Fog between double-glazed panes: replace the unit, not the window

Double-glazed windows are two panes of glass with a sealed gap between them. When that gap fogs up, misty, streaky or milky and impossible to wipe, the unit has failed. The good news is that the frame usually hasn't, and you rarely need a new window.

Where's the fog? Check the side first

Before anything else, work out which surface the fog is on:

• Room side: you can wipe it off with a cloth. That's ordinary condensation from moist indoor air: cooking, showers, drying clothes, people breathing. Better ventilation and wiping in the morning help.
• Outside: dew on the outer pane on a cold, clear morning. It clears as the sun hits it, and it's harmless.
• Between the panes: you can't reach it from either side. That's a failed seal, and it's what this guide is about.

1. 0–4 s Room-side condensation sits on the surface facing the room.
2. 4–8 s Outside condensation sits on the exterior surface, not in the cavity.
3. 8–12 s Moisture inside the sealed cavity cannot be wiped from either accessible surface.
4. 12–16 s The complete sealed unit is replaced; a suitable existing frame remains.

Why double-glazed units fog

Around the edge of every double-glazed unit is a spacer and a seal. They keep the gap between the panes dry, and the spacer holds a drying agent that soaks up any moisture trapped when the unit was made. Over the years, sun, heat, movement and water sitting on the seal break it down. Moist air gets in, the drying agent fills up, and the moisture condenses on the inside of the glass whenever it cools.

Once the unit is fogging, it has also lost much of what you paid for. The gap no longer holds dry air, so the window insulates less well.

Why drilling it is a stopgap

You'll find services that drill small holes in the glass, dry the gap and plug the holes. It can clear the view for a while. But the original seal is still broken, so moist air keeps getting back in, and the fog tends to return. Drilling can also void any remaining warranty on the unit.

The fix: a new sealed unit in the same frame

If the frame is sound, the fix is a new double-glazed unit made to the exact size of the old one:

1. We measure the unit and note the glass type: thickness, any toughened or laminated glass, any Low-E coating.
2. The new unit is made to size.
3. We take out the fogged unit and fit the new one in the same frame, with new glazing seals or beads where needed.

1. 

   1

   Step 1 of 3

   Confirm where the fog is

   Fog on the room side or the outside surface can be wiped away. Fog trapped between the panes cannot, and means the seal has failed.

2. 

   2

   Step 2 of 3

   Replace the sealed unit

   The whole double-glazed unit is replaced, not just one pane.

3. 

   3

   Step 3 of 3

   Same frame, new unit

   If the frame is in good condition it stays, and the new unit goes into the same frame.

It's a good moment to upgrade the glass. A laminated pane cuts noise and holds together if broken, and a Low-E coating keeps more heat in. Our guide to laminated vs toughened glass explains the difference.

What it costs

Each unit is priced once it's measured, because the cost depends on its size and glass. Replacing a fogged unit typically costs about $600 to $2,000 per window in Melbourne. We quote in writing before any work starts.

Repair the unit or replace the window?

A new unit is the better call when the frame is in good order and the window works properly. Think about a new window when the frame is corroded, the window no longer opens or locks, or several units have failed at once and you want to upgrade the lot. If you're weighing up that bigger job, our double glazing cost guide covers retrofit and new windows.

Check the warranty first

Many double-glazed units carry a warranty against seal failure from the maker or the installer. Find the paperwork before you pay for a replacement, and don't let anyone drill the unit while it's covered.

Book a replacement unit

We replace fogged and broken double-glazed units in aluminium and timber frames across our Melbourne service areas. Send a photo of the window and a rough size if you can. See our window and door repairs or ask for a glass replacement quote.