Misted double glazing

Misted double glazing means the seal around the edge of the glass unit has failed, so damp air has got into the gap between the panes. You can’t wipe it away, and it won’t clear on its own. However, you rarely need a new window: a glazier can swap the sealed unit inside your existing frame, which typically costs £90 per unit.

How to tell misted double glazing from ordinary condensation

Condensation on the room side of the glass is normal in a damp room, and it wipes off. Condensation on the outside pane on a cold, clear morning is a good sign, because it means the window is keeping the heat in. Misting between the panes is different. It sits where no cloth can reach, it often leaves a milky film or streaks once it dries, and it comes back with every change in the weather.

If you’re not sure which you have, wipe both faces of the glass. Whatever is left is inside the unit.

Why the seal fails

A sealed unit is two panes held apart by a spacer bar, with a desiccant inside the spacer to soak up any moisture. Over the years, sunshine and temperature swings work the edge seal loose. Once the desiccant is saturated, moisture shows as mist. As a result, units of fifteen years or more are the usual suspects, though a poorly made or badly glazed unit can fail much sooner.

Some things make failure more likely:

• South and west-facing windows, which heat up and cool down the most.
• Water sitting in the frame, because blocked drainage holes leave the bottom edge of the unit wet.
• Unbeaded or poorly packed units, where the glass presses unevenly on the seal.

Misted double glazing: repair or replace the unit

For a sound frame, a new sealed unit is the proper fix. The glazier measures the old unit, orders a new one to the same size and specification, and fits it on a second visit. You keep the frame, handles and colour. See replacement glass units for the full price breakdown, from small fanlights to large picture panes.

Demisting services drill tiny holes in the glass, dry the cavity and seal the holes. That can clear the view for a while. However, the gas fill and the insulation don’t come back, so it is a cosmetic fix rather than a repair.

When the whole window is the better buy

A new unit only makes sense if the window around it is worth keeping. Replace the window instead if:

• the frame is warped, cracked or discoloured;
• the hinges and locks are worn out, or the sash has dropped;
• several units in the same window have failed;
• the window is single glazed elsewhere, or so old that its energy rating is poor.

A full uPVC window costs several times as much as a single unit, so weigh it up window by window. Our guide to repairing or replacing windows walks through the decision.

Check the guarantee before you pay

Many installers guarantee their sealed units for ten years, and an insurance-backed guarantee can carry on even if the firm has closed. Look through the paperwork from the original installation first. If a FENSA or CERTASS registered installer fitted the window, the FENSA certificate will name the firm.

What to ask for

When you get prices for misted double glazing, ask for the size and glass of each unit, whether the unit needs toughened glass (doors and low panes do), the warm-edge spacer, and the guarantee. Then use our quote checker to see whether the figure is reasonable for your area.