- Interior condensation is usually a humidity issue; warmer triple-glazed glass greatly reduces it.
- Exterior condensation is actually a sign of an efficient window — it's harmless.
- Fog between the panes means a failed sealed unit that needs glass replacement.
- Persistent interior condensation with frost or mould signals it's time to replace.
Why your windows get condensation in winter
Condensation on windows is one of the most common winter questions Ontario homeowners ask us. The short version: condensation forms when warm, moist indoor air meets a cold surface. In winter, your window glass is often the coldest surface in the room, so moisture collects there first. Whether that's a harmless nuisance or a warning sign depends on where the condensation appears.
The three types — and what each one means
- Interior condensation (on the room-side of the glass): caused by high indoor humidity meeting cold glass. Common in winter, and usually a humidity-and-ventilation issue rather than a window fault — though warmer, better-insulated glass greatly reduces it.
- Exterior condensation (on the outside of the glass): actually a sign of an efficient window — the outer pane stays cold because so little heat is escaping. It's harmless and clears as the day warms.
- Condensation between the panes: this one is a problem. It means the sealed insulated glass unit has failed and lost its gas fill. The unit can't be dried out — the glass needs replacing.
How to reduce interior condensation
- Manage indoor humidity: use bathroom and kitchen exhaust fans, run an HRV if you have one, and avoid over-humidifying in deep cold.
- Improve air circulation: open blinds and curtains during the day so warm air reaches the glass; move furniture off large windows.
- Upgrade the glass: triple-glazed, Low-E, warm-edge-spacer windows keep the interior pane warmer, so moisture is far less likely to condense on it.
- Fix drafts and cold spots: airtight European windows eliminate the cold zones where condensation forms first.
When condensation means it's time to replace
Persistent interior condensation on old single- or double-pane windows — especially with frost, water pooling on the sill, or peeling paint and mould around the frame — is a clear signal the windows are under-performing. And any fog trapped between the panes means the seal has failed and that unit is done. In both cases, upgrading to modern triple-glazed European windows solves the problem at its source: the glass stays warm, the frame seals tight, and the cold surface that drove the condensation is gone.
If you're seeing winter condensation across several windows, a free in-home assessment will tell you quickly whether it's a humidity issue, a few failed units, or windows that have simply reached the end of their life.
Frequently Asked Questions
Is window condensation bad?
It depends where it is. A little interior condensation in deep cold is usually a humidity issue. Exterior condensation is a sign of an efficient window. Condensation between the panes means a failed seal and needs glass replacement.
How do I stop condensation on the inside of my windows?
Lower indoor humidity (exhaust fans, HRV), improve air circulation to the glass, and upgrade to warmer triple-glazed windows. Warmer interior glass is far less likely to collect moisture.
Why is there fog between my window panes?
The sealed insulated glass unit has failed and lost its gas fill, letting moisture in. It can't be dried out — the glass unit needs to be replaced.
Do new windows stop condensation completely?
They dramatically reduce it by keeping the interior glass warmer, but very high indoor humidity can still cause some condensation. Managing humidity and ventilation handles the rest.
Sources & further reading
This article is for general information and reflects typical Ontario conditions; final specifications and pricing depend on your specific home and project.
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