Condensation on windows: what it means and how to stop it

You come downstairs on the first properly cold morning of autumn and the bedroom windows are running with water. It looks like a fault, and the usual first thought is that the glazing has gone. Usually it has not. Condensation on windows is nearly always a moisture problem in the room, and the glass is simply the first place it shows up.

The useful thing is that there are only three kinds, they have three different causes, and you can tell which one you have in about five seconds.

Which kind of condensation have you got?

Put a finger on the misted glass.

• You can wipe it away from indoors. This is condensation on the room side of the glass. It is by far the most common, and it is about the moisture and ventilation in your home.
• You cannot reach it from indoors, but it wipes off outside. This is condensation on the outer face of the glass, usually on a still spring or autumn morning. It is a sign your windows are insulating well.
• You cannot wipe it off at all. The misting is sealed inside the gap between the two panes, where you cannot reach it from either side. This one is about the glass unit itself.

Everything below is organised in that order. Skip to the one that matches your windows.

How to stop condensation on the inside of your windows

Every home makes water.

Showering, cooking, washing, drying clothes and simply breathing all put vapour into the air, and it adds up faster than most people imagine. The Glass and Glazing Federation puts two adults asleep for eight hours at roughly a litre of water between them, and cooking on gas at about a litre an hour.

That water has to go somewhere.

If it cannot get out of the house, it collects until it meets the coldest surface in the room, and in most homes that is a window.

So the fix has three parts:

1. let the moisture out
2. make less of it
3. keep your surfaces warmer

Let the moisture out

This is the part that does the most work and costs the least.

Open windows for a while each day, even in winter. Ten or fifteen minutes with a couple of windows on the vent position clears a surprising amount of damp air and barely touches the room temperature, because the air changes over quickly while the walls and furniture stay warm. If your windows have trickle vents in the head of the frame, leave them open. They are there to move moisture out steadily in the background, and taping them over is usually what lies behind a room full of streaming glass.

Use your extractor fans and let them run on for a few minutes after you finish. Keep the bathroom door closed while you shower and for a while afterwards, and the kitchen door closed while you cook. That way the vapour leaves through the fan before it drifts into the cooler bedrooms and settles there.

Make less moisture in the first place

Put lids on your pans. Vent your tumble dryer properly outside, or use a self-condensing model.

Try not to dry washing on radiators or on an airer in a closed room, because all the water still in it has to end up somewhere in the house. If you have no choice, dry it in one room with the door shut and a window or vent open.

Keep the glass and the walls warmer

Condensation forms where surfaces are coldest, and a low background heat through the day gives you fewer cold spots than a short blast in the evening.

Full length curtains and closed blinds trap a pocket of cold air against the glass. On a bad night you may find the worst misting hiding behind them. Pulling furniture a few centimetres away from external walls lets air move behind it, which is where mould appears long before anyone notices.

If it still will not shift

A dehumidifier is worth trying in a room that stays damp despite all of the above, and it is the sensible answer in a flat with limited outside walls or a bedroom that never quite dries out. It deals with the symptom while the ventilation deals with the cause. Keep both going.

Why condensation happens at all

Air holds water as an invisible vapour, and warm air holds far more of it than cold air. When that air touches a surface cold enough, it can no longer hold what it is carrying and the water appears on the surface as droplets.

The temperature at which this starts is called the dew point, and it moves with the humidity in the room.

Glass is usually the coldest surface in a room, which is why it shows first. The same thing is happening on the walls, in the corners of wardrobes and behind the sofa, where nobody sees it until a mark appears. Your windows are giving you an early warning.

Condensation on the outside of the glass is a good sign

External condensation catches people out, and it is nothing to worry about.

Why it happens on good windows

On modern low emissivity glazing, so little heat escapes through the window that the outer pane stays cold. When the night air is damp and still, dew forms on that cold glass exactly as it forms on car roofs and garden furniture.

It is the same effect as frost on a roof. A well insulated loft keeps a roof white until mid morning, while the badly insulated house next door has cleared by nine, because heat is leaking up through it.

Cold outer glass means your heat is staying indoors.

Why one pane mists and the one beside it stays clear

You may notice a clear border around the edge of each pane. That is warmth conducted out through the frame and the spacer bar keeping the perimeter of the glass a fraction warmer, and it is a fair sign the unit is doing its job.

You may also find one pane misted and the one beside it clear, because a tree, a fence or a neighbouring wall has changed how much heat that pane radiated to the sky overnight.

It clears on its own once the sun reaches the glass or a breeze picks up, and it disappears altogether as the weather settles into winter or summer. Specialist coatings that reduce it are available, though for most homes the few hours of morning mist in spring and autumn are not worth designing around.

What misting between the panes is telling you

A double glazed unit is sealed around its edges and holds a desiccant, a drying agent that soaks up the small amount of moisture trapped inside at manufacture. Once that edge seal breaks down, damp air works its way in, the desiccant saturates, and you are left with permanent misting or a milky bloom in the cavity.

This is the one kind of condensation that nothing you do in the room will change.

The water is sealed inside the glass, so ventilation, heating and dehumidifiers all leave it exactly as it was. If your windows are still within their guarantee, your paperwork is the first thing to look at. If you are not sure what you are seeing, send us a photograph and we will tell you.

When condensation turns into damp and mould

Government guidance on damp and mould, published in September 2023 and last updated in April 2026, describes condensation as the most common form of damp in UK homes and sets out the effects that persistent damp and mould can have on the airways.

None of that makes a misty window on a cold morning alarming, though it is a fair reason to wipe the sills, treat any black spots while they are still small, and sort the ventilation out before winter properly sets in.

Will new windows stop condensation?

Partly, and it is worth being straight about the limits.

New windows do not remove moisture from your air.

What they do is raise the temperature of the surfaces in the room, so the inner face of the glass sits well above the dew point on a normal winter morning and the water no longer has an obvious cold surface to collect on. Warm edge spacer bars keep the perimeter of the glass warmer too, and the edge of the pane is where mould usually starts.

Since 15 June 2022, most replacement windows in England have had to include trickle vents under Approved Document F. They are not always popular, and they are there to provide background ventilation and to help avoid condensation and mould growth. Fit new windows, keep the vents shut and carry on drying washing indoors, and the condensation will simply move to the next coldest surface in the room.

Come and talk it through

If you are not sure which kind of condensation you have, give us a ring on (020) 8868 1133 or email office@ruislipwindows.co.uk.

You are also very welcome at our Watford showroom, where you can see the glass and the frames for yourself and ask as many questions as you like. Ruislip Windows has been fitting windows across Middlesex, Hertfordshire and Buckinghamshire since 1986, our aftercare is handled by our own service engineer rather than a subcontractor, and there is no hard sell at any point.

The Glass and Glazing Federation publishes a free consumer guide to condensation, updated in April 2025, which covers the same ground in more detail: GGF Consumer Guide to Condensation.