How do I deal with condensation on basement windows in Vancouver?

Condensation on basement windows in Vancouver is caused by warm, moist indoor air meeting the cold glass surface — and in Metro Vancouver's marine climate, where outdoor humidity regularly exceeds 80% from October through April, it is one of the most common basement complaints. The solution involves a combination of reducing indoor humidity, improving air circulation near windows, and in some cases upgrading the windows themselves.

The first step is understanding why your basement windows are sweating. Below-grade spaces are naturally cooler than the rest of your home, and concrete foundation walls radiate cold. When warm air carrying moisture contacts a cold window surface, the water vapour condenses into droplets. In Vancouver's climate, this is worse than in drier regions because the outdoor air is already saturated with moisture — opening a window for ventilation during the wet season can actually increase indoor humidity rather than reduce it. This is the opposite of what many homeowners expect.

Reduce indoor humidity as your first line of defence. A quality dehumidifier rated for your basement's square footage is essential — look for a unit rated for at least 50 pints per day for a typical 800–1,200 square foot Metro Vancouver basement. Set it to maintain relative humidity between 35% and 50%. A basic hygrometer ($15–$30 at any hardware store) lets you monitor levels. Make sure your clothes dryer is properly vented to the exterior, not leaking humid air into the basement. Check that bathroom exhaust fans are ducted to the outside and running during and after showers — a minimum 50 CFM fan is required by the BC Building Code. If you have a crawlspace connected to your basement, ensure it has a proper vapour barrier (6 mil poly) on the ground.

Improve air circulation around windows to prevent moisture from settling on the glass. Keep curtains or blinds slightly open so warm room air can reach the window surface. If you have forced-air heating, make sure at least one heat register is near the problem windows — the warm air flowing across the glass raises its surface temperature above the dew point and prevents condensation. A small fan directed at the window can also help in stubborn cases.

Upgrade single-pane windows if your home still has them. Many post-war homes across Burnaby, New Westminster, and North Vancouver were built with single-pane basement windows that are extremely prone to condensation because the glass temperature drops nearly to outdoor temperature. Double-pane or triple-pane windows with low-E coating dramatically reduce condensation by keeping the interior glass surface warmer. Replacement basement windows in Metro Vancouver cost $400–$1,200 per window installed, depending on size and type. If you are planning egress window installation for a bedroom ($3,000–$8,000 per window), the new window will be modern double-pane and the condensation problem disappears in that location.

Check your window wells if you have below-grade windows. Poorly draining window wells allow water to pool against the glass, and in Vancouver's heavy rainfall — over 1,200mm annually — clogged window well drains are a common source of both condensation and actual water leaks. Clean debris from window wells each fall, ensure the drain at the bottom is clear, and consider adding a clear polycarbonate window well cover to keep rain out while letting light in. Covers cost $30–$80 each and are a simple DIY project.

If condensation persists despite these measures, you may have a more serious moisture issue — water infiltration through foundation walls or floor slab, inadequate or missing vapour barriers behind finished walls, or a disconnected perimeter drain system. Persistent condensation that leaves puddles on window sills or creates visible mould around window frames warrants a professional assessment. A basement waterproofing specialist can evaluate whether your perimeter drainage and foundation waterproofing are adequate for Metro Vancouver's rainfall and groundwater conditions.

Need help finding a basement professional to assess your moisture issues? Vancouver Basement Finishing can match you with experienced local contractors for a free consultation.

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