Condensation Inside Double Glazing: What It Means and What to Do About It

Reviewed by Apex Windows · Last updated June 2026

TL;DR: Condensation inside double glazing shows up in three places, and only one is a fault. If it’s on the room-facing pane, you’ve got a ventilation issue rather than a window problem. The outer pane misting up means your windows are insulating well. But moisture trapped between the panes is different: the seal has failed, and only the glass unit needs replacing.

You’ve probably walked past it for weeks before you noticed: a hazy patch on one of your windows that won’t wipe away properly, or maybe it does, and comes straight back the next cold morning. Condensation inside double glazing tends to go unnoticed until you’re getting the house ready to sell, and you start worrying a surveyor will read it as damp.

Not all condensation on double glazing means something’s wrong. Where it’s forming tells you exactly what’s going on, and whether you need to do anything at all.

Where Is Your Condensation? Three Locations, Three Different Meanings

Misting on the room side points to humidity and poor ventilation indoors. On the cold outer pane, misting usually means your windows are insulating well. The one worth worrying about sits trapped between the two panes inside the sealed unit, where it means the seal itself has failed.

Condensation on the Inside Pane (Room Side)

This is the condensation most people run into, especially in kitchens and bathrooms where cooking and showers pump moisture into the air. FENSA attributes it to the water vapour in the air, indoor temperature, and outdoor temperature. Once warm, moist air meets glass that’s colder than its dew point, the temperature at which water vapour turns back into liquid, it condenses straight onto the surface.

Condensation on the Outside Pane

This one looks the most alarming the first time you spot it, usually on a cold, clear morning. It happens because modern double glazing insulates so well that the outer pane sits close to the outdoor air temperature, rather than being warmed by heat escaping from inside your home. Older, less efficient windows rarely show this, so outer-pane misting is often a sign your glazing is doing its job.

Condensation Between the Panes

This is the one that actually means something’s gone wrong. Double glazed glass sits inside a sealed unit, sometimes called an IGU, where two panes are joined by a spacer bar packed with desiccant, a drying agent that keeps the gap between them free of moisture. Once the seal around that spacer bar fails, damp air gets in and the desiccant eventually saturates, leaving moisture with nowhere to go.

If you’re not sure which type you’re dealing with, wipe both sides of the glass. If the haze clears from the inside or the outside, it’s a ventilation issue. If it won’t shift from either side, the moisture is between the panes and you’ll need a glazier to sort it.

What Causes Condensation Between Double Glazing Panes?

Condensation between double glazing panes means the seal around the sealed unit has broken down, letting moist air into the gap and saturating the desiccant that’s meant to keep it dry. Once that moisture is inside, wiping the glass or running a dehumidifier in the room won’t help. The seal has already failed, so nothing on the room side can reach it.

Seals fail for a few different reasons. The most common is simple ageing: the sealant around the spacer bar dries out and cracks after years of expanding and contracting as temperatures shift through the seasons. Poor installation can speed this up, particularly if a unit was fitted slightly out of square or the frame wasn’t properly sealed against the weather.

Double glazed windows typically last 20 to 35 years if they’re good quality, well installed, and properly maintained (Energy Saving Trust, accessed 2026). If your casement windows or other double glazing are creeping toward that age and you’ve started noticing misting between the panes, the seal failing isn’t surprising; it’s roughly when you’d expect it.

For more on how seals are built and why they eventually give out, our guide to broken window seals goes into the mechanics in more depth.

Can You Fix Condensation Between Double Glazing Yourself?

You can’t fix condensation between double glazing panes yourself. Once the seal has failed and moisture is trapped inside the unit, there’s no DIY method that removes it or restores the window’s insulation. That said, some confident people do successfully replace the glass unit itself, as long as their windows are internally beaded, meaning the bead holding the glass in place sits on the room side rather than outside.

Two fixes get suggested a lot online, and neither one works:

• Drilling a small hole and pointing a hairdryer at it. The mist might clear for a few days, but the seal is still broken, so it comes back.
• Flushing the gap with a drying agent. This treats the symptom on the surface, not the failed seal causing it underneath.

