with your furnace humidifier set no higher than 30–35% relative humidity, running bathroom and kitchen exhaust fans diligently, and improving air circulation near windows (keep blinds slightly open to allow warm air to reach the glass) can significantly reduce condensation.

Costs for professional condensation damage repair around a single window in the GTA typically run $400–$900 including drywall removal, mould cleaning if needed, insulation upgrade, new moisture-resistant drywall, taping, finishing, and primer. Multiple windows bring the per-window cost down to $300–$600 each. If the damage is extensive — multiple windows with mould throughout the framing — expect $1,500–$4,000 for a comprehensive repair across the affected areas.

This is definitively a professional repair. The combination of mould assessment, proper insulation, moisture-resistant materials, and finished drywall work around the complex geometry of a window opening requires experience. Find a qualified drywall contractor through Toronto Drywall Installers for a free estimate on your window condensation repair.