Why Windows Mist Up on the Inside

Wake up to wet glass and streaming sills, and it is natural to blame the windows. Most of the time, though, the glass is simply the coldest surface in the room, and warm moist air from cooking, showers, drying laundry and breathing condenses on it overnight.

This guide explains the causes, what you can change, and the one situation where misting really does mean a window problem.

First, find which side the water is on

Wipe the inside of the pane. If it clears, the moisture was on the room side and the cause is indoor humidity meeting cold glass. If the mist stays, it is between the two panes, which means the seal of the unit has failed. That is a different problem with a different fix, covered in our misted double glazing guide.

Common causes of inside condensation

• Laundry dried indoors on radiators or airers
• Bathrooms and kitchens without an extractor, or with it switched off
• Trickle vents closed or taped over
• Bedrooms with doors shut and heating off overnight
• Old single glazing or double glazing with aluminium spacer bars

Why it is worse in the South West

Coastal and river valley homes across Devon, Cornwall and Dorset sit in humid air for much of the year, so the moisture load indoors starts higher. Older solid wall cottages add to it. Ventilation matters more here than in drier parts of the country.

What helps

Keep trickle vents open, run extractor fans during and after showers and cooking, and dry laundry outside or in a vented space. A little background heat overnight also keeps glass above the dew point.

Where windows are old, modern units with warm edge spacers make a clear difference because the glass and its edge stay warmer. That is a performance upgrade, not a cure for a damp house, and we will say so on survey.

Outside condensation is a good sign

Mist on the outside of the glass on a cold, clear morning means the unit is insulating well, keeping heat in so the outer pane stays cold. It clears as the sun comes up and needs no fix.

A quick checklist for damp mornings

Open trickle vents in every room and leave them open through winter. Run the bathroom fan for twenty minutes after a shower. Put lids on pans and use the cooker hood. Move furniture a little away from cold external walls. If one room is always worse than the rest, look for a hidden moisture source such as a tumble dryer without a vent, or a leak.

Questions people ask