New Windows, More Condensation? Why It Happens

You paid for new windows partly to stop the dripping, and the first winter they fog more than the old ones did. That’s a common complaint, and Natural Resources Canada explains why.

Why it gets worse

NRCan’s Keeping the Heat In guide says many homeowners buy new windows only to find the problem gets worse, “because the old, leaky windows actually helped to reduce humidity.” Drafty old windows let moist indoor air escape and dry winter air in. Seal them, and the moisture you produce from cooking, showers, laundry, and breathing stays inside and finds the next-coldest surface.

The new glass is usually warmer than the old glass, but the house is now wetter. Together that can still mean more water on the windows, at least until humidity is brought back down.

What the government guides recommend

• Know your number. CMHC’s Moisture and Air guide says indoor RH in the heating season shouldn’t exceed 45%, and in very cold weather 30% or lower may be needed to prevent window condensation. Measure it with a hygrometer.
• Reduce moisture at the source, and stop using a humidifier if the windows are wet (CMHC).
• Ventilate. NRCan’s fix for the new-window problem is to reduce humidity and improve house ventilation. If you have an HRV, make sure it’s on.
• Let warm air reach the glass. NRCan notes that deep sills, closed drapes and blinds, and furniture can let cool air pool at the window.
• Expect some edge condensation in extreme cold. NRCan says glazing edges can fog and freeze in harsh climates, and triple glazing helps.

Health Canada (cited in NRCan’s Moisture Problems booklet) recommends winter RH between 30% and 55% for health. In practice, cold Canadian nights call for the low end to keep windows dry.

Sources: NRCan, Keeping The Heat In: Section 8; NRCan, Condensation; CMHC, Moisture and Air; NRCan, Moisture Problems.

Originally published at https://dry-glass-notes.surge.sh/articles/new-windows-more-condensation.html (AI-assisted, sources cited). If this helped, zaps go to hollisvarrow3753@coinos.io.