The heat came on a few weeks ago, and now the bedroom window is fogged every morning. You wipe it, and by evening it is wet again. Before you blame the windows, know this: in most Moncton houses, condensation is not a window problem at all. It is a humidity report, written on the coldest surface in the room.
Your windows are the messenger, not the problem
Warm air holds more moisture than cold air. When warm, damp indoor air touches cold glass, it cools below its dew point and the water it was carrying has to go somewhere, so it lands on the glass. Windows fog first because they are the coldest visible surface in the house, not because they are broken. That is why the same window can be clear in October and streaming in January: the glass got colder, and the house got tighter once the heating season closed everything up.
This is the most common misunderstanding in homeowner forums on the subject. People shop for new windows to fix condensation, when the real question is how much moisture is in the air. New windows can even make it worse: Natural Resources Canada notes that old, leaky windows helped dry a house out, and sealing them tight traps the moisture inside.
First, figure out which condensation you have
Where the water sits tells you what is wrong:
- On the inside surface: indoor humidity is too high for the glass temperature. The fix is drier air or warmer glass.
- Between the panes: the sealed unit has failed and damp air got inside. No amount of dehumidifying fixes this; the glass unit or the window needs replacing.
- On the outside surface on a clear morning: usually a sign your windows are doing their job, because the outer pane stayed cold instead of leaking heat. Nothing to fix.
If you are unsure, touch it. If you can wipe it from inside the room, it is indoor condensation.
Why it got worse after you sealed the drafts
This one comes up constantly in forums, and it feels backwards. You draft-proofed in the fall and now the windows sweat more than before. The draft-proofing did not fail. It worked. The leaks that used to carry moist air out of the house are closed, so the moisture from showers, cooking, laundry and breathing now stays inside until it finds the coldest surface. Do not undo the sealing. Give the moisture a proper way out instead.
This is a common Moncton setup: many local houses now heat with a ductless mini-split, which heats and cools but does not bring in fresh air. A tight house with a mini-split and no other ventilation is a classic condensation house in January.
How humid should a Moncton house be in winter?
Health Canada's general guidance is 30 to 50 percent relative humidity. But Natural Resources Canada publishes a colder truth for standard double-glazed windows at 21 degrees indoors: you can hold about 58 percent humidity at 0 degrees outside, 44 percent at minus 10, 34 percent at minus 20, and only about 25 percent at minus 30 before condensation appears. In a real Moncton cold snap, the window sets a lower ceiling than the general guidance. A cheap hygrometer tells you where you actually stand, room by room. Triple-glazed, low-E windows tolerate much more, about 67 percent at minus 20.
Five fixes that actually dry a house out
- Run the bathroom fan for every shower, and let it run 15 to 20 minutes after. Make sure it vents outside, not into the attic.
- Run the kitchen range hood when cooking, especially when boiling water.
- Stop drying laundry indoors on racks in winter, and do not store firewood inside the house.
- Open blinds and curtains during the day so warm room air reaches the glass, and keep furniture and heavy drapes off the vents under windows.
- Wipe standing water off sills promptly. Pooled water rots paint and trim and feeds mould.
If you have done all of that and the windows still stream, the house likely needs real ventilation. A heat-recovery ventilator, sized and balanced by a certified technician, brings in fresh air without throwing away the heat. That is the permanent fix for a tight, modernised house.
💡 Use your windows as the gauge
If condensation keeps appearing on double-glazed windows even after you have cut moisture sources, treat it as a signal, not a nuisance: check the hygrometer, then look at ventilation. Persistent condensation can mean moisture is also condensing where you cannot see it, inside walls and the attic.
Foggy windows are annoying, but they are also useful. They are the one moisture gauge every house already has. Read them, dry the air out, and the glass will clear on its own.