What causes condensation between double glazing?

Condensation between double glazing is caused by a failed seal in the glass unit. When the airtight seal around the edge of a sealed double glazed unit breaks down, moist air gets into the gap between the two panes, and when that trapped air cools it forms condensation on the inside of the glass where you cannot wipe it away. If you can see misting or fogging between the panes rather than on the room-facing surface, the sealed unit has failed and the glass needs replacing.

This is different from condensation on the inside or outside of your windows, which is normal and usually harmless. Condensation trapped between the panes is the one that signals a genuine fault. Below we explain exactly why sealed units fail, how to tell the difference between the types of condensation you might see, and what can and cannot be done about each, because the right response depends entirely on where the moisture is forming.

Why the seal fails in the first place

A double glazed unit is two panes of glass held apart by a spacer bar, sealed all the way around the edge, with dry air or insulating gas in the gap between them. That seal is what keeps moisture out. When it works, the gap stays perfectly dry and clear. When it fails, the unit is no longer airtight, humid air works its way in, and condensation appears inside the sealed cavity.

Seals fail for a few reasons, and usually it is a combination rather than a single cause. Age is the main one. The seal is a bonded edge that slowly degrades over many years of use, and no sealed unit lasts forever. Constant temperature change speeds this up, because the glass and the seal expand and contract every time the window heats in the sun and cools overnight, and that repeated movement gradually works the seal loose.

Standing water is another major factor. If water is allowed to pool along the bottom of the frame, it sits against the edge of the sealed unit and attacks the seal over time. This is why blocked drainage channels are such a common hidden cause. Piper Windows has been fitting and repairing double glazed units across Lincolnshire since 1983, and in our experience the units that fail earliest are very often the ones where water has been sitting in a frame with blocked drainage, rather than the glazing simply reaching the end of its natural life.

Why coastal homes see it sooner

Where you live affects how quickly seals fail, and this is where local knowledge counts. A large part of our work is along the Lincolnshire coast around Skegness and Winthorpe, and coastal conditions are harder on sealed units than sheltered inland ones.

The constant temperature swings, the exposure and the relentless weather all put extra stress on the seals, and wind-driven rain tests every part of the frame and its drainage. From working with clients on this coast for over 40 years, we have seen plenty of units on exposed plots mist up sooner than the same glazing would inland. It is not that coastal air causes condensation directly, but that the harsher conditions wear seals and overwhelm poor drainage faster, bringing forward the day the unit fails.

The three types of condensation, and what each one means

It genuinely matters where the condensation is forming, because each type means something different.

Condensation between the panes, inside the sealed unit, is the fault. You cannot wipe it away because it is sealed inside the glass. This always means the unit has failed and the glass needs replacing. Nothing you do around the house will clear it, because the problem is the broken seal, not the air in your room.

Condensation on the inside of the glass, the surface facing your room, is the most common and is not a window fault. It happens when warm, moist indoor air meets the cooler glass, most often on cold mornings, in kitchens and bathrooms, or in bedrooms overnight. It points to humidity in the home rather than a failed window. Better ventilation, using extractor fans, opening trickle vents and reducing moisture at source usually keeps it under control. That said, if you are getting heavy internal condensation on newer windows, it can sometimes be a sign the glazing is underperforming, which is worth weighing up if you are already asking whether higher-rated glazing is worth the investment.

Condensation on the outside of the glass, the external surface, actually indicates the opposite of a problem. It tends to appear on cold, clear mornings on well-insulated modern windows, precisely because the efficient glazing is keeping so much heat inside that the outer pane stays cold enough for dew to form on it. It clears as the day warms up and is nothing to worry about.

We have found that a lot of homeowners assume any condensation means their windows have failed, when in most cases the misting they are seeing is on the room side and is about household humidity rather than the glazing. Knowing which surface the moisture is on saves a great deal of unnecessary worry, and occasionally unnecessary expense.

Can condensation between double glazing be fixed?

Once a sealed unit has misted internally, the honest answer is that the unit needs replacing. You may see suggestions online about drilling and drying out a blown unit, but in our experience this works better on paper than in practice, because it does not restore the airtight seal or the insulating gas, so the unit keeps failing and its energy performance stays compromised. Replacing the sealed unit is the proper fix.

The good news is that this is very often a straightforward and relatively affordable repair rather than a full window replacement. In many cases the frame and hardware are still perfectly sound, and only the glass unit itself needs swapping. Our repairs service covers misted and broken double glazed units on our own and other companies’ installations, so a blown unit does not automatically mean a whole new window.

In our experience, replacing the affected sealed unit works better than replacing the entire window when the frame is still in good condition, because you resolve the fault and restore the insulation at a fraction of the cost and disruption. Where the frames themselves are old, draughty or failing too, that is the point at which full replacement starts to make more sense, and we will always tell you honestly which situation you are in rather than push the bigger job.

It is also worth knowing that replacement glazing must comply with building regulations, and a registered installer certifies and registers the work for you. We are a FENSA Approved Installer, so the work is documented and compliant.

The bottom line

Condensation between double glazing means one thing, a failed sealed unit letting moist air into the gap between the panes, and it is usually driven by age, constant temperature change and standing water from poor drainage, with coastal exposure bringing it on sooner. If the misting is on the room side or the outside of the glass instead, that is about humidity or efficient glazing rather than a fault. The fix for a genuinely blown unit is to replace the glass, which is often a simple repair with the frame left in place.

If you have windows misting up between the panes and want to know whether you need a simple reglaze or something more, the best step is to have them looked at properly. You can get a free no-obligation quote, or call us directly on 01754 898 998. We have spent more than 40 years helping Lincolnshire homeowners sort condensation problems, and we will always tell you straight whether it is a repair or a replacement you actually need.