Winter Window Condensation Ontario 2026: Why It Happens and What to Do

Why windows sweat in winter

Take a cold glass of beer outside on a humid August day and watch it sweat in thirty seconds. That is the exact same mechanism happening to your living room windows in January. The air inside your house carries water vapour. When that air touches a surface colder than its dew point, the vapour condenses into liquid water. In summer it is an iced glass. In a Canadian winter it is the inside face of your windows, because the glass surface sits far closer to the outdoor temperature than to the indoor one. Health Canada's guidance on residential moisture frames condensation as the most visible symptom of a broader indoor humidity problem, not as a problem in itself.[1]

Ontario conditions make it worse than almost anywhere else in the country. A January evening in Toronto can sit at -12 C outside with 70 percent RH, Ottawa can hit -25 C overnight, and the indoor air is typically held at 21 C so the house is comfortable. That 30 to 50 C temperature gradient across a piece of glass is huge. Single-pane or older aluminum-framed double-pane windows can run an interior glass surface temperature of 2 to 6 C when it is -20 C outside. At 21 C and 40 percent RH, the dew point of the indoor air is about 7 C. The math is brutal: glass cooler than the dew point will always condense water, every time, until you change the math.

CMHC's homeowner guidance on condensation puts it plainly: if you are seeing liquid water on your windows, either the indoor humidity is too high for how cold the glass is, or the glass is too cold for how much humidity you are producing. Those are the only two variables you can realistically move in an occupied house.[3]

The dew point math

The formal formula is the Magnus approximation, but the kitchen-counter version is good enough for most households. Dew point in Celsius is roughly room temperature minus ((100 minus relative humidity) divided by 5). At 21 C and 40 percent RH, dew point is about 21 minus 12, which is 9 C. At 50 percent RH, it is 21 minus 10, so 11 C. At 60 percent RH, it is 21 minus 8, so 13 C. Every 10 percent of humidity you add to the house raises the dew point by roughly 2 C, and every 2 C you raise the dew point is one more notch of windows that will start fogging.[2]

To measure your own house, buy a $20 digital hygrometer and set it about a metre from the window you are worried about. Let it stabilize for an hour. Buy a $40 infrared thermometer (the kind contractors use to point-and-click surface temperatures) and measure the centre of the glass and the lower edge where condensation usually starts. If the glass is colder than your calculated dew point, you have your answer. Window manufacturers rarely quote interior surface temperature directly; they quote U-factor. But Fenestration Canada's homeowner material on condensation and performance converts that into plain English: low U-factor windows keep the interior glass surface warmer, which raises the humidity level the house can tolerate before fogging begins.[4]

Reducing indoor humidity first

The cheapest fix is almost always the right first move: turn the humidifier down. Health Canada's residential moisture guidance is explicit that winter indoor RH should be held in the 30 to 50 percent band, and that in very cold weather the bottom of that range is healthier and safer for the building.[1] Many Ontario furnaces ship with whole-house humidifiers set at 40 or 45 percent, and homeowners never touch the dial. On a -5 C evening, that is fine. On a -22 C evening, the glass is too cold for 45 percent and water runs down the sill.

Other moisture sources that add up faster than people expect: bathroom showers without the fan running (add 0.5 to 1 litre of water per long shower), drying laundry indoors on a rack (2 to 4 litres per load), cooking without the range hood vented outside (especially boiling and simmering), an unvented gas fireplace, a large aquarium, and a lot of houseplants. None of these is worth banning, but if you have three or four of them and your windows are dripping, ventilation usually matters more than the humidifier setting.

The step-by-step: drop the humidifier 5 percent and wait a day. Run bathroom fans for 20 minutes after showers. Run the range hood (vented outside, not recirculating) during cooking. Open a window for five minutes when you remember to. Watch the same problem window the next morning. If condensation lessens, you are on the right track. If nothing changes, the issue is not a simple humidity adjustment.

When windows themselves are the problem

The other side of the equation is the glass temperature. An old single-pane sash window with weather stripping from the 1970s runs an interior glass temperature that is essentially the outdoor temperature plus a few degrees. No amount of humidifier tuning makes that window stop fogging on a cold night without drying the air to uncomfortable levels. Natural Resources Canada's "Keeping the Heat In" material on windows and condensation lays out the same point in engineering language: low-performance windows force the household to choose between comfortable humidity and dry, clear glass, and the honest answer is that you cannot have both without upgrading the glass.[6]

Modern double-pane low-E argon windows raise the interior glass surface temperature significantly, often 8 to 12 C warmer than a single pane under the same conditions. Triple pane raises it another 3 to 5 C on top of that. This is why airtight, well-insulated new builds can hold 40 percent RH comfortably at -25 C while a 1965 bungalow with original windows cannot hold 25 percent without the windows weeping. It is not the humidifier's fault. It is the glass.

