Waking up to water beading, or a layer of frost, on the inside of the windows every cold morning feels like it must mean the windows are failing. In the vast majority of cases the windows are fine and the actual problem is how much moisture is in the air inside the house relative to how cold the glass surface is.
Why it happens on the inside, not between the panes
Condensation forms wherever warm, moist indoor air meets a cold surface, and in winter the coldest surface in most rooms is the glass, particularly near the bottom edge and in the corners of the frame where airflow is weakest. If the moisture is on the inside face of the interior pane, wipeable with a cloth, that is a humidity issue in the house, not a seal failure in the window itself. Moisture or fogging trapped between the two panes of a sealed double-glazed unit, which cannot be wiped away because it is sealed inside the unit, is a different problem entirely, a failed seal, and that genuinely does mean the glass unit itself needs replacing rather than anything to do with indoor humidity.
Where the excess humidity is coming from
Cooking without using the range hood, long showers without the bathroom fan running, drying laundry indoors on a rack, and even a large number of houseplants all add measurable moisture to indoor air over the course of a day. In a newer, tightly sealed home especially, that moisture has nowhere to go without mechanical ventilation actively removing it, which is a common reason newer, more energy-efficient builds sometimes see more window condensation in winter than older, leakier houses did, simply because the old house was ventilating itself by accident through gaps the new one no longer has.
What to check first
Before assuming the worst, work through these in order over a few days.
- Run the kitchen exhaust fan during and for a few minutes after cooking, every time, not just for strong-smelling meals.
- Run the bathroom fan during showers and leave it running for fifteen to twenty minutes afterward.
- Confirm both fans actually vent outside rather than into the attic, which is a surprisingly common installation shortcut worth checking once.
- Buy an inexpensive hygrometer and place it in a main living area, away from the kitchen or bathroom directly.
- Track the reading for a few days. Winter indoor humidity is generally recommended to stay in the 30 to 40 percent range, and anything reliably higher makes condensation on the coldest surfaces in the house likely.
When a dehumidifier or ventilation upgrade is worth it
If humidity is reliably testing high on the hygrometer and running the exhaust fans consistently is not bringing it down, a whole-home dehumidifier or a heat recovery ventilator, which brings in fresh outdoor air while recovering heat from the air being expelled, are both real solutions for a persistently humid house rather than something to just live with every winter and wipe down every morning. Either is a bigger step than the checklist above, and worth trying the checklist first for a couple of weeks before deciding the house actually needs one.
If the condensation turns out to be a genuinely failed window seal rather than humidity, our window repair and replacement service in Kitchener, Waterloo and Cambridge can confirm which one it is and quote the fix.