Will new windows stop condensation and mould?

What this usually means

Condensation forms where humid indoor air meets a surface below its dew point. On old single glazing the glass is the coldest surface, so it condenses there first — which is why people blame the windows. New, warmer glazing raises the glass temperature, so condensation on the glass itself usually decreases. To that extent, new windows do help.

But the moisture in the air has not gone anywhere. If the home is under-ventilated, that moisture remains and simply condenses on the next-coldest surface instead — typically the window reveals, the wall behind furniture, or cold corners. Worse, the old draughty windows were providing accidental ventilation; sealing them up removes that, so indoor humidity can actually rise, and wall mould can appear or worsen even as the glass stays clear.

So the honest answer is that new windows can reduce condensation on the glass but will not, on their own, cure a condensation-and-mould problem driven by too much moisture and too little ventilation. The reliable fix combines warmer surfaces with controlled ventilation — and that is best decided by measuring the home's humidity and ventilation, not by assuming the windows are the cure.

Common causes

Signs and symptoms

Condensation is governed by the relationship between indoor humidity and surface temperatures. New glazing raises the coldest surface (the glass) above the dew point more of the time, so the glass condenses less. But the indoor humidity — set by moisture production and ventilation — is unchanged, so the dew point is unchanged; any surface still below it will condense. With the glass warmer, that surface is now the reveal, the wall behind furniture, or a cold corner.

Ventilation is the lever that actually removes moisture. Old, leaky windows ventilated accidentally, keeping humidity (and the dew point) lower than it would otherwise be. Replacing them with sealed units removes that accidental ventilation; unless trickle vents or mechanical ventilation are provided, humidity rises and condensation risk increases overall — even though the glass is warmer. This is the mechanism behind the common complaint of mould appearing after new windows.

The durable solution treats both halves of the dew-point balance: warmer surfaces (better glazing and insulated reveals) and lower humidity (controlled ventilation). Measuring the home's humidity and ventilation alongside the glazing shows whether windows will help, and ensures any window work is paired with the ventilation needed to remove the moisture rather than relocate it.

How to actually stop condensation and mould

Pair warmer surfaces with controlled ventilation. New windows can be part of the answer, but only alongside removing the moisture.

1. 01

   Measure humidity and ventilation

   Log humidity and assess ventilation to confirm whether the problem is moisture-and-ventilation rather than glazing.

2. 02

   Provide controlled ventilation

   Ensure trickle vents, continuous extract or MVHR remove household moisture so the dew point stays low.

3. 03

   Warm the cold surfaces

   Improve glazing where it is genuinely cold, and insulate cold reveals so condensation has nowhere to form.

4. 04

   Fit trickle vents with new windows

   If replacing windows, include trickle vents so you do not remove ventilation along with the draughts.

5. 05

   Address moisture sources

   Reduce indoor drying and use extract while cooking and bathing.

6. 06

   Verify

   Re-measure humidity and check that condensation and mould have gone, not just moved.

How to prevent it coming back

• Always provide ventilation when fitting sealed new windows.
• Keep indoor humidity down with extract and good background ventilation.
• Insulate cold reveals so condensation does not migrate there.
• Treat condensation and mould as a moisture-and-ventilation issue, not just glazing.