Condensation on the Inside of Windows: Causes and Fixes

Every GTA winter, the same question comes in as soon as the first cold snap arrives: why are my windows wet on the inside? The short answer is that warm indoor air holds moisture, and when that air touches a surface colder than its dew point, the moisture turns back into water. In a heated home in January, the coldest surface in most rooms is the window glass, so that is where the water collects first.

Condensation on the inside of a window is not a sign that the window is broken. It is a sign that the air in the room carries more moisture than the glass can tolerate at that outdoor temperature. Fix the humidity and the condensation stops. Ignore it, and the water that runs down the glass every morning soaks into sills, frames and the drywall below, which is where mould starts.

This guide covers where the moisture comes from, the indoor humidity level to aim for as the weather gets colder, practical ways to stop the condensation, and how to tell when wet window sills have already become a mould problem.

Where the water forms tells you what is going on, and only one of the three is a humidity problem inside your home.

A family home produces a surprising amount of water vapour every day. In summer it leaves through open windows. In winter the house is closed up, so it builds up unless it is removed on purpose. The usual sources:

Health Canada recommends keeping indoor relative humidity between 30 and 50 percent, and closer to 30 percent in winter. The reason for the lower winter target is the window glass: the colder it is outside, the colder the inside surface of the glass becomes, and the less moisture the air can hold before it condenses on it.

A long-standing Canadian rule of thumb gives a maximum indoor humidity for each outdoor temperature. Treat it as a starting point rather than an exact limit, because older single-pane windows and aluminium frames run colder and will sweat at lower humidity than modern double or triple glazing.

Condensation on the inside of windows in the morning is one of the most searched versions of this problem, and the timing is not a coincidence. Overnight the outdoor temperature reaches its lowest point, so the glass is at its coldest. Many homes turn the thermostat down at night, which lowers the air temperature and raises the relative humidity of the same air. And everyone in a closed bedroom has spent eight hours breathing moisture into it.

Closed curtains and blinds make it worse. They trap a layer of cool, still air against the glass, cut the window off from the warmer room air, and let the moisture condense undisturbed. Opening them overnight, or at least leaving a gap at the bottom, often makes a visible difference by the next morning.

Most homes can solve this without new windows. Work through the list from the top, because the first few changes are free and fix the majority of cases.

Mould needs moisture and something organic to feed on. A window that drips every morning provides the moisture, and wooden sills, painted frames, the paper facing on the drywall return and even the dust that collects in the track provide the food. Mould can start growing within 24 to 48 hours of a material staying wet, so a winter of daily condensation is more than enough. Our guide on how long mould takes to grow after water damage explains the timeline.

Signs that condensation has already caused damage:

For small patches on hard surfaces such as glass, vinyl frames and sealed, painted sills, Health Canada's guidance is to scrub with water and a little unscented dish detergent, wipe with clean water and dry the surface completely. Bleach is not recommended. Wear gloves, eye protection and a dust mask, and keep the room ventilated while you work.

Caulking and rubber gaskets that are stained right through usually need to be replaced rather than cleaned, because the growth is inside the material. Soft or rotted wood and stained drywall cannot be cleaned either; they have to be cut out and replaced.

Toronto and Mississauga condo units see more window condensation than most houses. Floor-to-ceiling glass gives a huge cold surface, units are small and tightly sealed, and many suites rely on a bathroom fan and a recirculating kitchen hood as their only ventilation. Window frames in many towers are aluminium, which conducts cold far more than vinyl or wood.

If you live in a condo, check that your in-suite HRV or ERV is running and its filters are clean, run the bathroom fan longer than feels necessary, and keep blinds open at night. If the glass units themselves are fogging between the panes, or water is coming in around the frame rather than condensing on the glass, report it to your property manager, because window assemblies are usually the corporation's responsibility under the declaration.

Condensation on the inside of your windows is fixable, and catching it before it soaks into the sills and walls saves a much bigger repair. If it has already left black spots on the frames, stained the drywall or brought a musty smell into the room, Rosehill Restoration Services provides mould inspection and removal across Toronto, North York, Scarborough, Mississauga, Brampton and the wider GTA.

Our guide to how much mould removal costs in Toronto gives a sense of budget, and you can book a free mould estimate when you are ready.