Condensation on windows: why it happens and when it signals a problem

Why a window mists up

Room air carries moisture. How much it can carry depends on temperature — warm air holds more, cold air less. When warm, humid room air touches a cold surface it cools, can no longer hold all that moisture, and the excess comes out as droplets. That is condensation, the same thing that happens on a cold bottle taken out of the fridge.

Glass is the coldest surface in the room. That is why condensation appears there first, usually along the bottom edge where the glass is coldest because of the spacer behind it.

Why it happens more after new windows go in

Old timber or badly sealed joinery leaked air. Moisture escaped through those gaps and dry outside air came in — the house ventilated itself, twenty-four hours a day, at a cost in heat you paid through your heating bill.

New joinery with two gasket lines stops that. The heat stays in, but so does the moisture produced by cooking, showering, drying laundry and simply breathing. A household of four releases several litres of water into the air every day.

Put another way: condensation on a new window is usually proof that the window is doing its job. The problem is not the window — it is that the moisture now has nowhere to go.

When condensation is normal

• It appears on the inside face of the glass, in the morning, and clears during the day.
• It is worst in the bathroom and kitchen, right after a shower or cooking.
• It shows up in winter, when the temperature difference inside and out is large.
• It clears after a few minutes of airing.

When it is not normal — three cases

These three need attention:

• Condensation between the panes. A haze you cannot wipe off, because it is inside the sealed unit, means the edge seal has failed and the desiccant is saturated. The glazing unit is replaced; the frame and hardware stay.
• Water running down the reveal, or mould around the frame. That is no longer condensation on glass but moisture in the wall, and it usually points to a badly sealed perimeter joint or a sill without proper fall.
• The frame mists, not the glass. On aluminium that means a profile without a thermal break in a heated room — the wrong material for that position.

What actually helps

• Short, hard airing instead of leaving a window on the latch all day. Five minutes with the sash wide open clears the moisture without chilling the walls. A window left ajar all day cools the wall and wastes heat with little effect.
• Use the micro-ventilation setting. WINKHAUS hardware has an intermediate position that leaves the sash open by a few millimetres — enough for steady air exchange in winter.
• Do not dry laundry in a closed room, and run the extractor while cooking.
• Do not block the window. Floor-length curtains and deep sills trap cold air against the glass and make condensation worse.
• Heat evenly. A room heated in bursts sees bigger swings in glass temperature.

If it persists

If condensation stays all day despite airing, or shows up on one window while the others stay dry, that opening is worth a look. Usually something specific turns up: hardware out of adjustment so the sash does not seat all round, a broken perimeter seal, or a sill throwing water back at the wall.

We come out, look and tell you what it is — with no obligation, and regardless of who fitted the windows.