Insulated Glass Units
What it is
An insulated glass unit is two or three panes sealed around a spacer with a dry gas cavity between them. When that seal fails, moisture gets into the cavity and fogs the inside faces — which no cleaner can reach, from either side. It is the single most common thing a window cleaner gets wrongly blamed for.
Almost every house in this region built or re-glazed in the last forty years has insulated glass, and almost nobody thinks about it until one unit goes cloudy and stays cloudy.
This page is mostly about that moment, because it is where the largest gap sits between what a homeowner expects and what is physically possible.
What an insulated glass unit actually is
A sealed unit is two panes of glass — sometimes three — held apart by a spacer bar around the perimeter and bonded to it with a sealant, forming a cavity.
That cavity is not simply air. It is filled with dry air or an inert gas such as argon, and the spacer bar itself is hollow and packed with a desiccant, which absorbs whatever trace moisture is present at manufacture. The insulating performance comes from the gas and from the coatings on the inner faces, not from the glass, which is a poor insulator on its own.
Everything about the unit depends on the perimeter seal holding. It is doing two jobs: keeping the fill gas in, and keeping water vapour out.
How a seal fails, and what you see
Seals fail from fatigue. The unit heats and cools every day, the gas inside expands and contracts, and the sealant flexes with it — thousands of cycles a year, for decades. Eventually it develops a path.
Once there is a path, moist air enters, the desiccant absorbs it until it is saturated, and after that every cold night condenses moisture onto the inner faces of the glass.
The progression is fairly consistent. First, intermittent fogging visible only in particular conditions — a cold morning, a sunny afternoon after rain — that clears by itself. Then fogging that persists longer and appears more often. Then a permanent haze. Finally, mineral deposit: the condensation runs, dries, runs again and leaves etched tidemarks and a white bloom low on the pane, which is the stage where the unit is beyond looking acceptable even on a dry day.
Two things make units fail sooner here. Standing water — a sill that holds rain because it is blocked, badly sloped, or has lost its drainage path — keeps water against the seal permanently. And heat cycling on south and west elevations, which is why those go first on most houses.
Why nobody can clean it
The haze is on surfaces two and three — the inner faces — inside a sealed assembly. From the room you are looking through pane one. From outside you are looking through pane four. There is no access to the cavity that does not involve breaching the unit.
There are services that drill the unit, flush it and fit vents. It removes the visible fog and it also converts a sealed insulating unit into a vented one, so the insulating performance is gone and the glass is now open to outside air. It is a cosmetic intervention on a dead unit, not a repair, and it is worth understanding as that before paying for it.
The ordinary answer is replacement of the sealed unit alone, which is much cheaper than replacing the window, and which a glazier handles from a measurement.
How to tell a dead unit from a dirty window
Useful, because the two get confused constantly and the diagnosis decides who you should be calling.
Wipe both faces. If the haze is unchanged from either side, it is not on a surface you can reach.
Watch it across a day. A failed unit changes with temperature and humidity — worse on a cold morning, better by afternoon, sometimes gone entirely in dry weather. Dirt is indifferent to the weather.
Look at the bottom edge. Tidemarks, a white mineral bloom, or a visible line where water has sat is condensation history. That is conclusive.
Compare with its neighbours. One hazy pane in a run of clear ones on the same elevation is a unit that has failed, not a cleaning that was missed.
The rest of the glass in the house
The other types show up on the same building and each behaves differently.
Annealed glass — older single panes, soft and forgiving to clean, usually in wooden sashes that need care rather than force.
Tempered glass — heat-treated for strength and required by code in specific locations: doors, glass beside doors, tub and shower enclosures, large low panes, stair glazing. Look for the small etched stamp in a corner. It is stronger than annealed and, counterintuitively, needs more care during cleaning, because the tempering process can leave fine particles fused to the surface that a blade or a hard pad will drag across the pane.
Low-E coated glass — a microscopically thin metallic coating, generally on an inner surface of the sealed unit, that reflects heat. It gives the pane a faint sheen or a slight colour cast that people routinely mistake for a film that will not come off. It is not a film and it is not removable; on the rare units where the coating is on an exposed face, it is also delicate enough that abrasives are out of the question.
