- Home
- Window damp
Why the windows stream, and what actually stops it
Condensation on the inside of the glass is arithmetic: the air in the room is warmer and damper than the glass it is touching. This works out where your own numbers sit.
The check
A cheap hygrometer from a hardware shop is accurate enough for this. The glass temperature is an estimate from the glazing type, not a measurement — treat the result as the shape of your problem, not as a survey.
Your numbers
Dew point 51°F · estimated glass 57°FThe glass is colder than the point at which this air gives up its moisture, so water forms on it.
- Getting the room to about 45% relative humidity would stop it at these temperatures.
- Run the extract fan during and for twenty minutes after every shower, and while cooking.
- Leave blinds and curtains open by day so room air can reach the glass.
- These windows are a large part of the problem — until they change, ventilation has to do the work.
You are within a few degrees of condensation. A shower, a pan without a lid or a colder night will push it over.
- Small changes are enough here: lids on pans, laundry dried outside or vented, fans actually used.
- Check that each extract fan moves air — a sheet of paper should hold against the grille.
- Watch the coldest room, usually a north-facing bedroom, before the rest of the house.
The glass is comfortably warmer than the dew point of this air, so it should stay clear.
- If windows still stream, the moisture is probably higher in that room than the figure you entered.
- Bedrooms overnight and bathrooms after a shower are the usual exceptions.
- Persistent damp with numbers like these points at a leak or rising damp rather than ventilation.
On mould: we do not diagnose or treat mould, and nothing here is a health assessment. Persistent black growth on walls or in cupboards is worth a specialist's opinion as well as a ventilation fix.
There are only three things you can change
- Less moisture in the airLids on pans, the extract fan actually running during and after a shower, laundry not dried indoors, and a fan that moves what its label claims. This is the cheapest lever and usually the effective one.
- Warmer glassBetter glazing, or curtains and blinds left open so room air can reach the glass. A heavy curtain in front of a cold window makes condensation worse, not better.
- More ventilationTrickle vents, an extract fan on a humidistat, or a proper ventilation unit in a tightly sealed house. This is where a heating and ventilation contractor is genuinely useful.
- What does not workTurning the heating up on its own. It raises the dew point of the room air as moisture is added, and the glass stays the coldest surface either way.
