Why PVC windows fog up and what to do about it
Misted glass worries many people who have just installed new windows. But most of the time the window is not at fault: it simply shows that the room is too humid or lacks fresh air.
How condensation forms
Warm room air contains water vapour. When it touches a cold surface, the vapour turns into droplets. The coldest surface in a room is usually the glass or the frame. The higher the humidity and the colder it is outside, the more noticeable the effect.
The main causes
- high humidity indoors: drying laundry, cooking, lots of houseplants
- poor ventilation: new windows are tighter than old ones, so air no longer leaks in by itself
- floor-length curtains or blinds covering the radiator under the sill
- cold spots in the installation joint if it was done wrongly
- a glazing unit whose insulation does not suit the room
What you can do yourself
- Ventilate briefly and intensively for 5–10 minutes instead of leaving a window ajar for hours.
- Use the sash's micro-ventilation position, especially at night.
- Clear the sill and do not cover the radiator with thick curtains.
- Switch on the extractor when cooking and do not dry laundry in a closed room.
- Watch the humidity: roughly 40–60% is comfortable for living rooms.
When you need a technician
If condensation appears inside the glazing unit, between the panes, the seal has failed and the unit is replaced. If the reveals are wet and mould appears around the frame, the problem is most likely in the installation joint.
Where condensation shows up and what it means
| Where | What it means |
|---|---|
| Outside on the glass in the morning | Normal: low-e glazing keeps the heat in, so the outer pane stays cold |
| Inside on the glass in frost | High humidity or too little ventilation |
| Between the panes | The seal has failed and the unit is replaced |
| On the reveals and around the frame | A cold bridge, possibly a fault in the installation joint |
What not to do
- seal every gap and keep all windows tightly shut all winter
- dry laundry in a closed room
- put many plants right against the glass
- wipe off condensation but leave the sills and reveals wet
Need advice on your project?
Call us or book a measurement: we will come, look at the opening and tell you honestly what fits.
