What Causes Misted Double Glazing?

Misted double glazing means the seal around the edge of the sealed unit has broken down and moist air has worked its way into the gap between the two panes. The haze you can see is condensation forming on the inner faces of the glass, where nothing can reach it. It cannot be cleaned off, it will not clear on its own, and it does not improve in dry weather.

The more useful news is that a misted unit rarely means a whole new window. Across Epsom, Cobham and Walton-on-Thames we've found the frame is very often in perfectly sound order and only the glass needs to come out. People tend to assume the whole thing is finished, when what has actually failed is one component inside it that can usually be replaced on its own for a fraction of the cost.

What is actually inside a double glazed unit?

Rather more than most people picture, and understanding it explains everything about the failure.

Two panes of glass are held apart by a hollow spacer bar running around the perimeter. That spacer is filled with a desiccant, usually silica granules, whose job is to absorb the small amount of moisture sealed in during manufacture and keep the cavity permanently dry. The edge is then sealed twice. A primary seal does the real work of keeping air and gas where they belong, and a secondary sealant bonds the assembly together and gives the unit its structural strength.

If the cavity is filled with argon rather than plain air, that gas is held in by those same seals. So is the performance of the low-emissivity coating on the inner face, which only does its job in a dry, sealed cavity. Break the seal and you lose all three at once.

Why does the seal fail?

Age is the honest headline, but it is very rarely the whole story.

• Thermal cycling wears the sealant out. The unit expands every time the sun lands on it and contracts again every night. After enough cycles the sealant loses its flexibility and a hairline path opens up.
• Ultraviolet light degrades the edge sealant wherever it is exposed. South and west facing elevations reliably fail before north facing ones on the very same house, and that is the clearest pattern we see.
• Standing water at the bottom of the frame is the cause nobody expects. Every outer frame has drainage slots designed to let rainwater escape. When those slots block with dirt, moss or paint, the unit sits in a shallow puddle and the bottom edge seal is permanently wet. This is the most common cause of early failure we get called out to, and it is entirely preventable with a brush and five minutes.
• Poor glazing at installation shortens the life considerably. A unit bedded without proper setting blocks carries its own weight on the edge seal rather than on the frame, and a unit packed too tightly has nowhere to expand when it warms.
• Manufacturing faults reveal themselves early. Insufficient desiccant or an incomplete factory seal usually shows within the first year or two rather than after a decade.

Once air is getting in, the desiccant absorbs it for a while and nothing visible happens. The misting appears at the point where the silica is completely saturated and can hold no more, which is why the fault seems to arrive overnight when it has in fact been developing quietly for months.

Can a misted unit be cleaned, drilled or repaired?

No, and this is worth being blunt about, because there are services that suggest otherwise.

Drilling a hole and injecting a drying agent, or fitting a small vent into the unit, will often clear the visible haze for a while. What it cannot do is restore the seal. It cannot put the argon back, and it cannot return the cavity to a dry sealed state. What you are left with is a vented unit that breathes damp outside air in and out with every temperature change, so the insulation performance is permanently gone and the misting commonly returns. There is usually a visible drill mark left in the glass as well.

We've found the honest framing helps people decide quickly. The haze is a symptom. The failure is the seal, and a seal cannot be repaired from the outside.

Does a misted unit still insulate?

Partly, but nothing like it should, and the loss is considerably bigger than the appearance suggests.

The cavity is now open to outside air, so any argon fill has long since gone. Moisture sitting in the cavity conducts heat far more readily than dry gas does. And the low-emissivity coating, which does much of the thermal work in a modern unit, degrades once it is sitting in a damp environment rather than a sealed one. A misted unit performs closer to old-fashioned double glazing than to the specification it was originally sold as.

There is a knock-on effect worth knowing about. A colder inner pane means condensation forming on the room side of the glass as well, so a failed unit frequently arrives with a second and apparently separate damp complaint attached to it.

Do you have to replace the whole window?

Usually not, and this is the part genuinely worth getting an opinion on before spending anything.

If the frame is sound, the hinges and locks still work, the drainage is clear and the beading will come out cleanly, then replacing the sealed unit on its own is a straightforward job. The new unit is measured, made to size and glazed into the existing frame.

In our experience, replacing the sealed unit works better than replacing the whole window where the frame is genuinely sound, because the frame is the expensive part and a new unit lets you upgrade the glass specification at the same time. On a 1930s semi in Epsom where two or three units on a south elevation have gone while the rest of the house is untouched, that is far better value than a full replacement, and it buys the frames several more years of useful life.

Where that logic stops working is when the frame itself has become the problem.

• Units failing across several different elevations usually means the whole set is of an age, and replacing them one at a time turns into a rolling cost with no end to it.
• Frames that have distorted or dropped will not hold a new unit square, so the replacement tends to fail early too and the money is wasted twice.
• Timber frames that are soft at the bottom rails cannot be reglazed reliably, because there is nothing sound left to bed the new unit against.
• Obsolete systems where the beading or gasket profile is no longer manufactured can make reglazing impractical even when the frame looks perfectly acceptable from the outside.

It is worth weighing all of that against how long a modern frame should last, because putting money into reglazing a set of frames already near the end of their life is a false economy that only postpones the decision.

Is it covered by a guarantee?

Often, and it should be the first thing you check before paying anybody.

Sealed units are typically guaranteed separately from the frame and for a shorter period, so a window carrying a long frame guarantee may have a much shorter one on the glass itself. If the original installer is still trading and the installation was registered at the time, that is where to start. Where the work was carried out under a competent person scheme, the registration paperwork will confirm who installed it and when.

How do you stop it happening again?

Some of it is down to the unit and outside your control. A useful amount of it is not.

• Keep the drainage slots clear. They sit along the bottom of the outer frame and they block gradually rather than suddenly. Clearing them takes minutes and it directly addresses the most common early failure we see.
• Do not seal the gap under the beading or paint over the drainage. We have been called to units sitting in water because a well-meant decorating job closed the only route out.
• Ask what the unit guarantee actually is at the point of quoting rather than afterwards, and get the period for the glass stated separately from the period for the frame.
• On a very sunny elevation, ask about the edge seal specification. South and west facing glass works harder, and it is a reasonable question to put to any installer.

So, what should you do about misted double glazing?

Work out whether the frame is worth keeping, because that is the only decision that really matters. The misting itself is not repairable, and no amount of cleaning or drilling will genuinely fix it, so the unit is coming out either way. What you are actually choosing is whether the unit alone comes out or the whole window goes with it.

If it is one or two units on a sunny elevation and the frames are sound, reglazing is very often the sensible answer and we will say so. If units are failing across the house, or the frames have moved, or they are timber and going soft at the bottom, replacement is likely to be better value across the next decade.

Our team has over 30 years of combined experience with both jobs, and we are perfectly happy to tell you when reglazing is the better call. If you would like someone to look at yours and say which it is, get in touch.