Why Have I Got Mist Inside My Sealed Unit?

If you can see what looks like condensation, mist, fog or droplets of water trapped between the two panes of glass — and you can’t wipe it away from either side — your sealed unit has “blown.” It’s one of the most common faults we’re asked about, and the good news is it’s almost always the sealed unit at fault rather than the window or door itself.

What Is a Sealed Unit?

A sealed unit (also called an insulated glass unit, or IGU) is the part that does the insulating in a double or triple glazed window. It’s made up of two or more panes of glass held apart by a spacer bar around the edge, with the cavity between them filled with dry air or an inert gas such as argon.

Two things keep that cavity clear. A continuous seal around the perimeter — typically a butyl primary seal backed by a secondary sealant — keeps moisture out and gas in. Inside the spacer bar sits a desiccant, a drying agent that absorbs any tiny amount of moisture left in the cavity, holding the glass crystal clear.

So Why Does It Mist Up?

Misting appears when that perimeter seal is breached. Once the seal lets go, damp outside air starts working its way into the cavity. The desiccant soaks it up at first, but it has a finite capacity. When it becomes saturated it can no longer cope, and moisture begins to condense on the inside faces of the glass — the fog you can see but can’t reach.

At the same time, any argon gas escapes and is replaced by ordinary air, so the unit quietly loses much of its insulating performance even before the misting becomes obvious. A blown unit is therefore both a cosmetic and a thermal problem.

Common Causes of Sealed Unit Failure

Seal failure usually comes down to one of the following — and often a combination of them:

• Age and natural wear – Every sealed unit has a finite life. The perimeter seal slowly hardens and the desiccant gradually fills up, so misting becomes more likely the older a unit gets.
• Poor fabrication – Inconsistent seal application, contaminated glass surfaces or the wrong spacer/desiccant spec at the point of manufacture can cause a unit to fail years earlier than it should.
• Sealant incompatibility – Certain installation sealants and solvents attack the butyl primary seal, breaking it down chemically and letting moisture migrate into the cavity.
• Weather and exposure – Constant UV, driving rain, heat cycling and standing water all accelerate seal breakdown — south-facing and coastal units tend to go first.
• Trapped water – Blocked drainage or a poor fit lets water sit against the bottom edge of the unit, which is the single biggest enemy of a long-lasting seal.

A Closer Look at the Main Culprits

Age. No seal lasts forever. As a unit gets older the butyl hardens, the secondary seal becomes brittle, and the desiccant works through its capacity, so the probability of misting climbs with every year. This is why most manufacturers cap their guarantee at 10 years: it’s a sensible safeguard that covers early-life failures (which point to a fabrication fault) while a unit reaching the back end of its natural life sits outside the warranty. Independent industry guidance generally puts the working life of a good-quality sealed unit at around 20 to 35 years, though exposed or poorly drained units can fail in as little as 10 to 12 years, and sheltered, well-made units can run beyond that range.

Poor fabrication. Quality at the point of manufacture matters enormously. If the glass isn’t perfectly clean when the seal is applied, if the butyl is laid down unevenly or too thin, or if the spacer and desiccant aren’t to spec, the unit carries a built-in weakness from day one. These units are the ones that tend to blow within the first few years — squarely inside the warranty period — which is exactly what a 10-year guarantee is designed to protect the customer against.

Sealant contamination. This one catches a lot of otherwise good units out. The primary seal is usually polyisobutylene (butyl), and it’s chemically vulnerable to certain solvents and to incompatible sealants used during glazing or installation. If the wrong product comes into contact with the edge of the unit, it can degrade the butyl, soften the seal and open a path for moisture — effectively causing a failure that has nothing to do with age or the quality of the glass itself. Using glazing tapes and sealants that are compatible with IGU edge seals is essential to avoid this.

Weather, exposure and trapped water. Sunlight, heat cycling and standing water all speed up seal breakdown. South-facing and coastal units take the most punishment, and any window where drainage is blocked or the unit sits in water will fail sooner. Good installation — with the unit correctly seated, packed and drained — makes a real difference to how long the seal survives.

Can a Misted Unit Be Repaired?

The frame is almost always fine — it’s the sealed glass unit that needs replacing. Because the glass itself doesn’t wear out, swapping the failed unit for a new one restores both the clarity and the insulation at a fraction of the cost of a whole new window. You can also see our SevenDay How To Guide on changing a sealed units Hyperlink to ensure you undertake the work to give a lasting solution.

How SevenDay Can Help

As a supply-only specialist, SevenDay manufactures replacement sealed units to your exact sizes and specification across 22 trade counter depots. Whether you’re a trade customer ordering in volume or a homeowner just replacing a single blown unit, your local depot can advise on the right glass spec, spacer and gas fill for the job. Bring your sizes to your nearest depot and the team will sort the rest.