Condensation Between Window Panes: Why Your Double Glazing Has Misted Up

Is it really between the panes?

Condensation can form in three places on a double-glazed window, and only one of them is a fault:

• On the room side: it wipes off from inside. That's moisture in the room and not enough ventilation, not a broken window.
• On the outside: it wipes off from outside, and it clears as the day warms up. That's a sign the glass is keeping the heat in well.
• Between the panes: it won't wipe off from either side. The Glass and Glazing Federation is clear that condensation won't form inside a sealed unit that's working properly, so mist in the gap means the unit has failed.

Why it happens

A double-glazed unit is two panes of glass held apart by a spacer, with the gap between them filled with air or an inert gas such as argon, and sealed round the edge. While the seal holds, the gap stays dry. Once it fails, the unit lets air in, moisture with it, and the window insulates less well.

Window seals can fail from:

• weather: rot, warping or swelling in very damp conditions or extreme temperatures;
• condensation that builds up on the window over time, where mould can form and damage the seals;
• cleaning the windows with a pressure washer, which can weaken them.

Age plays its part too: double glazing lasts about 20 to 35 years if it's good quality, well fitted and looked after. If a unit has failed within the first couple of years, Which? says it may be down to a mistake in making or fitting it, which is one more reason to go back to the installer.

Can you fix it yourself?

• A hair dryer: no. Warming the glass doesn't mend the seal, so air and moisture keep getting into the gap.
• Cleaning between the panes: you can't get in there without breaking the seal, which is the problem in the first place.
• Resealing it yourself: not on a sealed unit. Which? says seal and glass repairs are jobs for a professional installer.

What you can do yourself is everything around it: check whether it's still under guarantee, find the paperwork, and count how many units have misted (below).

Demisting or a new unit?

Some companies offer to demist a unit: they drill a small hole in the glass, draw the moisture out and plug the hole. It's cheaper than a new unit, but Which? warns that, even when it works, it's likely to be a short-term fix, and the plug often comes loose again in 6 months.

What we'd do: replace the unit. A new sealed unit is the only fix that puts back a properly sealed gap, gas filling and all, and it comes with its own guarantee. We'd only consider demisting for a pane you don't mind redoing later, and only after asking the company what it guarantees and for how long.

Check your guarantee first

Before you pay anyone, find out whether yours are still covered. When Which? asked nine of its Trusted Trader installers what a good warranty looks like, they said glass should be covered for 5 to 10 years, frames for 10, with a 10-year insurance-backed guarantee that still pays out if the installer stops trading.

• Find the paperwork: the guarantee from the company that fitted the windows, and the FENSA certificate (or one from another competent person scheme). FENSA says certificates issued after 6 June 2014 confirm the installer's warranty is insured.
• Lost it? You can search for your FENSA certificate by address and order a copy, for a fee. Your house purchase documents may include it too.
• Contact the installer in writing, even if you ring first, with photos of each misted unit. Which? says to keep as much as possible in writing.

Even outside the guarantee, the Consumer Rights Act 2015 says the materials must be of satisfactory quality and the work done with reasonable care and skill. If the installer won't help, schemes such as FENSA have complaints processes, and the Glass and Glazing Federation offers free conciliation if the company is a member.

What to do, in order

1. Check it's between the panes (wipe both sides) and that it isn't secondary glazing.
2. Check the guarantee and contact the installer if you're covered.
3. Count every misted unit in the house, including any just starting to cloud at the edges. Much of the price of a single unit is the glazier's visit, so having them all done at once costs much less per pane.
4. Get quotes for new sealed units, with the questions below. Prices are on misted double glazing repair costs.

What to ask the glazier

• Low-E glass? It reflects heat back into the room, and the Energy Saving Trust calls it the most efficient kind. A new unit is the chance to upgrade.
• Warm edge spacers? The spacers that hold the panes apart: more efficient windows generally have ones with little or no metal.
• The right glass for the spot: obscure glass to match in a bathroom, and safety glass where the rules require it, such as low down and in or beside doors.
• What's the guarantee on the new unit, and is it insurance-backed?
• Does the price include measuring, fitting and taking the old unit away?

One unit or new windows?

• One or two misted units in sound frames: replace the units. The Energy Saving Trust says you can replace the glass rather than the whole window.
• Units misting all over the house, and the frames showing their age: price new windows too. Draughts, discolouration and windows that are hard to open and close are the Energy Saving Trust's other signs it may be time.
• Under guarantee: neither. Claim it.

For what new double glazing costs and how long it lasts, see double glazing costs.

Frequently asked questions