Dealing with misted double glazing
What has actually failed, why drilling and drying is a false economy, and how to tell whether you need a new unit or a new window.
Misted double glazing is one of the most common glass jobs we do. Here is what is happening inside the unit, why it matters, and what your options are.
What is happening
A double-glazed unit is two panes of glass sealed together with a spacer bar around the edge. The cavity inside is filled with dry air or argon and held there by the seal. When the seal fails, moisture from the outside air gets into the cavity and condenses on the inside of the panes when temperatures drop.
You see it as a milky haze, water droplets between the panes, or in bad cases visible water trails. The window has lost its insulating performance, the gas has leaked out and been replaced by humid air.
Is it definitely a failed unit?
Worth ruling out the alternatives first, because the fix is completely different.
Condensation on the inside face, the surface you can touch from indoors, is not a unit failure. It is humidity in the room meeting cold glass, and it wipes off. The answer is ventilation, extractor fans and heating, not new glass.
Condensation on the outside face is also not a fault. It is dew forming on cold glass on a clear morning, and it is actually a sign the unit is performing well: so little heat is escaping that the outer pane stays at air temperature. It burns off by mid-morning.
Condensation you cannot wipe off from either side is between the panes. That is a failed seal, and that is the one that needs replacing. If you are unsure, try wiping both faces. If the haze persists, it is inside.
Why seals fail
- Age. Most sealed units last 10–25 years. Older units have older sealants.
- Sun exposure. South-facing units fail faster, UV breaks down the sealant.
- Movement. Frame movement (in timber frames especially) puts stress on the seal.
- Drainage. Standing water at the bottom of the unit accelerates seal breakdown.
Blocked drainage is the one most within your control. uPVC and aluminium frames have small weep holes along the bottom of the outer face that let rainwater out of the frame. When those clog with dirt, moss or paint, water sits against the bottom edge of the sealed unit, which is exactly where the seal is. Clearing them takes a couple of minutes and materially extends unit life. It is worth doing on every window once a year.
South and west elevations along the Sussex coast take a lot of UV and a lot of wind-driven salt, and units on those elevations do tend to fail first.
Your options
1. Replace the sealed unit (recommended)
The frame is fine, only the glass unit has failed. We measure the existing unit, order a new one to match, and swap it. Most jobs are a single visit and take 30–60 minutes per unit.
The frame, hinges, handles and locks all stay. You get a new unit with an intact seal, a fresh gas fill and, if you want it, better glass than you had before. Since you are paying for a new unit anyway, it is usually worth specifying low-emissivity glass and an argon fill, which will outperform a 2005 unit noticeably for very little extra.
2. "Defogging" services
Some companies drill a hole, dry the cavity, and reseal. We do not recommend this. The thermal performance is gone, the gas filling has been lost, and the long-term seal is unreliable. It is a cosmetic fix only.
To be specific about why: the drilled vent has to stay open to let moisture out, which means the cavity is now permanently connected to outside air. It can never hold an inert gas fill again, so the U-value stays poor, and humid air continues to enter. It often looks clear for a season and then hazes again. The cost is usually not far below a replacement unit that actually fixes the problem.
3. Replace the whole window
Only worth it if the frame itself is also failing. Otherwise you are paying for the most expensive part of the window again.
Signs the frame has gone as well: timber that is soft or rotten at the bottom rail, uPVC that has become brittle or discoloured beyond cleaning, hinges and locks that no longer align, or draughts you can feel around the frame rather than through the glass. If two or three of those apply across most of the house, replacement starts to make sense. For one hazy pane in an otherwise sound window, it does not.
On a listed or period property, replacement may not be permitted at all. Our secondary glazing comparison covers what to do instead.
How long will a replacement last?
A well-made unit in a well-drained frame should give 15 to 20 years and often more. What shortens it is standing water, poor initial installation with the unit sitting directly on the frame rather than on setting blocks, and heavy shading from film or internal blinds on south-facing glass, which can cause thermal stress.
Units we supply carry a manufacturer guarantee against seal failure. We will confirm the current term in writing on your quote rather than quoting a figure here that may change.
What we need from you
Photos of the affected windows from inside, the frame type if you know it (uPVC, aluminium, timber), and rough sizes. We can usually quote from photos alone for standard sizes.
If several windows have gone at once, mention it. Units installed at the same time tend to fail at the same time, and doing them together in one visit is cheaper than three separate call-outs.
Want a quote? Visit our glazing restoration service page or request a free quote.
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