Why Is My Double Glazing Misty, and Can It Be Fixed?

If your window looks cloudy and no amount of cleaning makes a difference, the problem is almost certainly inside the glass. A double-glazed window is two panes held apart by a spacer and sealed around the edge. When that seal fails, damp air creeps into the gap and condenses on the inside faces, where no cloth can reach it.

It’s one of the most common faults on older double glazing, and the good news is that it rarely means a whole new window.

How to tell if it’s a failed seal

Run a dry finger over the misty patch, on both the room side and the outside. If the mist stays exactly where it was, it’s between the panes. You may also notice:

• Mist that comes and goes with the weather and the sun
• White, chalky marks or streaks on the inside faces of the glass
• A dark or damp look along the edge of the unit
• Small droplets sitting between the panes on a cold morning

If the moisture wipes away, you’re looking at ordinary condensation instead, which has different causes. We cover that in what condensation on your windows is telling you.

Why seals fail

Sealed units don’t last forever. The glass and frame expand and contract every time the temperature changes, and years of that slowly tire the seal. Water sitting in the bottom of the frame speeds things up, which is why blocked drainage holes can shorten a unit’s life. South-facing windows take the biggest temperature swings, so they often go first.

Does it matter?

A misted unit won’t fall out, and it isn’t dangerous. But the view gets steadily worse, the staining inside can’t be cleaned off, and the unit has lost the sealed gap it was built around, so it won’t insulate as well as it did.

The usual fix: a new sealed unit

In most cases, only the glass unit needs replacing. The frame, hinges and handles stay where they are. The glazing beads come off, the old unit comes out, and a new one made to the exact size goes in. It’s far less disruptive than replacing the window, and there’s no making good around the frame.

It’s also a chance to improve what’s there. A new unit can use better-insulating glass, or acoustic glass if road noise bothers you.

When a new window makes more sense

Replacing just the unit is the wrong answer if the frame itself has had it. Look for rot in timber, frames that have warped so the window no longer closes square, or uPVC that has cracked at the corners. If several units in a very old window have failed and the frames are poor as well, it’s worth comparing the cost of a full replacement.

Check your paperwork first

If your windows are fairly new, look for the original guarantee before paying for anything. Sealed units are often covered for a number of years, and a failed seal may be included.

Not sure what you’ve got? Call us on 0800 1956 955 or send us the details and we’ll tell you whether it’s a unit swap or something bigger.

Photo: Nicole López, CC BY-SA 4.0, via Wikimedia Commons.