What causes basement windows to fog up in the winter, and how do I fix it?

Basement window fogging in winter is caused by warm, humid indoor air hitting the cold glass surface, creating condensation. This is especially common in Ottawa's harsh winters when outdoor temperatures drop well below freezing while basements remain heated.

The primary culprit is humidity imbalance in your basement. During winter, cold air outside can't hold much moisture, so when warm basement air (which can hold more moisture) contacts the frigid window glass, the water vapor condenses into droplets. This problem is amplified in basements because they're naturally more humid due to concrete foundations that can wick moisture from the ground, especially during Ottawa's freeze-thaw cycles.

Poor ventilation makes the issue worse. Many Ottawa basements lack adequate air circulation, allowing humid air to stagnate near windows. If you're running a humidifier upstairs or have activities like laundry, cooking, or showering that add moisture to the air, that humidity often settles in the basement where it has nowhere to go.

Window quality and age also play a significant role. Older single-pane windows common in many Ottawa homes built before the 1980s offer virtually no insulation value. Even newer windows can fog if they have failed seals in double-pane units, allowing moisture between the glass panes.

To fix the problem, start by controlling humidity levels. Use a dehumidifier to maintain basement humidity between 30-50% during winter months. Ensure your dryer vents outside (not into the basement) and fix any plumbing leaks or foundation moisture issues. Improve ventilation by running bathroom fans longer after showers and consider installing a basement ventilation fan if the problem persists.

For immediate relief, increase air circulation around windows using small fans, and ensure heating vents aren't blocked by furniture. If you have older single-pane windows, consider upgrading to quality double or triple-pane units with low-E coatings - this is especially worthwhile if you're planning a basement renovation or secondary suite conversion.

Window replacement should meet Ontario Building Code requirements and may require permits depending on size changes. In Ottawa's climate, investing in proper basement windows isn't just about comfort - it prevents mold growth and protects your investment.

If condensation appears between glass panes rather than on the interior surface, the window seals have failed and replacement is necessary. For comprehensive basement renovations including window upgrades, contact Ottawa Basements for a free consultation to address both the symptoms and underlying moisture issues.