Condensation on windows in winter happens when warm, humid indoor air touches glass that is colder than the air’s dew point, so water vapour turns into droplets on the surface. In most Toronto homes the cure is a mix of lower indoor humidity, better ventilation and better air movement across the glass. If moisture is trapped between the panes, or your windows fog up even at sensible humidity levels, the window itself has become the weak point.
It is October, the furnace is coming on, and many GTA homeowners are about to see the first beads of water along the bottom of their glass. Here is why it happens, what each type means, and what to do about it.
The Simple Physics: Warm Air, Cold Glass, Dew Point
Warmer air can hold more water vapour than cold air. The dew point is the temperature at which air becomes saturated and starts releasing that water as liquid. When room air drifts against a surface colder than its dew point, it cools, cannot hold its moisture, and leaves droplets behind.
At a typical room temperature of about 21°C, air at 50 percent relative humidity has a dew point of roughly 10°C. Drop the humidity to 30 percent and the dew point falls to roughly 3°C. So a window that stays clear at 30 percent can stream with water at 50 percent on a cold January morning.
That gives you two levers: lower the dew point (reduce indoor humidity) or raise the temperature of the glass (improve air circulation or install a better-insulated window).
The Three Types of Window Condensation
Where the moisture appears tells you almost everything: on the room side, between the panes, or on the outside.
| Where it appears | What causes it | What fixes it |
|---|---|---|
| Inside surface (room side), often along the bottom edge or frame | Indoor humidity too high for the outdoor temperature, poor air circulation at the glass, or a cold window that drags the inside pane below the dew point | Lower humidity, run exhaust fans or an HRV/ERV, open blinds and curtains; if it persists at sensible humidity, upgrade to warmer glazing and frames |
| Between the panes (inside the sealed unit) | A failed insulated glass unit (IGU) seal: moist air has entered and the desiccant is saturated | Replace the sealed glass unit; replace the whole window only if the frame, hardware or seals are also worn out |
| Outside surface (exterior pane) | The outer pane cools below the outdoor dew point on clear, calm, humid mornings, because so little indoor heat escapes | Nothing to fix: it is a sign of an efficient window and clears as the sun and air warm the glass |
Condensation on the Inside of Windows: Usually a Humidity Problem
Condensation on the inside of windows is the most common complaint, and in most homes the root cause is moisture, not the window. Showers, cooking, laundry, plants and breathing all add water vapour, and in winter, with the house shut tight, it accumulates.
The colder it gets outside, the colder your glass becomes, and the lower indoor humidity needs to be. As general guidance, many homes do well at roughly 30 to 40 percent relative humidity in cold weather, sliding toward the lower end, or slightly below it, when outdoor temperatures drop well below freezing. If you run a furnace humidifier, turn it down as a cold snap arrives.
Typical warning signs that humidity is the culprit:
- Condensation appears on many windows at once, especially bedrooms in the morning.
- It is worst in bathrooms and kitchens, and after showers or cooking.
- A hygrometer reads above 45 percent on cold days.
Do not ignore it: water running onto sills can stain wood, lift paint and encourage mould around the opening over time.
How to Stop Window Condensation: Practical Fixes That Work
If you are wondering how to stop window condensation without buying new windows, start here. These steps cost little and solve the problem in many homes.
- Measure first. A basic digital hygrometer tells you your real indoor humidity.
- Run exhaust fans properly. Use the bathroom fan during and after every shower and the range hood whenever you cook, and make sure both vent outdoors.
- Use your HRV or ERV. Many newer GTA homes have a heat recovery ventilator (HRV) or energy recovery ventilator (ERV), which swaps humid indoor air for fresh outdoor air while recovering much of the heat. Keep it running with clean filters. An HRV is generally better at removing excess moisture; an ERV retains some humidity and suits homes that run dry.
- Let air reach the glass. Heavy drapes, cellular shades and closed blinds trap cold air against the glass. Open coverings during the day and keep furniture off the heating registers below windows.
- Avoid drying clothes indoors. A load hung to dry releases a surprising amount of water. Use a dryer vented outdoors.
- Be patient with new construction. New or heavily renovated homes release construction moisture through the first heating season.
One honest note: after replacing old, draughty windows you may see more condensation than before. The old units leaked humid air outdoors; your tighter house now needs controlled ventilation instead.
