To stop condensation on windows in a Winnipeg winter, lower your indoor humidity as the temperature outside drops, keep warm room air moving over the glass, and fix the window itself if it’s the cold spot. Here’s why windows sweat and frost, the right humidity for every outdoor temperature, and ten fixes, most of which you can start tonight.
What to do tonight
- Turn the humidifier down, or off if the forecast is −25 °C or colder.
- Open blinds and curtains a few centimetres, so warm room air can reach the glass.
- Run the bathroom fan during evening showers and for at least 30 minutes after.
- Set your HRV to high, if your home has one.
- Wipe the glass and sills dry before bed and again in the morning.
Then use the table below to find the right humidity for the week ahead.
Why windows sweat and frost in winter
Warm air holds more moisture than cold air. When room air touches a surface colder than its dew point, the extra moisture turns to water on that surface, or to frost when the surface is below freezing. Air at 21 °C and 40% humidity, for example, has a dew point of about 7 °C. Any part of the glass colder than that will fog. At 30% humidity the dew point drops to about 3 °C.
The colder it gets outside, the colder the inside of the glass gets. So the house can hold less and less humidity before the windows sweat. The coldest part of the glass is the bottom edge, and metal frames are colder still. That is why condensation and frost start there (National Research Council Canada).
What the condensation is telling you
Where the water or frost shows up points to the cause.
| Where you see it | What it usually means, and what to do |
|---|---|
| On the inside of the glass, most mornings | Indoor humidity is too high for the temperature outside. Lower it to the level in the table below. |
| Only along the bottom edge, on the coldest mornings | The edge of the glass is the coldest spot. Common in a cold snap. Lower the humidity a little and let warm air reach the glass. |
| In one room only | Moisture from that room, such as a bathroom, a bedroom with the door closed, plants or a humidifier, or a blocked heat register. Ventilate that room and open the door. |
| Frost on the frame or trim | A cold frame or an air leak. Aluminum frames stay very cold, and warm, moist air leaking into a gap around the window freezes there. Seal the gaps from the inside first, around the interior trim: caulking only the outside can trap moist air in the wall (NRCan). An old aluminum window is a candidate for replacement. |
| Between the panes | In a sealed double- or triple-pane window, the seal has failed. It can’t be wiped away, and the window insulates less well. The sealed glass unit or the whole window needs replacing; we replace whole windows. On an old window with a separate storm window, house air is leaking past the inside window: seal the inside window more tightly than the storm. |
| On the outside of the glass on cool mornings | A sign of an efficient window. Little heat escapes, so the outer pane stays cold enough for dew to form. It clears as the day warms up. |
The right humidity for the temperature outside
Health Canada recommends keeping indoor humidity between 30% and 50% (Health Canada). In a Manitoba winter, that is more than most windows can handle on cold nights. CMHC notes that in very cold weather, 30% or lower may be needed to keep windows clear (CMHC, Moisture and Air). The window industry’s guideline lowers the target step by step as it gets colder:
| Outside temperature | Indoor humidity, at most |
|---|---|
| −7 to 4 °C | 40% |
| −12 to −7 °C | 35% |
| −18 to −12 °C | 30% |
| −23 to −18 °C | 25% |
| −29 to −23 °C | 20% |
| Colder than −29 °C | 15% |
For a house at about 20 °C. Source: Fenestration and Glazing Industry Alliance, based on the CSA A440.2 user guide.
- Treat it as a starting point. Energy-efficient windows can carry more humidity than the table shows before they fog; old single-pane or aluminum windows may need less. If the glass stays clear, you can let the humidity creep up for comfort.
- Measure it. A digital hygrometer from a hardware store shows the humidity in a room. Check it in the room with the worst windows, away from the humidifier, first thing in the morning.
- Adjust with the forecast. A furnace humidifier has a control on the duct or the wall: turn it down as a cold snap arrives, and off on the coldest nights. Some controls do this for you with an outdoor temperature sensor.
Why the glass matters
The warmer the inside of the glass, the more humidity a room can hold before it fogs. National Research Council Canada calculated this for clear glass on a cold, windy night:
Modern triple-pane glass with Low-E coatings, argon gas and warm-edge spacers keeps the inside pane warmer still. That is why upgraded windows can support a higher humidity without condensation (CMHC). The edge of the glass and the frame stay colder than the centre, though, so even a good window can show a line of frost at the bottom on a −35 °C morning.
10 ways to stop window condensation
- Turn the humidifier down. Match the table above, and switch it off on the coldest nights. Health Canada’s advice for wet windows includes unplugging portable humidifiers too.
- Run the bathroom fan during every shower, and for at least 30 minutes after. That is Health Canada’s guidance. Keep the bathroom door closed while the fan runs, and check that the fan vents outdoors, not into the attic. A timer switch makes it automatic.
- Use the range hood on high whenever you cook, and put lids on pots. Boiling water for pasta or soup puts a lot of moisture into the air.
