Why does condensation keep forming on my bathroom window in winter, and will it eventually cause the window frame to rot?

Condensation on your bathroom window during Toronto winters is caused by warm, humid indoor air hitting the cold glass surface, and yes, persistent condensation can absolutely cause window frame rot if not addressed.

The physics are straightforward: your heated bathroom air holds moisture from showers, baths, and daily use, while your window glass becomes extremely cold during GTA winter temperatures that regularly drop to -10 to -20 degrees Celsius. When that warm, moisture-laden air contacts the cold glass, it instantly condenses into water droplets. This is the same process that creates fog on your car windshield, but in your bathroom, it happens daily throughout Toronto's long winter season from December through March.

The real concern is what happens to that condensation water. If it's running down the glass and pooling on the window sill, soaking into wooden window frames, or collecting in the corners where the frame meets the wall, you're creating perfect conditions for wood rot and mould growth. Wooden window frames in older Toronto homes are particularly vulnerable because they were often installed before modern moisture barriers and thermal breaks became standard. The repeated wet-dry cycles from daily condensation gradually break down the wood fibers and protective finishes.

Your bathroom's ventilation is likely inadequate for Toronto's climate conditions. The Ontario Building Code requires mechanical ventilation (exhaust fan) in every bathroom, with a minimum capacity of 50 CFM for standard bathrooms. However, many GTA homes have undersized fans or fans that aren't being used properly. During Toronto's humid summers and cold winters, bathroom humidity levels can easily exceed 70-80% without proper ventilation, making condensation inevitable on single-pane windows or even older double-pane units with poor thermal performance.

Check your exhaust fan capacity and usage habits. Calculate your bathroom square footage and ensure your fan provides at least 1 CFM per square foot (minimum 50 CFM total). Run the fan during every shower or bath and for 30 minutes afterward to remove moisture before it can condense on surfaces. Many Toronto homeowners make the mistake of turning off the fan immediately after showering, leaving all that humid air in the bathroom to condense on the coldest surfaces — which is always the windows in winter.

Window upgrades may be necessary for a permanent solution. Single-pane windows or older double-pane units with failed seals will always condensate heavily in Toronto winters because the interior glass surface temperature is too close to the dew point of bathroom air. Modern triple-pane windows or quality double-pane units with low-E coatings and argon gas fill keep the interior glass surface warmer, dramatically reducing condensation. If you're planning a bathroom renovation, window replacement should be considered alongside the other improvements.

Immediate steps to prevent frame damage: Wipe down condensation daily with a squeegee or towel, ensure proper caulking around the window frame to prevent water infiltration, and inspect wooden frames regularly for soft spots, discoloration, or peeling paint that indicate moisture damage. If you notice any of these signs, address them immediately — wood rot spreads quickly in the consistently moist bathroom environment.

When to hire a professional: If you're seeing persistent condensation despite running an adequate exhaust fan, if window frames show signs of rot or mould, or if you're considering window replacement as part of a bathroom renovation, consult with professionals. A bathroom renovation contractor can assess your ventilation needs, while a window specialist can evaluate whether your current windows can be improved or need replacement.

The combination of Toronto's extreme winter temperatures and high indoor humidity makes bathroom condensation a serious concern that goes beyond just wiping down glass — it's about protecting your home's structure and your family's health through proper ventilation and appropriate window performance.