Why is there frost on the inside of my windows?

Interior window frost forms when warm, humid indoor air meets a cold window surface, causing water vapor to freeze directly onto the glass. This typically happens when your windows aren't providing adequate insulation against Ottawa's harsh winter temperatures.

The main culprit is usually single-pane windows or older double-pane windows that have lost their insulating properties. When outdoor temperatures drop below -10°C (common in Ottawa winters), even double-pane windows with poor seals can become cold enough for frost formation. The problem is made worse by high indoor humidity levels from cooking, showering, and everyday activities.

Poor window seals are another major factor. If the seal between glass panes has failed in double-pane windows, moisture can enter the airspace and the window loses its insulating value. You might also notice condensation between the glass panes, which is a clear sign the sealed unit needs replacement.

In Ottawa's Climate Zone 6, triple-pane windows with Low-E coatings are the most effective solution for preventing interior frost. These windows maintain a warmer interior glass surface even when it's -25°C outside. The inner pane stays closer to room temperature, preventing the condensation and frost formation that occurs with less efficient windows.

Immediate steps you can take include reducing indoor humidity with exhaust fans, ensuring proper ventilation, and checking that your windows are properly sealed around the frames. However, if frost formation is persistent, it's usually a sign that window replacement is needed for both comfort and energy efficiency.

For a comprehensive assessment of your windows and frost issues, Ottawa Window Pros can evaluate whether your current windows can be improved with weatherstripping or if replacement with energy-efficient triple-pane units would be more cost-effective.

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