Is window condensation a sign of a bad window?

Not on the inside surface — that's a humidity/ventilation issue. Moisture trapped between the panes, though, means the window's seal has failed; that's a different fix.

Will condensation cause mould?

Persistent condensation keeps frames and nearby walls damp, which is exactly what mould needs. Controlling humidity is how you prevent the mould.

Condensation on your windows — what it's telling you

Condensation on the inside of your windows means the indoor air is too humid for the cold glass — common in winter, kitchens, and bathrooms. Fix it by lowering humidity: run exhaust fans, open trickle vents, don't dry laundry indoors, and aim for 30–50% relative humidity. Persistent condensation feeds window-frame and wall mould, so it's worth solving. (Fog between the panes is different — that's a failed seal; see the foggy-window page.)

Why it happens

Warm indoor air holds moisture; when it hits cold glass it cools and releases that moisture as condensation. Everyday activities — cooking, showering, drying clothes indoors, even breathing — add humidity, and modern airtight homes trap it. It's worst on the coldest windows and in the least-ventilated rooms.

How to stop it

Lower the humidity and improve airflow: use kitchen and bathroom exhaust fans (and keep them running a while after), open trickle vents or crack a window, avoid drying laundry indoors, and consider a dehumidifier in problem rooms. Aim for 30–50% relative humidity. If condensation is heavy and constant, or black spots are forming on frames and walls, treat it as a ventilation and mould problem, not just wet glass.

When to call a professional

• Condensation and mould keep returning despite better ventilation — you may need a ventilation assessment.
• Damp is spreading into walls, not just on glass — investigate the moisture source.

Common questions

Is window condensation a sign of a bad window?: Not on the inside surface — that's a humidity/ventilation issue. Moisture trapped between the panes, though, means the window's seal has failed; that's a different fix.
Will condensation cause mould?: Persistent condensation keeps frames and nearby walls damp, which is exactly what mould needs. Controlling humidity is how you prevent the mould.

Sources

Keep indoor humidity around 30–50% and dry any wet materials within 24–48 hours to limit mould growth (EPA). Tier 1 (official)
U.S. Environmental Protection Agency (EPA) · verified 2026-07-26
Mould always grows from a moisture problem — unless the water source is fixed, cleaned mould returns (EPA / Health Canada). Tier 1 (official)
Health Canada · verified 2026-07-26