Why are my windows wet on the inside in the morning?

Natural Resources Canada (NRCan) puts it simply: condensation forms when moist indoor air touches a surface that is colder than it is, and the risk rises as outdoor temperatures drop and indoor humidity rises. A window is the coldest surface in most rooms, and its glass and frame edges are colder still. Overnight the outdoor temperature falls, the heat is often turned down, and people breathing in a closed bedroom add moisture, so by morning the glass is wet. The first cold nights of October are when it starts, and January is when it is worst.

What humidity should my house be to stop window condensation?

Health Canada recommends keeping indoor relative humidity between 30% and 50%. In an Ontario winter, the right number for your windows is lower the colder it gets: what is fine at 0 °C will stream down the glass at minus 20. Buy a hygrometer for a few dollars and watch it with the windows. If the glass is wet in the morning, the house is too humid for those windows that night. Turn the furnace humidifier down, or off, during cold snaps, and run ventilation. Better windows tolerate more humidity, because their inside surface stays warmer.

Which windows are wet, and where? Narrow it down

Why are my windows wet even though the humidity is low?

Because the reading in the hallway is not the air at the glass. A bedroom with the door shut and two people asleep can be much more humid than the hygrometer downstairs, and blinds or heavy curtains keep the glass much colder than the room. Older windows, aluminium frames and the bottom edge of any sealed unit run colder too. Open blinds slightly at night, leave bedroom doors ajar, keep furniture off the heat registers under windows, and run the furnace fan on low or the HRV continuously in the coldest weeks so air moves over the glass.

Does an HRV help with window condensation?

Yes, if it is running and maintained. A heat recovery ventilator (HRV) swaps stale, moist indoor air for drier outdoor air and recovers most of the heat on the way. NRCan's HRV guide describes adjusting ventilation through the seasons to keep windows clear. An HRV left on its lowest setting, or with filters that have not been cleaned in years, does little. Clean the filters and the core as the manual says, make sure the outdoor hoods are not blocked by leaves or snow, and turn the speed up when windows start to fog. A technician can balance the airflows if it still struggles.

In Toronto and York Region homes, what makes it worse?

• Newer, tighter houses: about half of Vaughan's homes, 47% of Markham's and 44% of Richmond Hill's were built in 2001 or later (Statistics Canada, 2021 Census). Tight houses hold moisture in, and in their first winters they are still drying out, so the HRV and bath fans matter more.
• Older houses: 29% of the City of Toronto's homes were built in 1960 or earlier, and about 30% of Newmarket's before 1981. Older windows have colder glass and frames, so they fog at lower humidity.
• Gas furnaces with humidifiers: Enbridge Gas supplies natural gas across Toronto and York Region, and many furnaces here have a humidifier on the ductwork. Its setting is often left where it was in October all winter.
• Damp basements add moisture to the whole house when the furnace circulates air. A musty basement and wet windows often share a cause.

What to tell the pro

• Which windows: all of them, bedrooms only, bathroom and kitchen, one floor, or one window.
• On the glass or between the panes, and on which mornings (only the coldest nights, or every day).
• A hygrometer reading, and the humidifier setting if the furnace has one.
• Whether the house has an HRV or ERV, its age, and when its filters were last cleaned.
• Whether bathroom fans run, and where they vent.
• Photos of the window, the sill, any mould and the HRV or humidifier label.

What does it cost on Doyo?

Most wet-window cases are fixed with settings, not repairs. If the humidifier will not shut off or the HRV needs attention, book an HVAC visit: Furnace Maintenance on Doyo is $206.50, all-in with Doyo's fee included, and you can ask the technician to look at the humidifier and HRV while they are there. A furnace repair is priced by written quote. A bathroom fan that needs replacing or ducting outside is electrical or handyman work; General Repairs by a handyman is $59 an hour, with the estimated time and total shown before you book.