Mould After New Windows: Why It Happens & How to Fix It

The symptoms

New double or triple glazing, then mould appears on walls, corners and reveals
Less condensation on the glass, but more on cold walls
A stuffier, more humid feeling indoors
Old draughty windows that 'never had this problem'

The building physics: you removed the accidental ventilation

Old windows leaked air constantly — around the frames, through worn seals, via sash boxes. That accidental ventilation was wasteful, but it quietly removed moisture and kept indoor humidity down. New, well-sealed windows stop that leakage. Unless ventilation is deliberately replaced — trickle vents, continuous extract or MVHR — the moisture from cooking, washing, drying and breathing now stays in the home. Humidity rises, and it condenses on the next-coldest surfaces: the walls, corners and reveals, where mould grows.

There is a second effect. With warmer glass, the windows no longer condense — so the moisture that used to appear harmlessly on the glass (where you wiped it away) now condenses invisibly on cold walls instead. The total moisture has not changed; it has just moved to where it does more harm.

The likely causes

Loss of accidental ventilation when leaky windows were replaced
No replacement ventilation strategy (trickle vents disabled or absent)
Cold walls and reveals now becoming the coldest surfaces
Existing high indoor moisture load (cooking, washing, drying, breathing)

Why this is not a reason to keep old windows

New windows are a genuine improvement — warmer, quieter, more secure and more efficient. The problem is not the windows; it is that ventilation was not upgraded alongside them. The right response is to add the controlled ventilation the old draughts used to provide, so you keep all the benefits of the new windows without the condensation. This is exactly the kind of joined-up thinking a building-performance approach brings.

Common mistakes homeowners make

Taping up or refusing to open trickle vents on the new windows
Blaming the windows rather than the missing ventilation
Bleaching the new mould without controlling humidity

How RetrofitIQ resolves post-glazing mould

Humidity logging to confirm indoor moisture has risen since the windows changed
Thermal imaging and surface-temperature readings to find the new cold spots
A ventilation assessment to size continuous extract or MVHR
A combined plan: controlled ventilation plus, where needed, warmer surfaces
Verification that humidity is back in the healthy 40–60% band

Common questions from clients.

Should I have kept my old windows?

No — new windows are a genuine upgrade. The issue is that ventilation was not upgraded alongside them. Adding controlled ventilation gives you the best of both.

Will opening the trickle vents fix it?

It often helps significantly. If the moisture load is high, a designed continuous extract or MVHR system gives more reliable control without the heat loss of open windows.

Is this the window installer's fault?

Not exactly — it is a building-physics consequence of tightening the home. The remedy is to add the ventilation that the leaky windows were inadvertently providing.

Why is the mould on the walls now, not the windows?

With warmer glass, the windows no longer condense, so the moisture condenses on the next-coldest surfaces — the cold walls, corners and reveals — where it grows mould instead of sitting harmlessly on glass.

What ventilation should I add after new windows?

It depends on the moisture load and how tight the home now is — anything from properly used trickle vents and continuous extract in wet rooms to MVHR in a well-sealed home. We measure and recommend the right level.

Mould after new windows — why upgrading glazing can backfire.