What causes window condensation?

Window condensation forms when warm, moist indoor air contacts a cold glass surface — it's essentially the same principle as a cold drink sweating on a humid day. While it looks like a window problem, it's almost always a sign of excess indoor humidity or inadequate window insulation.

There are two types of condensation worth understanding. Interior condensation (on the inside of the glass) is the most common and is caused by high indoor humidity levels. Exterior condensation (on the outside) is actually harmless and just means your windows are energy-efficient — the glass is cooler than the outdoor dew point. The one that signals a real problem is between-pane condensation, which means the sealed unit has failed and argon or air has leaked out, destroying the window's insulating value.

What drives interior condensation in Ottawa homes is usually a combination of everyday moisture sources — cooking, showering, breathing, houseplants, and humidifiers — combined with insufficient ventilation. In winter, Ottawa's cold outdoor temperatures (regularly hitting -20°C or colder) make this worse because the temperature differential between inside and outside is extreme. The colder the glass, the lower the humidity level needs to be before condensation forms. Older single-pane or even older double-pane windows are much more prone to this because their glass surface temperature is significantly colder than modern low-e triple-pane units.

Ideal indoor humidity levels for Ottawa winters are roughly:

• -20°C or colder outside: keep indoor humidity at 20–25%
• -10°C to -20°C: 25–30% is acceptable
• Above -10°C: up to 35–40% is generally fine

If you're consistently seeing condensation, the first step is picking up an inexpensive hygrometer (available at any hardware store for $15–30) to measure your actual humidity levels. If you're above those thresholds, improving ventilation — running bathroom fans longer, using a kitchen range hood, and ensuring your HRV (Heat Recovery Ventilator) is set correctly — usually solves the problem. Many Ottawa homes have HRVs that are either set too low or not maintained, and a simple adjustment makes a significant difference.

When to call a professional: If the condensation is between the panes of glass, the sealed unit needs replacement — this is typically a $150–350 repair per window for just the glass unit, rather than a full window replacement. If you're seeing condensation alongside frost buildup, mold near window frames, or persistently high humidity despite good ventilation habits, it may be worth having an HVAC contractor assess your home's ventilation system. An HRV service call in Ottawa typically runs $100–200.

Browse verified HVAC and window contractors in our Ottawa contractor directory if you need a professional assessment.