Condensation complaints arrive in October and peak around the New Year. Before you spend anything, work out which of three different problems you actually have, because two of them are about how you run your house and only one is about the window.
Water on the room side of the glass
This is the common one, and it is a humidity problem rather than a defect. Warm indoor air holds moisture; when it touches glass that is much colder, the moisture drops out as liquid. The glass is simply the coldest surface in the room.
The fix is to run drier air as it gets colder outside. A level that feels fine at −5 °C will stream down the glass at −25 °C. As a working guide for Canadian winters:
- Outdoor temperature above −10 °C — keep indoor relative humidity around 35–40%
- Between −10 °C and −25 °C — around 30%
- Below −25 °C — 25% or lower
Practical steps: run bathroom and range fans for a good twenty minutes after use and confirm they vent outdoors rather than into the attic, turn down or turn off a whole-house humidifier, open blinds and curtains so room air can wash across the glass, and check that the HRV is running if you have one. A $15 hygrometer tells you more than any amount of guessing.
Frost or water on the frame, not the glass
If the wet part is the frame — particularly the bottom rail and the perimeter — the frame is conducting cold from outside. Non-thermally-broken aluminum does this badly, which is why so many 1960s and 70s units in Regina and Winnipeg ice up along the edge while the middle of the pane stays clear.
Humidity control helps here but rarely solves it. What solves it is a frame that does not conduct — PVC, fibreglass, or a hybrid with a proper thermal break — and a warm-edge spacer at the glass edge instead of a bare aluminum one. The spacer matters more than most quotes suggest: it is the single biggest factor in whether the bottom edge of your glass stays above the frost point at −30 °C.
Fog sealed between the panes
This one is a failure, and no amount of ventilation touches it. If the haze is inside the glass unit where you cannot wipe it, the perimeter seal has gone and the insulating gas has been replaced by damp air. Once that happens the unit has lost most of its thermal value and the fogging is permanent.
The good news is that you may not need a whole window. If the frame and hardware are sound, many installers can replace just the sealed glass unit for a fraction of a full replacement. That is worth asking about explicitly, because a full-frame quote is the default answer. Note that glass-only work does not qualify for most rebate programs — see our cost breakdown for how the two compare.
When condensation means it is time to replace
Fogging between the panes on several units at once usually means the whole generation of glass is at end of life — sealed units typically run 20 to 25 years. Persistent frame frost on a non-thermally-broken frame is the other clear signal. If you are weighing it up, our guide to how long windows actually last covers what wears out first, and energy-efficient windows for Canadian winters covers which specs are worth paying for.
One caution about a common upsell: adding a third pane will not fix condensation caused by indoor humidity or a conductive frame. Fix the humidity first, then judge the window.