Condensation Between Window Panes: A Failed Seal
Condensation between window panes is a different problem, and no amount of ventilation will fix it. Double- and triple-glazed windows use a sealed insulated glass unit: panes held apart by a spacer, sealed at the edge, often argon-filled, with desiccant inside the spacer to absorb residual moisture.
Over years of temperature cycling and sun exposure, that seal can fail. Humid air leaks in, the desiccant saturates, and you see fogging or a white film that cannot be wiped from either side. The insulating gas has usually escaped too.
A failed seal does not always mean a new window. If the frame, hardware and perimeter seals are sound, replacing only the glass unit is usually the sensible, lower-cost repair. If the frame is also deteriorating or draughty, a full window replacement in Toronto is the better long-term investment.
Condensation on the Outside: Why It Is Actually Good News
Dew on the outside of new windows alarms many homeowners, and fall is when it appears most. On clear, calm, humid mornings the outer pane radiates heat to the sky and cools below the outdoor dew point, just like your lawn or windshield.
Older windows leak enough indoor heat to keep the outer pane slightly warm. A high-performance unit keeps that heat inside, so exterior condensation is a visible sign your window is insulating well. It burns off as the morning warms.
When the Window Is the Cause
If humidity is sensible, ventilation works and coverings are open, yet you still see heavy condensation or frost on the room side, the window is the cold spot. Common culprits in older Toronto homes:
- Aluminium frames without thermal breaks. Aluminium conducts heat readily; without an insulating barrier, the inside frame runs cold enough to collect water and even ice.
- Double-glazed units with aluminium spacers. The metal spacer bridges heat out at the edge, which is why condensation often starts as a band along the bottom of the glass.
- Failed seals and lost gas fill. A unit that has lost its argon insulates less well, so the inside pane runs colder.
For more on U-values and frame materials, our guide to thermal performance in windows for Ontario homes goes deeper.
How Better Windows Raise the Inside Glass Temperature
High-performance windows keep room-side surfaces warmer, above the dew point of normal indoor air. Three features matter most.
Warm-edge spacers
Replacing the conventional aluminium spacer with a less conductive warm-edge spacer raises the temperature at the edge of the glass, exactly where condensation tends to form first.
Triple glazing
A third pane and second gas-filled cavity cut heat loss, so the inside pane stays closer to room temperature. Triple-glazed windows suit bedrooms and large glazed walls where condensation persists. Whether they pay off house-wide is covered in are triple-glazed windows worth it in Canada.
Thermally broken frames
Premium European aluminium systems insulate between inner and outer frame so the room-side profile stays warm. The Schüco AWS 90.SI+ reaches U-values down to 0.8 W/(m²·K), the Reynaers MasterLine 8 down to 0.9, and the Cortizo COR 80 Passivhaus system is PHI certified at 0.8. For a warm interior finish, the wood-clad Weather Shield casement offers a timber interior with protective exterior cladding.
For context, ENERGY STAR Canada windows certified since 2020 must reach a U-factor of 1.22 W/(m²·K) or lower, or an Energy Rating of 34 or higher; see Natural Resources Canada’s ENERGY STAR Canada page. Systems well below that threshold, such as those used for Passive House windows, keep inside glass noticeably warmer.
To be balanced: you do not need premium European windows to solve condensation. A well-made ENERGY STAR certified North American window with warm-edge spacers will do it in many homes once humidity is controlled. Premium systems make most sense where you also want slim sightlines, large spans or Passive House-level performance. Our energy-efficient windows page compares the options.
Window Condensation in Toronto: Use This Winter to Diagnose
Toronto’s winter is your best diagnostic tool. Note which windows fog, at what outdoor temperature and indoor humidity. If condensation clears once humidity drops to the low 30s, it is a ventilation fix. If a few units stay wet, fog between the panes or ice at the frame, they are candidates for glass or full replacement. With European systems typically on an 8 to 14 week lead time, documenting problem windows now helps you plan upgrades for warmer months.
For a second opinion, our team can assess whether you need a sealed-unit repair or new windows, and show you warm-edge, triple-glazed and thermally broken systems side by side in our North York showroom. Book a consultation or call +1 (647) 409-8922.
