Misted Double Glazed Windows: Causes and Fixes

Double glazing works by sealing two panes of glass together with an insulating gap between them, usually filled with argon gas. As long as that seal holds, the unit keeps the heat in and moisture out. When it fails, moisture gets into the gap and the glass starts to mist up from the inside, and no amount of wiping will clear it.

Here's how to tell whether your windows have a failed seal, what causes it, and what your options are for fixing misted double glazing.

Not all condensation is a problem

Condensation can form in three places, and only one of them means your window has failed:

• On the inside of the glass (the room side). This is caused by moisture in the air, from cooking, showers, drying clothes and breathing, meeting cold glass. It's most common in kitchens, bathrooms and bedrooms on cold mornings. It means the room needs more ventilation, not that the window is faulty. Opening trickle vents, using extractor fans and airing rooms will usually help.
• On the outside of the glass. Condensation on the outside, especially on cold, clear mornings, is actually a good sign. It happens because modern energy-efficient glass keeps so much heat inside that the outer pane stays cold. It clears as the day warms up.
• Between the panes. Misting or fogging inside the sealed unit, which you can't wipe away from either side, means the seal has failed. This is the one that needs fixing.

Signs of a failed sealed unit

• Misting, fogging or droplets between the panes that come and go with the weather.
• A cloudy haze inside the glass that no longer clears, which usually means moisture has been getting in for some time.
• Water collecting at the bottom of the gap between the panes.
• The area near the window feeling colder than it used to, because the insulating gas has escaped.

Draughts around the edges of a window are a different problem. They're usually caused by worn or damaged weather seals around the frame or opening sash, or by hinges and locks that need adjusting, rather than by the glass unit itself.

What causes double glazing seals to fail?

• Age. Sealed units don't last forever. The unit warms up and cools down every day and every season, and over the years that movement can weaken the seal.
• Water sitting in the frame. Windows have drainage channels to let rainwater escape. If these get blocked, water can sit against the edge of the unit and break down the seal over time.
• Poor installation. Units need to be properly supported and packed within the frame. If they're not, the seal can be put under strain.
• Damage. Knocks, harsh cleaning chemicals and pressure washers can all damage the seal.
• Quality. Cheaper units with lower-grade sealants and spacers tend to fail sooner.

Keeping the drainage slots clear and cleaning your windows with mild soapy water will help your units last longer.

Can misted double glazing be fixed?

Once moisture has got into a sealed unit, the seal can't be reliably repaired. Some companies offer a demisting service that drills small holes in the glass to dry out the unit, but it doesn't restore the insulating gas or fix the failed seal, so it's a short-term fix at best.

The reliable options are:

• Replacing just the sealed unit. If your frames are in good condition, a new glass unit can be made to measure and fitted into the existing frame. It's usually the quickest and most cost-effective fix, and a modern unit with low-E glass and a warm-edge spacer may insulate better than the one it replaces.
• Replacing the whole window. If the frames are old or damaged, or several units are failing, a new window is often better value in the long run. You'll get the latest energy efficiency and security features, and a new guarantee.

If your windows are still under guarantee, check with the original installer first, as failed sealed units are often covered.

Should you fix it straight away?

A misted unit won't cause any harm overnight, but it will get worse. The window won't insulate as well as it should, and the view through it will get cloudier over time. It's worth getting it looked at before the colder months.

Got misted windows?

We can replace misted glass units in your existing frames, or supply and fit new windows if that's the better option, and we'll give you honest advice on which makes more sense.

Not sure what state your windows are in? We also offer a whole-house window and door check, whoever fitted them. We'll spot failing units and other problems early, so you can deal with them before they get worse.

Bennbrook Windows is a FENSA-registered installer based in Harlow, working across Essex, Hertfordshire and North and East London. Contact us for a free, no-obligation quote, or call us on 01279 964009.

Thinking about replacing your windows altogether? Read our guide to choosing new windows or browse our windows range.