Replacing the glass unit itself is a different job entirely, separate from trying to fix the moisture in place, and it’s one plenty of competent people manage on internally beaded windows:

1. Measure the gap between the bead and the frame.
2. Allow for the spacer bar’s thickness when ordering.
3. Order a new sealed unit from a local glazing manufacturer.
4. Fit the new unit into the existing frame.

If your windows are externally beaded instead, the glass can be popped out from outside the house. That’s exactly why some people choose a professional even for a job that’s otherwise straightforward, because of the access point it leaves on the outside of their home.

How Much Does It Cost, and Should You Check Your Warranty First?

Before you pay anyone, check whether your windows are still covered by their original guarantee. Many reputable installers, including Apex through our 10-year insurance-backed guarantee, cover failed seals free of charge within that period, and an insurance-backed guarantee keeps that protection in place even if the original installer goes out of business.

If you’re outside the warranty, here’s what each route costs.

Cost to Repair a Single Blown Unit

Replacing a single pane costs around £55 to £145. If the whole window unit needs replacing, expect £150 to £600 depending on size, from £150 for a small window up to £600 for a large one (Checkatrade, 2024). This route costs less when the rest of the window, the frame, hinges and locking mechanism, is still in good shape.

Cost to Replace the Whole Window

A full window replacement through Apex starts at around £819 per window, all-inclusive. It costs more upfront, but it makes sense once several units in the same window have failed, or the frame itself is showing wear alongside the seal. For a full breakdown across different window types and house sizes, our double glazing costs guide covers it in more depth. You can also get an instant online quote to see real numbers for your own property.

What Happens If You Leave It?

Leaving a failed double glazing unit isn’t an emergency, but it costs you money over time. Pre-2002 double glazing is around 50% less energy efficient than current-standard windows (GGF & British Glass, 2023), and a failed seal drags a unit back toward that older level of performance. The longer it’s left, the more it shows up on your heating bills.

There’s a soundproofing cost too. The gas-filled gap between the panes is part of what blocks outside noise, and once a seal fails, that gap stops doing its job properly. You’ll often notice traffic or neighbours sounding closer than they used to.

Left long enough, the moisture trapped inside can leave permanent mineral staining on the glass once it eventually dries out. At that point, cleaning won’t restore it. Replacing the unit is the only way to fix the view as well as the insulation.

Conclusion

Only one of the three spots means real trouble. Misting on either outer surface is normal, sometimes even a sign your windows are doing their job. Moisture trapped between the panes is the exception: that’s a failed seal, and it needs a glass unit replacement.

If that’s what you’re dealing with, get it looked at by someone who’ll check the rest of the window too, not just quote you for the obvious failed unit. A loose hinge or worn frame often gets missed when the focus is only on the glass.

If you’re weighing up repair against full replacement, the instant online calculator gives you a real figure for your own windows in under a minute, no obligation either way. Or if you’d rather talk it through first, speak to our team.

Frequently Asked Questions

Does Putting the Heating On Reduce Condensation?

Heating the room raises the glass surface temperature, which helps stop inside-pane condensation from forming in the first place. Condensation between the panes sits in a sealed gap the warm air never reaches, so it stays exactly as it was regardless of how warm the room gets.

Should I Worry About Condensation on the Inside of Double Glazing?

Some inside-pane condensation in a humid room like a kitchen or bathroom isn’t unusual and isn’t a sign of a faulty window. If it’s constant, heavy, or appearing in rooms with no obvious moisture source, improve ventilation before assuming anything’s wrong.

Does Condensation on Windows Mean the House Is Too Cold?

Not necessarily. It usually means a mismatch between indoor humidity and the surface temperature of the glass, which can happen in a perfectly warm home if a room is poorly ventilated, such as after cooking or drying clothes indoors.

Will Condensation Between the Panes Put Buyers Off If I’m Selling?

It can, mainly because buyers and surveyors sometimes read it as damp rather than a simple failed seal. Replacing the affected unit before viewings clears up both the look of the window and any confusion in a survey.