You come down on a cold morning and the windows are running with water on the inside. Puddles on the sill, maybe a bit of black mould creeping into the corner of the frame. It looks like the windows are failing. Before you worry, though, the first thing to know is that condensation on the room side of the glass usually isn’t a fault with the window at all. It’s the window telling you something about the air in your house.
That’s a different problem from misting between the panes, and it matters which you’ve got, so let’s start there.
First, work out where the water is
Run your finger over the wet glass. If you can wipe the moisture away, it’s on the inside surface of the glass, in the room, and that’s ordinary indoor condensation. This article’s for you. If the fog is sealed inside the glass and you can’t wipe it off no matter what, that’s a blown double glazed unit, which is a repair job. We’ve written about that separately in the misted double glazing article.
Assuming it wipes away, read on.
Why it forms
Warm air holds moisture. Cold air can’t hold as much. Your glass is the coldest surface in the room, so when warm, damp indoor air touches it, it cools and dumps its moisture out as water, the same way a cold drink sweats on a summer’s day. Everything you do indoors adds moisture to the air: showers, baths, cooking, drying washing on radiators, even breathing overnight in a closed bedroom. In winter, with windows shut and heating on, that damp air has nowhere to escape, so it condenses on the coldest thing it can find, which is your windows.
So it’s really a combination of three things: how much moisture is in your air, how cold the glass gets, and how little ventilation there is to carry the damp away.
What actually helps
You can nearly always cut it down without spending much:
- Ventilate the moisture at source. Use an extractor fan in the bathroom and kitchen if you have one, and open a window a crack after showering or cooking. Getting the damp air out before it spreads through the house is the single biggest win.
- Dry washing outside or in a vented room rather than on radiators. Wet clothes on a rad can put litres of water into the air of a room over a day.
- Crack the trickle vents. Those little slotted vents at the top of many uPVC windows are there for exactly this. Leave them open to let a slow trickle of air keep things moving.
- Let air circulate. Pull furniture a few inches off cold external walls, and open internal doors so the house breathes rather than trapping damp in one cold room.
- Keep a gentle background heat in rooms that suffer worst. A warm room has warmer glass, and warmer glass sweats less.
Do a few of those and most people find the morning puddles shrink right down.
When the window is part of the problem
Here’s where it does tie back to your glazing. Condensation forms on the coldest surface, and the colder your glass, the more you’ll get. So if you’ve still got single glazing, or a blown double glazed unit that’s lost its insulating gas, that glass runs much colder than healthy double glazing, and it’ll gather condensation far more readily than the windows next to it. If one particular window streams while the others stay dry, that’s often a clue the unit in it has failed.
In that sense, sorting a blown unit or upgrading old glazing does two jobs at once: warmer glass means less condensation and a warmer room. It won’t cure condensation caused by a genuinely damp, under-ventilated house, because nothing but ventilation will do that, but it takes the coldest, worst-offending surfaces out of the equation.
Why it’s worth dealing with
Left alone, persistent condensation leads to black mould around the frames and on the sealant, which is unpleasant, bad for the plaster and not great to be breathing in. It’s much easier to manage the damp air now than to be scrubbing mould off the reveals every winter.
Not sure which you’ve got?
If you can’t tell whether it’s room-side condensation or a blown unit, or one window’s much worse than the rest and you suspect the glass, send us a photo on WhatsApp and we’ll take a look. If it’s a failed unit we can sort it. If it’s genuinely just indoor damp, we’ll tell you that too and point you the right way, no charge for an honest opinion.
We’re a family-run local firm covering Wishaw and Lanarkshire. Give us a ring on 01698 604 278 whenever it’s handy.
If it turns out the moisture is inside the glass rather than in the room, that is a failed unit, and it needs a sealed unit replacement.
Related advice
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Stuck on something else? Our window and door advice section covers around fifty of the faults we see most.
Written by the team at Wishaw Window Repairs. Over thirty years repairing uPVC windows and doors across Lanarkshire.