Condensation between the panes: replacing a failed double-glazed unit

If there is mist or droplets between the two panes of a double-glazed window, the unit's edge seal has failed and moist air has got into the cavity. It can't be wiped or dried from inside. The usual fix is to replace the sealed unit in the existing frame: NZ glaziers publish $220–600 per unit (Window Doctor; Handyman Wellington, Sept 2026), against $1,500–2,500+ for a whole new window (Wellington Glass).

How do I know the condensation is between the panes?

Wipe both sides of the glass. If the fog stays, it is inside the unit. A failed unit often looks worst on cold mornings and in the sun, can leave a white haze or tide marks once the moisture dries, and doesn't clear when the room is aired. Condensation you can wipe off the room side is a ventilation and moisture problem, which our bedroom condensation guide deals with.

Why do double-glazed units fail?

A sealed unit is two panes held apart by a spacer, with a desiccant in the spacer to soak up the little moisture sealed in at the factory. When the edge seal breaks down, outside air carries moisture in faster than the desiccant can take it, and it condenses on the inside faces of the glass. Common causes are age, a frame that holds water against the bottom edge of the unit, blocked drainage in aluminium joinery, and units that were glazed without proper support.

Once it has failed, the cavity is no longer still, dry air, so the unit insulates less well as well as looking bad. Window Doctor's guidance makes the same point: a failed seal makes the window foggy and less efficient.

What does replacing a failed unit cost?

For a typical domestic window, published New Zealand prices for a new sealed unit fitted into the existing frame sit between about $220 and $600 per unit. Size moves it most, then glass type: a large low-E or toughened unit costs more than a small clear one. Access is the next variable, because a second-storey window may need scaffolding, which CheckMyBuilder puts at $1,000–5,000 for a home of two storeys or more.

If one unit has failed, ask the glazier to look at the others. Units of the same age from the same supplier often go in clusters, and replacing several on one visit usually costs less per unit than several separate call-outs.

Should I replace the unit or the whole window?

• Replace the unit when the frame is sound: timber that isn't rotten, aluminium that still slides, drains and seals. This is the cheapest fix by a wide margin.
• Fix the cause too if water is sitting in the bottom of the frame or the drainage holes are blocked, or the new unit will go the same way.
• Replace the window when the frame itself has failed, or when you want a thermally broken frame. That's a full window replacement, at $1,500–2,500+ per opening.
• Re-spec while you're there. A replacement unit doesn't have to match the old one. Low-E glass costs about 5–10% more (Wellington Glass) and EECA says it can improve performance by up to 30%.

Is it covered by a warranty?

Possibly. Sealed units are usually sold with a manufacturer's warranty on the seal, and some glaziers offer 10 years (Window Doctor publishes a 10-year seal warranty). Look for the manufacturer's mark printed on the spacer bar inside the unit, find the original invoice if you have it, and ask the supplier before you pay for a replacement. A warranty is not a lifespan, though: plenty of units outlast theirs.

Can a fogged unit be repaired without replacing it?

Some companies drill the unit, dry it and reseal the holes. That can clear the fog, but the cavity is no longer the sealed, dry gas space the unit was made with, so it won't insulate the way a new unit does. If what you want is warmth as well as a clear view, a new sealed unit is the fix that matches what you paid for.

What to do next

1. Wipe both sides and confirm the fog is between the panes.
2. Count the failed units and note each window's rough size and frame type.
3. Check for a warranty: invoice, spacer-bar markings, the original installer.
4. Look at the bottom of the frame for standing water or blocked drainage.
5. Get it measured, so the price is for your units rather than a published range. Our calculator prices a whole house if you're thinking of doing more than the failed units.