Why did I get mould after having new double glazing fitted?

This is one of the most common and most misunderstood sequences in UK housing: a house that streamed with window condensation for years gets new windows, the glass stays clear, and within two winters there is black mould in the corners of every bedroom. Nothing has gone wrong with the windows. The building has simply lost the accidental ventilation it was relying on.

Step by step

1. Check the trickle vents.Modern replacement windows normally have them, and they are frequently closed on the day of installation and never opened. Open every one and leave them open through the winter. This alone fixes a large share of these cases.
2. Check whether trickle vents were fitted at all.If the windows were fitted without them, and the house has no other purpose-provided ventilation, that is a real deficiency. Retrofitting vents into the frames or the glass is possible and is not a large job.
3. Check the extract fans.A kitchen and a bathroom that used to leak air through the windows now depend entirely on their fans. Test each one: hold a sheet of paper against the grille and see whether it holds. A fan that will not hold paper is not moving air.
4. Look at where the mould has appeared.Reveals, corners, ceiling lines and behind furniture. That is the map of the coldest remaining surfaces, and it tells you what to insulate if ventilation alone does not solve it.
5. Change the airing habit.Windows wide for a short period each morning, particularly in bedrooms, moves far more moisture than a window left ajar. The house no longer does this for itself.
6. Reduce the moisture load.The same house with the same washing, cooking and showering now has much less air leaving it. Dealing with the input is the other half of the answer.
7. Deal with the reveals if mould persists there.New windows often make the reveal the coldest spot. An insulated reveal board raises it above dew point permanently.
8. Take it up with the installer if vents were omitted.Purpose-provided ventilation is part of a compliant replacement window installation. If your certificate says one thing and the windows have no vents, that is worth raising.

Tips

• If you are about to have windows fitted, ask for trickle vents on every openable window and an insulated cavity closer at the reveals. Both are cheap at installation and expensive afterwards.
• The same thing happens after external doors are replaced, after a house is draught-proofed, and after loft insulation is topped up without attention to the hatch.
• Clear window condensation on the glass was never the problem — it was the symptom being handled where you could see it. Losing it is not the same as fixing it.

Common mistakes

• Closing the trickle vents because the house feels draughty — Trickle vents provide the small continuous air change the house needs to shift moisture. Closing them in October is exactly why mould appears in December.
• Blaming the windows and asking for them to be replaced again — The units are usually performing correctly. The building has lost its ventilation, and that is what needs adding back.

Questions people ask