Why do my windows fog up only on the coldest mornings?

Window fogging on cold mornings happens because of the temperature difference between your warm, humid indoor air and the cold window surface. When warm air hits the cold glass, the moisture in that air condenses into water droplets - just like how your breath fogs up a car window in winter.

Condensation forms when the window surface temperature drops below the dew point of your indoor air. On the coldest Ottawa mornings, single-pane windows or older double-pane windows with poor insulation get extremely cold on the interior surface. If your home has typical winter humidity levels (30-50%), that cold glass becomes a magnet for moisture. The colder the outdoor temperature, the more dramatic this effect becomes.

This is actually a sign that your windows aren't providing adequate insulation. Quality triple-pane windows with Low-E coatings keep the interior glass surface much warmer, dramatically reducing condensation. In Ottawa's Climate Zone 6, where we regularly see -20°C to -30°C temperatures, proper window insulation is crucial for comfort and preventing moisture issues.

The fogging typically happens in the morning because overnight temperatures are coldest, and humidity builds up from cooking, showering, and even breathing while you sleep. Once the sun comes up and outdoor temperatures rise slightly, or your heating system warms the glass surface, the condensation evaporates.

If you're seeing heavy condensation regularly, it's time to consider window replacement. Modern ENERGY STAR certified windows with U-factors of 1.22 or lower will virtually eliminate this problem while reducing your heating costs. For a free assessment of your windows' energy performance and condensation issues, Ottawa Window Pros can evaluate your specific situation and recommend the best solution for your home.

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