White uPVC casement window in a bright room

Misted double glazing means the seal on your sealed glass unit has failed, so moist air now sits between the panes. However, you can usually keep the frame, because replacing the unit costs only around £55 to £145.

First, you cannot wipe the fog away. It sits between the two sheets of glass, so it will not clear for good until you repair or replace the unit.

Bathroom uPVC window, a common place to find misted double glazing

What causes double glazing to mist up?

A failed seal lets moisture in and lets the insulating gas escape. As a result, the glass fogs up. Several things cause this to happen:

  • Poor original installation. For example, a fitter who does not support the unit correctly can cause an early failure.
  • Wear and tear, because sealant simply ages over the years.
  • Frame movement, since a warping, shrinking or rotting frame puts stress on the unit.
  • Damage, because knocks or cracks to the glass or frame can break the seal.
  • Harsh cleaning products, as abrasive or solvent-based cleaners attack the sealant.
  • Repeated temperature changes, which weaken the seal as the unit expands and contracts.

Is it condensation inside, outside or between the panes?

First, check where the fog appears. Then use this table to decide what to do.

Where the fog appears What it means Fix
Between the panes Failed seal Replace the sealed unit
On the inside surface Too much moisture in the room Ventilate, use extractor fans, reduce humidity
On the outside surface Normal on cold, clear nights Usually no action, because it shows the glass is insulating well

How much does it cost to fix misted double glazing?

Prices depend mainly on the size of the window. Also, a reseal costs less than a new unit, but it may not last as long.

Option Typical UK cost
Replace a sealed unit (small window) £55–£75
Replace a sealed unit (large window) £115–£145
Reseal or defog a window £50–£100
Replace all units in a 3-bed semi £1,400–£2,500
Full uPVC casement window replacement (for comparison) £350–£650

Source: MyJobQuote, based on real UK quotes (updated September 2026). For more detail on whole-window prices, read our double glazing cost guide.

Replace the glass or replace the whole window?

  • Replace the unit only if the frame is straight, sound and under about 15 to 20 years old. This is the cheapest permanent fix.
  • Replace the whole window instead if the frame is warped, rotting or draughty, or if the hardware is failing. Then you get a fresh frame, new seals and a new guarantee.
  • Replace several units at once if multiple windows mist up. In addition, this often works out cheaper per unit and shows whether your frames are near the end of their life.

If you are unsure how old your frames really are, read how long uPVC windows last.

Should you fix misted windows?

Yes, in most cases. Misted glass loses its insulation, so the room feels colder and your heating bills can rise. Also, it looks poor and can put buyers off. Finally, a misted unit never gets better on its own.

Get your misted windows sorted

Faron Home Improvements is FENSA-registered, and we replace sealed units and fit new windows across Sussex, Surrey and Kent. So call us on 01342 643075 or request a free quote, and we will tell you whether new units or new windows make more sense.

Misted double glazing FAQs

Why is there condensation between my double glazing panes?

The sealed unit has failed. Moist air has got in and the insulating gas has leaked out. Therefore, you cannot clean the fog off from the inside or the outside.

Can you fix misted double glazing without replacing it?

Sometimes. A defogging or reseal service typically costs £50 to £100 per window. However, it may not last as long as a new sealed unit, so most installers recommend replacing the unit.

How much does it cost to replace a sealed unit?

It typically costs £55 to £145 per unit, depending on size. Small windows sit at the lower end, while large windows sit at the higher end.

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

Not always. If the frame is straight, sound and works well, a new sealed unit is usually enough. However, if the frame is warped or rotting, a new window gives better value in the long run.

Does my guarantee cover misted double glazing?

Often, yes, if the windows are still within their guarantee period. So check your paperwork first, because many double glazing guarantees cover seal failure for a set number of years.

Will misted windows get worse?

Usually, yes. The fogging tends to spread as more moisture enters, and the unit’s insulation keeps declining.

Can I ignore misted windows?

They are not a safety risk. However, they lose insulation and look poor, so it is worth fixing them rather than leaving them.

How long does it take to replace a sealed unit?

A fitter can usually replace a sealed unit in under an hour per window. However, the unit itself is made to measure, so allow a week or two for it to arrive.

Can misted double glazing cause damp or mould?

The fog sits between the panes, so it does not cause damp by itself. Instead, mould usually comes from condensation on the inside surface, which poor ventilation causes.

Do I need a FENSA certificate to replace a sealed unit?

Usually not, because you are replacing only the glass and not the whole window. However, a full window replacement does need one, so ask your installer.

How do I stop my windows misting up again?

First, choose a quality unit and a FENSA-registered fitter. Then clean the frames with mild soap, and avoid harsh chemicals. Also, check your seals once a year.

Related guides and local services

Read more about windows and glazing from Faron Home Improvements:

Last updated: October 2026. Prices are UK averages including VAT and are a guide only.