Condensation on or Between Double Glazing: Why, and How to Remove It

Condensation on a double-glazed window tells you something very specific depending on exactly where it sits: inside the room, on the outer face, or trapped between the panes. The three situations have entirely different causes and entirely different responses. Getting the diagnosis right matters because one of them is a failed sealed unit that will never clear on its own, while another is a sign your windows are performing exactly as they should. If you already suspect a blown unit and want to know whether it is worth repairing or replacing, the repair cost calculator will give you a ballpark figure before you call anyone.

The three kinds of condensation

Most householders see condensation as one problem, but it is three separate phenomena that happen to look similar. Where exactly the droplets or misting sits is the only diagnostic test you need. Run your finger across the glass: if you can wipe it off, it is on a surface and therefore either inside condensation (room side) or outside condensation (outer face). If there is nothing to wipe, the problem is trapped between the panes, and no amount of wiping will clear it.

Source: DGCC 2026 research, based on UK glazing industry technical guidance.

Condensation on the inside: a humidity problem

Inside condensation is by far the most common complaint, and the most misdiagnosed. Homeowners often call an installer expecting to hear the window is faulty. In almost every case it is not. The window is simply doing what any cold surface does when warm, moisture-laden air meets it: the moisture drops out of the air as liquid water.

The sources of indoor moisture are easily overlooked because they are so ordinary. A pan of boiling water releases a significant amount of steam; a shower fills a bathroom with humid air in minutes; drying clothes indoors adds the full weight of water in the clothes to the indoor air over the course of a day. Even breathing contributes. In a well-sealed modern home, all of that moisture has nowhere to go unless ventilation moves it out.

The practical fixes are straightforward but require consistency. Open windows briefly after cooking and showering to flush moist air out. Use trickle vents (the small slot ventilators fitted in most modern window frames) rather than keeping them closed for warmth. Run kitchen and bathroom extractor fans during and for a few minutes after activity. Keep pan lids on while cooking. Dry clothes outside or in a vented tumble dryer. Wipe window sills dry in the morning rather than letting water sit and encourage mould. A portable dehumidifier in a bedroom where you notice persistent condensation is a practical short-term measure while you adjust habits.

The only time inside condensation genuinely points to a window problem is if a newer, better-insulated window produces more condensation than the old single-glazed unit it replaced. That is counter-intuitive but logical: the new window is so much better at keeping heat in that the room retains more warmth and therefore more moisture, which then settles on whatever the coldest surface is. The fix is still ventilation, not another new window.

Nine out of ten condensation complaints are inside condensation, and the homeowner is convinced the window is faulty because it has just been fitted. The window is doing its job. The problem is always the same: not enough ventilation. Open the trickle vents, run the extractor fan, stop drying clothes on the radiator. The condensation goes away.

Double Glazing Cost Calculator Team

Condensation on the outside: a good sign

Outside condensation surprises many people. They see misted glass first thing in the morning and assume something is wrong. In fact it is the opposite. Outside condensation forms when the outer pane of the window drops below the outdoor dew point, meaning the glass is cool enough for moisture in the outside air to settle on it. The reason the outer pane is cold is that very little heat is passing through the window from the warm room inside. Good thermal performance keeps the outer pane cold, just as it keeps your home warm.

This kind of condensation clears on its own as the morning sun warms the glass. It tends to be most noticeable on the best-performing windows in the house, which is why it often appears after new windows are fitted and rarely appeared on the old single glazing (because single glazing transmitted so much heat that the outer pane was never cold enough for dew to form). There is nothing to fix. If anything, a window that regularly shows outside condensation on cold mornings is confirming it has a well-functioning sealed unit.

Condensation between the panes: a blown unit

The third kind of condensation is the one that actually requires action. A foggy, milky or streaky appearance that you cannot wipe away means the problem is inside the sealed unit itself. The edge seal around the perimeter of the double-glazed unit has failed, outdoor air has entered the cavity, and the desiccant crystals packed into the spacer bar around the edge have absorbed all the moisture they can hold. Once the desiccant is saturated, any further moisture in the cavity condenses on the inner faces of the glass, producing the misting you see. That failure only makes sense once you know how a double glazed sealed unit works: the cavity is sealed and gas-filled by design, so anything entering it is a fault rather than wear.

Why does the seal fail? The most common causes are age, poor original installation, water pooling in the frame around the unit, and thermal cycling (the repeated expansion and contraction of the glass and frame over the seasons). Sealed units typically last 15 to 20 years. Units at the lower end of that range were often installed with cheaper spacer bars or thinner edge seals; the longer-lived units tend to use warm-edge technology and quality primary and secondary sealants.

