Condensation Inside Double Glazing: Causes and Fixes

If you can see mist or water droplets between the two panes of a double glazed window, the seal on that glass unit has failed and it can’t be wiped away. The usual fix is to replace just the sealed glass unit, not the whole window. Condensation on the inside face of the glass, on the other hand, is a ventilation and humidity issue. This guide explains how to tell which you have and what to do about it.

Where is the condensation?

Where you see it	What it means	The fix
Between the two panes	The unit’s seal has failed and moist air has got in	Replace the sealed glass unit
On the inside surface (room side)	Humid air inside the home is meeting cold glass	Improve ventilation and reduce moisture
On the outside surface	Cool, clear nights; often a sign of very efficient glass	Nothing; it clears as the day warms

A simple test: wipe the glass on both sides. If the mist is still there, it is inside the unit.

Why double glazed units fail

A double glazed unit is two panes separated by a spacer bar and sealed around the edge. Over time the seal can break down, letting in moist air. Common causes are:

• Age; seals gradually break down over many years
• Water sitting in the frame because of blocked drainage or poor glazing
• Movement in the frame, especially in older or poorly fitted windows
• Harsh cleaning products or pressure washing around the seals
• Manufacturing faults in the unit

Once the seal fails, the unit insulates less well and the misting usually gets worse.

Fix 1: replace the sealed unit

In most cases, only the glass unit needs replacing. A glazier or window company measures the unit, has a new one made and fits it into the existing frame, usually in a short visit. This costs far less than a new window and gives you full insulation again. While the window is being reglazed, consider an upgrade such as low-E or acoustic glass.

On timber sash windows, the unit is held in by beads or putty, and an experienced joiner should do the work to avoid damaging the sash. Our double glazed sash windows guide covers the glass options.

Fix 2: replace the whole window

Replacing the window makes more sense if:

• Several units have failed and the frames are also worn or rotten
• The frame is causing the failures, for example by trapping water
• You want to change the style, material or performance

Compare the options in sash window repair or replacement.

What about “demisting” services?

Some firms offer to drill, clean and reseal misted units. Results vary and the unit’s insulating gas is not restored, so replacing the unit is usually the more reliable fix.

Reducing condensation on the inside of windows

If the condensation is on the room side of the glass, the problem is moisture in the home:

• Open trickle vents or windows briefly each day
• Use extractor fans when cooking and showering
• Dry washing outside or in a ventilated room
• Keep the home evenly heated
• Wipe windows dry in the morning to protect sills and paint

Old single glazed sash windows suffer from this most. Secondary glazing or double glazing your existing sashes can reduce it a lot.

Is it covered by a guarantee?

Many sealed units come with a guarantee from the installer or manufacturer. Check your paperwork or ask the installer before paying for a replacement.

Find a local company to replace misted double glazed units and compare quotes. For other repair prices, see our sash window repair costs price list.

Frequently asked questions

Why is there condensation between the panes of my double glazing?

The seal around the sealed unit has failed, allowing moist air to get between the panes. It cannot be wiped away and usually gets worse.

Can you fix condensation between double glazed panes?

Yes. The usual fix is to replace the sealed glass unit, keeping the existing frame. Demisting services exist, but replacement is more reliable.

Do I need a new window if my double glazing is misted?

Usually not. Only the glass unit needs replacing, unless the frame is also damaged or rotten.

Is condensation on the inside of windows normal?

Some condensation on cold mornings is normal, especially on single glazing. Heavy or constant condensation means the home needs better ventilation or warmer glass.