Misted Windows in Double Glazing

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Finesse Windows Guide

Misted Double Glazing:
Repair or Replace?

A plain, honest guide for Birmingham homeowners with foggy or blown sealed units.

Birmingham & West Midlands 4 minute read Which? Trusted Trader

You can't wipe it away. That is the giveaway with a misted unit. The glass looks foggy, or there are little droplets of water sitting inside it, and when you clean both sides the haze is still there an hour later. That is because the moisture is trapped between the two panes, where no cloth can reach. If that is what you are looking at, you have a blown sealed unit, and the question is a simple one: do you repair it or replace it? Here is the honest answer.

The problem

What "misted" or "blown" really means

A double glazed window is two panes of glass with a sealed gap between them, usually filled with argon gas and held apart by a spacer bar around the edge. That sealed gap is what keeps the heat in.

When the seal around the edge fails, the gas slowly escapes and ordinary damp air works its way in. Warm days followed by cold nights make that trapped air condense against the glass, and you get the misting and fogging people call a blown unit.

The window still keeps the weather out. It just is not insulating the way it should any more, and it looks tired.

Inside a sealed unit: once the edge seal fails, moisture is trapped where no cloth can reach.

The decision

Repair or replace: how to decide

There are two routes, and they are not equal. Repairing tends to mean one of two very different jobs. Some firms drill the unit, draw the moisture out and reseal it. That is the "defogging" you will see advertised. Others mean taking out the failed glass and fitting a brand-new sealed unit into your existing frame. Only one of those lasts.

Temporary fix

Defogging

The unit is drilled, the moisture drawn out, and the hole resealed. It clears the view for a while, but the seal that failed is still failed. The misting usually comes back, sometimes within months.

We do not offer it. We would rather not take your money for a fix we would expect you to call us back about.

The proper repair

Replacing the sealed unit

Your frame stays put, the old blown glass comes out, and a new sealed unit goes in. The window insulates again and the misting is gone for good.

A lasting fix, at a fraction of the cost of a whole new window.

Add before publish: a real Birmingham job

Placeholder: drop in a recent job where blown units were swapped and the customer's existing frames kept, ideally with a before/after photo. Something like "We replaced three blown units on a bay in [area] last winter and left the frames exactly as they were" does more than any sales copy. Photo alt text: "Misted double glazed sealed unit before and after replacement, Birmingham".

The money question

What affects the cost

We can't put a price on this page, because no two jobs are the same. What we can do is tell you what moves the figure up or down, so nothing on the quote comes as a surprise.

Size of the glass

A small landing window and a large bay are very different amounts of glass.

How many units have gone

Sealed units often fail together, because they were fitted at the same time and have aged the same way. It is common to find two or three on the same side of the house.

The type of glass

Toughened, laminated, obscure or patterned glass, Georgian bars and decorative leadwork all cost more than plain glass.

Access

A ground-floor window is straightforward. Glass high up, over a conservatory roof or above a stairwell takes longer and sometimes needs extra kit.

A survey is the only way to give you a real number, and ours is free.

Being straight with you

New unit or new window?

Most of the time, a blown unit does not mean you need a new window. If your frames are sound, they open, close and lock properly, and the seals around the frame are intact, then replacing just the glass unit is the sensible, cheaper job. That is the answer most homeowners are glad to hear.

A whole new window makes more sense when the frame itself is the problem. If it is an older window that is draughty, the hinges or locks are failing, the uPVC has discoloured badly, or you are getting damp and mould around the frame as well as misting in the glass, then you would be repairing one part of a window that is near the end anyway. New glass in a worn-out frame is false economy.

If a company quotes you for full replacement windows when all that has gone is the glass, get a second opinion.

And if someone offers to defog a unit on a frame that is clearly on its way out, that is the wrong job too. The right call depends on the frame, not just the glass.

The quick check

Should I repair or replace?

Replace the sealed unit and keep the frame if:

The frame opens, closes and locks properly
Misting in the glass is the only real problem
The window is otherwise in good shape

Replace the whole window if:

The frame is draughty, sticking or hard to lock
The uPVC is cracked, warped or badly discoloured
You are getting damp or mould around the frame, not just misting in the glass
The window is old and other repairs are likely to follow soon

Not sure which camp you are in? That is exactly what a survey is for.

Common questions

Misted double glazing FAQs

Can misted double glazing be repaired?

Yes. The proper repair is to replace the failed sealed glass unit while keeping your existing frame. So-called defogging, where the unit is drilled and dried out, clears the view for a while but the broken seal remains, so the misting usually returns.

Should I repair or replace misted double glazing?

If the frame is sound and opens, closes and locks properly, replacing just the sealed glass unit is the cheaper, sensible choice. Replace the whole window when the frame is draughty, sticking, badly discoloured, or you have damp around the frame as well as misting in the glass.

Does defogging misted double glazing work?

Defogging can clear the misting temporarily, but the seal that failed is still failed, so the fogging often comes back within months. Replacing the sealed unit is the lasting fix.

Do I need a whole new window if the glass is misted?

Usually not. A blown unit normally means only the glass needs replacing, not the frame. A new window is only worth it when the frame itself is worn, draughty or failing.

Get it checked

Foggy glass somewhere in the house?

The first step is a proper look, so you know whether it is a quick unit swap or something bigger. Book a free, no-pressure window survey across Birmingham and the surrounding area. We will tell you straight: repair the glass, replace the window, or leave it alone if it does not need doing yet.

Book a free survey 0121 451 3724