Why is there condensation on our basement windows in winter?

Basement window condensation in winter is primarily caused by warm, humid indoor air meeting the cold glass surface, creating the perfect conditions for water droplets to form. This is especially common in Ottawa's harsh winter climate where outdoor temperatures can drop to -30°C while your basement stays around 18-20°C.

The science behind this is straightforward: warm air holds more moisture than cold air. When your heated basement air contacts the frigid window glass, it rapidly cools and can no longer hold all its moisture, causing water to condense on the surface. Single-pane windows and older basement windows with poor insulation are particularly prone to this issue because they transfer cold more readily than modern double or triple-pane units.

High humidity levels in your basement significantly worsen the problem. Ottawa basements often struggle with humidity from various sources: cooking, showering, laundry, houseplants, or even minor moisture infiltration through foundation walls. During winter, when homes are sealed tight for heating efficiency, this moisture has nowhere to go and accumulates indoors. Ideal basement humidity should be between 30-50% - anything higher creates condensation problems.

Poor ventilation compounds the issue by allowing humid air to stagnate around windows. Many Ottawa homes, especially older ones, lack adequate basement ventilation. Without proper air circulation, moisture-laden air sits against cold surfaces longer, increasing condensation formation.

Ottawa's extreme temperature swings make this worse than in milder climates. When outdoor temperatures drop from -5°C to -25°C overnight (common here), your windows become significantly colder while indoor humidity levels remain constant, dramatically increasing the temperature differential that causes condensation.

To address this issue immediately, increase ventilation with exhaust fans, reduce humidity sources where possible, and ensure your heating system is circulating air properly. Run a dehumidifier if humidity exceeds 50%. For long-term solutions, consider upgrading to energy-efficient windows with better insulation values - this is especially important in Ottawa where heating costs are significant.

Professional assessment is recommended if condensation is severe or accompanied by mold growth, as this could indicate broader moisture problems requiring waterproofing or ventilation improvements. Persistent condensation can lead to wood rot, mold growth, and structural issues that are costly to repair later.

For comprehensive basement moisture solutions and window upgrades as part of a basement renovation, Ottawa Basements can assess your specific situation and provide solutions that address both comfort and energy efficiency in our challenging climate.