- Run your HRV. If your home has a heat recovery ventilator, run it on low all winter and switch it to high when the windows start to sweat. Winter air outside is dry, so swapping stale indoor air for fresh air lowers the humidity. If it has a dehumidistat, set it lower, and clean the filters as the maker recommends; Lifebreath, for example, says every three months (Lifebreath manual).
- Let warm air reach the glass. Open blinds and curtains during the day and leave them open a few centimetres at night. Heavy curtains trap cold, moist air against the window. Keep sofas and dressers off the window wall, and keep heat registers under windows open and clear.
- Keep the heat steady. A room that cools down a lot at night has colder glass, and the same moisture in cooler air means higher relative humidity. In rooms with wet windows, skip deep temperature setbacks and leave the door open.
- Run the furnace fan on continuous during a cold snap. Moving air mixes the house, so damp air doesn’t sit against cold glass in bedrooms and the basement.
- Dry laundry in a dryer that vents outdoors. Hanging laundry inside raises the humidity. Clean the lint trap, and keep snow away from the vent hood outside.
- Cut other moisture sources. A cord of firewood stored indoors can release more than 270 litres of water (CMHC). Lots of houseplants and an open aquarium add moisture too, and a damp basement can add more than everything else combined. Run a dehumidifier in a damp basement.
- Wipe the glass and sills dry in the morning. Health Canada advises drying window frames and sills daily so the water doesn’t lead to mould. A small squeegee and a towel take a minute, and they save a wood sill from rotting.
Mould on the frame or sill
Water that sits on a sill every morning feeds mould, often in the corners of the glass and on wood or drywall around the window.
- Small areas (up to three patches, under one square metre in total) can be cleaned yourself. Health Canada recommends baking soda or a little unscented soap on a damp cloth, with safety glasses, an N95 mask and household gloves. Don’t use bleach.
- Larger areas should be assessed by a professional.
- Fix the moisture, or it will come back. If the wood under the mould is soft or crumbly, it has started to rot; a dark stain alone can be just mould or water marks.
Condensation on new windows
New windows greatly reduce condensation, but they don’t make it impossible.
- The house gets tighter. Old, drafty windows let in cold, dry outdoor air, which kept the humidity down. With new windows, less of that air leaks in, so the humidity can rise unless you run your fans and HRV.
- The coldest nights still test the edges. A thin line of condensation or frost at the bottom of the glass during a deep freeze usually means the humidity is a bit high for that temperature, not that the window is faulty.
- Dew on the outside of new triple-pane windows on cool spring or fall mornings is normal. The outer pane stays cold because so little heat gets through it.
When it’s the window, not the house
Look at the window itself if:
- The glass is wet or frosted even when your humidity is within the table above.
- There is fog or moisture between the panes.
- Frost builds up on aluminum frames, or you feel cold air around the sash.
- The sills are stained, soft or mouldy, or paint keeps peeling below the window.
- The windows are old single-pane or early double-pane units.
Triple-pane windows keep the inside glass much warmer, so they handle normal winter humidity far better. All the windows we install are triple-pane with Low-E glass and argon, ENERGY STAR certified and made in Canada. They meet the product rules for the Efficiency Manitoba rebate of $100 per window opening, for up to 20 openings, when they replace existing windows in a detached or semi-detached house that is your primary residence; you apply online within 90 days of installation and payment in full. You can also finance up to $7,500 on your energy bill through Manitoba Hydro, approved before anything is ordered. We replace windows all winter, one opening at a time, so the rest of the house stays warm: see how winter installs work.
Here’s what a new window costs installed:
| Window | Small | Medium | Large |
|---|---|---|---|
| Casement | $750 – $1,150 about 24″ × 40″ | $850 – $1,300 about 30″ × 48″ | $950 – $1,450 about 36″ × 56″ |
| Slider | $550 – $800 about 30″ × 17″ | $600 – $900 about 36″ × 22″ | $800 – $1,200 about 48″ × 36″ |
| Single hung | $600 – $900 about 28″ × 36″ | $700 – $1,050 about 32″ × 45″ | $900 – $1,300 about 36″ × 60″ |
| Picture | $550 – $800 about 24″ × 36″ | $700 – $1,050 about 36″ × 44″ | $950 – $1,400 about 48″ × 54″ |
Ranges are installed prices before tax and rebates, white frames, based on Arctic Star’s own current quotes — the same numbers as our price calculator. Your exact price comes after a free in-home measure.
For more, read why windows frost up in winter and the benefits of triple-pane windows. If the water comes in when it rains rather than on cold mornings, see leaking windows.
Common questions
Is condensation on windows normal in winter?
A little along the bottom of the glass on the coldest mornings is common. Water running down the glass every day, or thick frost, means the humidity is too high for the temperature outside or the window is too cold.
Why is there condensation on the outside of my windows?
On cool, damp mornings, dew can form on the outside of energy-efficient windows. It means very little heat is escaping through the glass. It clears as the day warms up.
Do new windows stop condensation completely?
They greatly reduce it, because the inside glass stays much warmer. Very high humidity can still cause some condensation at the bottom edge on the coldest days, so keep using your fans and HRV.
Thinking about new windows or doors?
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