New windows make a house more airtight, so the moisture that used to escape through draughty old frames stays inside and condenses on the coldest surfaces. Damp can also come from installation faults such as poor sealing, damaged reveals or a missing cavity closer, so the cause needs checking.
It’s a common story. Somebody spends good money on new double glazing, the house feels warmer, and then by the first winter there’s mould round the window reveals, on the walls and in the bedrooms. They think the windows are faulty. Sometimes they are, but usually the windows have changed how the house breathes.
Less air leakage, same moisture
Old single-glazed or tired windows let a lot of air in and out. That was wasting heat, but it was also carrying moisture out of the house. New windows stop most of that. The household still produces the same moisture from cooking, showers, washing and breathing, so with fewer ways out, the humidity rises. The moisture then finds the coldest surfaces, which are often the window reveals, the corners and the external walls.
Cold reveals
When windows are replaced, the reveals around the frame are often disturbed. On cavity walls, the cavity should be closed at the opening, and on solid walls the reveal can be a cold bridge. If there’s a gap in the insulation or the plaster has been patched with a cold, dense filler, you can get a strip of mould right around the frame.
Installation faults that let water in
- Poor or missing sealant around the outside of the frame.
- Missing or damaged cavity trays or lintel details above the window, letting water track down the cavity.
- Sills without a proper drip, or with the wrong fall, sending rainwater back towards the wall.
If damp patches appear after rain, especially below or beside the window, think water getting in rather than condensation.
What to do
- Open the trickle vents and keep them open.
- Use extractor fans properly and reduce moisture at source.
- Check the outside of the frames and sills for gaps and cracks.
- If the windows were installed recently, contact the installer and check the guarantee and the certification paperwork.
- If there’s mould around the reveals, have them looked at for cold spots.
New windows are a good thing. They just need the ventilation to go with them.
When to call a specialist
If you can't tell whether the damp around your new windows is condensation or water getting in, a survey will identify which and what needs fixing. Book a survey with All Dry on 01226 287181 or email info@alldrydampproofing.com.
Frequently asked questions
Can new windows cause condensation?
They can increase it by reducing air leakage, so moist air stays in the house longer.
Why is there mould around my new window frames?
Often the reveals are cold spots, or condensation is settling there because ventilation has been reduced.
Who do I contact about damp after new windows?
Start with the installer and check the guarantee, then get an independent diagnosis if needed.