Once a unit has blown, the argon gas fill (if it was present) has escaped, taking roughly a third of the cavity's insulating benefit with it, and any soft-coat Low-E layer is now sitting in water. It cannot be dried out or resealed; the fix is a new sealed glass unit in the existing frame, at roughly £55 to £145 for a standard pane. Whether the drilling and drying services are worth paying for, what a unit costs at each pane size, and how the job actually goes are covered in our guide to blown double glazing. For borderline cases where the frame is also ageing, the comparison of repair versus replacement sets out the decision rule, and the lifespan of your double glazing by material tells you how much life the frame has left.

Condensation on windows in winter

If your windows are dry from April to October and streaming from November, nothing has gone wrong with them. Air holds less water the colder it gets, and the dew point is the temperature at which it can hold no more and the vapour turns back to liquid on whatever surface it touches. In a heated room the glass is almost always the coldest surface, so it is the first thing to reach that temperature on a cold night, and the water in the air lands on it. Government guidance calls this condensation damp and describes it as the most common form of damp in UK homes.

Winter makes every part of that worse at once. The glass is colder, so the dew point is reached sooner. The windows are shut, so the moisture stays in. And the moisture load goes up: washing dries on radiators, showers run hotter and longer, pans steam with the extractor off. The Energy Saving Trust puts a typical household's output at up to 100 litres of moisture a week from cooking, bathing, drying clothes and breathing, and council housing teams quote roughly 12 to 17 litres a day for a family. In a well-sealed home with the trickle vents shut, all of it is looking for a cold surface.

That is also why new windows can bring condensation with them. Old leaky frames were ventilating the room whether you wanted them to or not. Tight new frames stop that, the moisture that used to escape through the gaps now stays in, and it settles on the glass. The fix is ventilation, not another set of windows; the trickle vents fitted to every replacement window under the Building Regulations exist for exactly this.

How to remove condensation from double glazing

For condensation on the room side, the kind you can wipe, the routine below is what the Energy Saving Trust and the government's damp and mould guidance recommend, and it forms the core of good window aftercare. It is habits, not products, and most of it is free.

• Wipe the glass and the sill each morning. A squeegee or a towel, then wring it into the sink rather than letting the water evaporate back into the room. Water left on the sill is what grows the black mould in the corners. If mould has already appeared, wipe it only while it is wet, with an antifungal cleaner and a disposable cloth; brushing it dry spreads the spores.
• Open the trickle vents and leave them open. Closing them to feel warmer is the commonest winter mistake. They are designed to move moist air out without a draught you can feel.
• Run extractor fans for 15 minutes after you finish. During the shower or the cooking and for at least 15 minutes afterwards, which is how long it takes to clear the moisture already in the air. A fan wired to the light switch with an over-run does this without anyone remembering. No fan? Open the window instead, for the same 15 minutes.
• Open windows for five minutes, several times a day. A short wide opening swaps the air in a room far faster than a long narrow one, and loses much less heat than it feels like it should.
• Keep doors closed while you shower or cook, so the moisture stays in the room with the extractor, and leave a 10 mm gap under internal doors so air can move between rooms the rest of the time.
• Dry clothes outside or in a vented dryer. Indoor drying is the single largest controllable source. If it has to be indoors, one room, door shut, window open or dehumidifier running.
• Pan lids on, furniture off the wall. Lids cut the steam at source. A gap between wardrobes or sofas and an external wall lets air reach the cold surface instead of letting moisture settle behind the furniture unseen.
• Use a dehumidifier where the habits are not enough. Which? puts the running cost of a good one at around 7p an hour at the October 2026 price cap rate, with the best at 3p and the worst at 17p. Rated capacities run from 7 to 40 litres a day, but that is a maximum at high humidity, not what it will collect in a bedroom. Keep the doors and windows shut while it runs and give it space to draw air.
• Do not leave the heating on all day to beat it. A cold, unheated room is prone to condensation, but the Energy Saving Trust is clear that heating a house constantly to avoid damp is not a sensible answer. Ventilate the cold room and heat it when it is in use.

For outside condensation, nothing is needed: it clears as the morning warms and is not a problem. For condensation between the panes, none of this applies; the only effective measure is a new sealed unit, and our blown double glazing guide covers that job from the price of the unit to whether you need a certificate.

Price-checkedLast reviewed 18 September 2026

Reviewed byDouble Glazing Cost Calculator TeamCross-checked against Checkatrade, the FMB & GreenMatch