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 structural failure that can raise heat loss by around 50 to 70%, 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.
| Where it forms | What it means | What to do |
|---|---|---|
| Inside (room side) | High indoor humidity; warm moist air meeting cold glass | Ventilate, use extractor fans, reduce moisture at source |
| Outside face | The window is working well; outer pane is cold because little heat escapes | Nothing needed; it clears as the day warms |
| Between the panes | The sealed unit has blown; edge seal failed and desiccant saturated | Replace the sealed glass unit (frame can usually be kept) |
Source: web/lib/research/condensation.md
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 calls I get about condensation are inside condensation. The homeowner is convinced the window is faulty because they have just had it fitted. I explain that 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.
Tom Bradley, FENSA-registered installer
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.
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 10 to 25 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, and the window's insulating capacity has fallen sharply. A failed sealed unit can raise that window's heat loss by around 50 to 70% compared with an intact unit. That is a meaningful penalty on your heating bills and a reason not to leave a blown unit in place.
The repair picture is clear: a blown sealed unit cannot be reliably fixed by injecting fresh desiccant or resealing the edges. The correct repair is to replace the sealed glass unit entirely. The good news is that the frame, in most cases, is perfectly serviceable. A glazier can extract the old unit from the frame and fit a new one, which costs considerably less than a full window replacement. Replacing a misted single pane costs roughly £55 to £145; a whole-window unit costs £100 to £850 depending on size, style and material. For borderline cases where the frame is also ageing, our comparison of repair versus replacement sets out the decision framework clearly.
Whether to repair or replace also depends on how long you expect to stay in the property and how the window fits into the overall lifespan of your double glazing. If the frames are already 20-plus years old, it may make more sense to budget for new windows than to keep repairing units in ageing frames.
People often ask me if we can inject something into the gap to fix a blown unit. There are services that offer this, but in my experience the fix is temporary at best. The seal has failed; the structural cause is still there. You will be calling someone again in a year or two. Fit a new unit, keep the frame if it is sound, and the job is done properly.
Tom Bradley, FENSA-registered installer
How to reduce condensation
For inside condensation, which is the type that responds to action on the homeowner's part, the following measures make the biggest practical difference.
- Open trickle vents. Modern window frames include small slot vents near the top of the frame specifically for background ventilation. Many homeowners close them in winter to feel warmer. Leaving them open provides a steady trickle of fresh, drier air that displaces moist indoor air without causing a draught.
- Run extractor fans properly. Kitchen and bathroom extractors should run during cooking and showering and for at least ten minutes afterwards to clear the moisture already in the air. Timers help if you tend to switch them off too early.
- Ventilate briefly after high-moisture activities. Opening a window for five to ten minutes after a shower or a long cooking session clears moist air far more effectively than a slow background trickle.
- Dry clothes outside or in a vented dryer. Clothes drying indoors on radiators or airers is one of the largest single sources of indoor moisture. Where possible, dry outside or use a vented tumble dryer with the duct run outside.
- Keep pan lids on. A simple habit that cuts steam release in the kitchen significantly.
- Use a dehumidifier. A portable dehumidifier in a bedroom or living room with persistent condensation can extract several litres of water a day in a damp home. It is not a substitute for proper ventilation, but it buys time while underlying habits change.
- Wipe sills dry. Water sitting on window sills evaporates back into the room and also encourages mould. Wipe them dry each morning as a routine.
For outside condensation, nothing is needed: it is self-correcting and is not a problem. For blown sealed units, the only effective measure is replacement of the unit.
Frequently asked questions
Inside condensation forms when warm, moist indoor air meets the cold glass surface. Everyday activities like cooking, showering and drying clothes indoors all add significant moisture to the air. It is a humidity and ventilation problem, not a fault with the window itself. Better ventilation and reducing moisture at source will stop it.
No, outside condensation is actually a good sign. It shows that very little heat is escaping through the window, so the outer pane stays cold enough for outdoor dew to settle on it. It clears naturally as the day warms and the sun rises. There is nothing to fix.
A foggy or misty patch between the panes that you cannot wipe off means the sealed unit has failed. The edge seal has let in moisture, the desiccant in the spacer bar is saturated, and the unit is effectively blown. It cannot be reliably repaired and the sealed glass unit needs replacing, though the frame can usually be kept.
Replacing a misted single sealed unit costs roughly £55 to £145 for a standard pane. A full window replacement, if the frame has also deteriorated, runs from £100 to £850 depending on size, style and material. Use the repair cost calculator to get a figure for your specific window.
Sealed units typically last 10 to 25 years, depending on the quality of the edge seal, the installer's workmanship and how exposed the window is. The frame usually outlives the glass unit by several years, which is why it is normally worth replacing just the glass rather than the whole window when a unit blows.
A failed sealed unit can raise that window's heat loss by around 50 to 70% compared with an intact unit. Once the argon fill has escaped and moisture has entered, the window's insulating performance drops sharply. Replacing the unit promptly limits both heat loss and the risk of further damage to the frame.