Ontario replacement cost ranges for 2026 are roughly $600 to $1,200 per window installed for a standard vinyl double-pane low-E unit, more for fibreglass or triple-pane. Our windows and doors cost guide breaks down the per-window and whole-house numbers by material and glazing. The Ontario Home Renovation Savings Program has returned rebates for ENERGY STAR certified windows on qualifying retrofits; check current program terms before booking the work.[7]

HRV balancing for envelope homes

Every retrofit wave in Ontario, from the 1980s R-2000 program forward, has tightened the building envelope. Modern construction is tighter still. A house that cannot leak air also cannot leak moisture, which is good for heating bills and bad for humidity control unless the ventilation system picks up the slack. This is exactly why building code now requires mechanical ventilation in new Ontario homes, and why retrofits that add insulation and air sealing without adding ventilation routinely create a new condensation problem where there was none before.

A heat recovery ventilator (HRV) exhausts humid indoor air and brings in drier winter outdoor air while reclaiming 60 to 85 percent of the heat. The CHBA builders' technical material on moisture control treats balanced mechanical ventilation as the standard prescription for airtight envelopes, because in a well-sealed house natural infiltration can no longer carry moisture out.[5] The key word is balanced: if your HRV is running more exhaust than supply (or vice versa), you are pressurizing or depressurizing the house, which creates its own moisture migration and backdraft risks.

For the practical decision between an HRV and an ERV in Ontario's climate, our HRV vs ERV in Ontario guide covers when each one actually earns its keep. The short version: in most of Ontario, an HRV is the right tool for winter condensation because it actively exhausts the moist air. An ERV transfers some moisture back into the incoming stream, which is great for summer dehumidification but counterproductive when you are trying to dry the house out in January.

Mould and wood-sill damage

Liquid water sitting on a painted wood sill for weeks does exactly what you would expect. The paint blisters. The wood swells and separates at the mitres. Dark grey or black staining appears first in the corners where the glass meets the frame, then migrates along the lower edge. Eventually the caulk releases, water gets behind the trim, and the problem that was a cosmetic nuisance in the room becomes a framing problem in the wall.

Health Canada's residential moisture guideline treats visible mould as a health issue and directs homeowners to correct the underlying moisture source before cleaning the surface, because cleaning without correction just restarts the cycle.[1] In practice that means: lower the humidity, ventilate the room, and only then strip the stained paint, treat the wood with a mould cleaner, and repaint. If the wood is soft or punky under a screwdriver, it is past cleaning and into replacement, and the bigger question is how far the damage has spread into the rough frame behind the drywall.

Indoor air quality broadly is its own topic, but window condensation is one of the single most reliable canaries that indoor humidity is off. Our indoor humidity in winter guide covers setpoints, measurement, and ventilation in more depth, and dovetails with this one.

When condensation means replace the windows

Most homeowners reach for window replacement too early (when the real fix was a humidifier dial) and too late (when the windows have been dripping for five winters and the sills are rotted). Here is the honest decision tree.

Replace the insulated glass unit (IGU), not the whole window, when you see fog, streaks, or haze between the panes of a sealed window. That is a failed seal and the inert gas is gone; no amount of cleaning from either side will clear it. IGU replacement is typically $250 to $600 per unit for standard sizes and can be done by a local glass shop without replacing the frame or trim.

Replace the whole window when: the frame is rotted or cracked, the window is a single-pane or very old aluminum double-pane that cannot practically be upgraded, or multiple seals have failed across a house full of similar-vintage units. If you have a single failed IGU in a 2015 vinyl window with a sound frame, swap the glass. If you have a dozen failed IGUs in 1970s aluminum sliders across a bungalow, it is window-replacement time.

Do not replace the whole window because of interior condensation alone, until you have ruled out humidity. A $200 hygrometer/thermometer investigation and two weeks of adjusting the humidifier costs almost nothing and will tell you definitively whether the windows are the problem or a scapegoat for an overfed humidifier in an otherwise reasonable house. Natural Resources Canada's consumer guidance is unusually direct on this point: do the humidity work first, measure, and only then decide whether the windows need to go.[6]

One last note on exterior condensation. If you see fog or dew on the outside of your windows on a cool fall morning, that is actually a sign of a well-performing window. The outer pane is cold because the interior pane is doing its job and keeping the warmth in, so the exterior surface is cooler than the outdoor air and the dew forms on it just like on a car hood. It will evaporate as soon as the sun warms the glass and it is not an issue.

Frequently Asked Questions

Related Guides

• Indoor Humidity in Winter: Ontario Setpoints and Measurement
• Windows and Doors Cost in Ontario
• HRV vs ERV in Ontario
• Indoor Air Quality in Ontario Homes
• Attic Insulation Cost in Ontario