Laminated glass — two panes bonded to an interlayer, used in skylights, overhead glazing and for security and sound. It stays in one piece when broken, and a delaminating edge — a cloudy fringe creeping in from the perimeter — is its own failure mode, distinct from a failed IGU seal.
Condensation that is not a seal failure
Three different kinds of condensation turn up on a double-glazed window, and only one of them means the unit is finished. Getting this wrong costs people money on replacement units that were working properly.
Inside the cavity — between the panes, unreachable, unchanged by wiping. The seal has failed. This page.
On the room side of the inner pane — wipes off, comes back. This is a house problem, not a window problem: indoor humidity meeting the coldest surface in the room. It is extremely common here through the wet months, worst in bedrooms and bathrooms, and worst of all on single glazing and older aluminium frames with no thermal break. The window is doing exactly what it should; it is simply the place where the moisture in the air becomes visible first.
On the outside face of the outer pane — a dew film on a clear cold morning that burns off by mid-morning. This one surprises people, and it is the most counterintuitive fact about modern glazing: it means the unit is working well. A high-performance unit keeps so little heat in the outer pane that the glass drops to dew point like any other outdoor surface. Exterior dew on new windows is a sign of good insulation, not a defect.
Worth running through before anyone is called, because two of the three have nothing to do with the glazing.
At a glance
| Property | Insulated glass unit |
|---|---|
| Construction | Two or three panes, perimeter spacer, sealed cavity |
| What is in the cavity | Dry air or argon, with desiccant in the hollow spacer |
| What the seal does | Keeps fill gas in and water vapour out |
| Why seals fail | Daily thermal cycling, accelerated by standing water at the sill |
| Which fail first | South and west elevations, and any sill that holds water |
| Can it be cleaned | No — the haze is on the inner faces, inside the assembly |
| The remedy | Replace the sealed unit alone, not the whole window |
Telling a failed unit from a dirty window:
- Wipe both faces — unchanged haze means it is not on a surface you can reach
- Watch it across a day — a failed unit changes with temperature, dirt does not
- Look at the bottom edge for tidemarks or a white mineral bloom
- Compare with neighbouring panes on the same elevation
- Check whether an internal grid is hiding early fogging at the edges
What we do at the quote
We flag failed units on the walk-round, before the work rather than after, and photograph them if you want a record for a glazier or a warranty claim. Many sealed units carry a manufacturer warranty measured in decades, and a surprising number of people never find out theirs is still live because nobody told them the unit had failed.
We do not sell glass, so there is nothing in it for us either way — which is precisely why the read is worth having.
Method, access and scheduling is on the window cleaning page.
Questions about this surface
- There is fog inside my window. Can you clean it?
- No, and neither can anyone else. The haze is on the inner faces of the two panes, inside a sealed cavity. There is no access from the room side or the outside without breaking the unit. Any service claiming to clean between the panes is either drilling and venting the unit — which ends its insulating function — or is describing something else.
- How do I know the seal has failed rather than the glass being dirty?
- Three tells. The haze does not change when you wipe either face. It varies with the weather — worse on a cold morning, sometimes gone by afternoon. And it often has a tidemark or a mineral edge low on the pane, which is condensation that has run and dried repeatedly. Dirt does none of those things.
- Is a failed unit an emergency?
- No. It is an appearance and efficiency problem, not a safety or water-intrusion one. The pane is still glazed and still weathertight against the outside. What it has lost is its insulating performance and its looks, and it can sit for years if you are content to look at it.
- Can just the glass be replaced, or the whole window?
- Usually just the sealed unit, which is far cheaper than a new window. A glazier measures the unit and the sash takes a replacement. It stops being straightforward with older, obsolete or unusually shaped frames, and with some clad and composite sashes.
- Why does one window fail and the ones beside it do not?
- Heat and water are the two drivers. A unit in full afternoon sun cycles through a much greater temperature range every day, which works the seal harder. A unit sitting in standing water — a blocked or badly sloped sill, a failed drainage path in the frame — has moisture against the seal permanently. South and west elevations and any sill that holds water go first.
- Does the fake muntin grid inside affect anything?
- Only your ability to see what is going on. Internal grids sit within the cavity, which means they cannot be cleaned either and they collect a little of whatever forms in there. They also hide early fogging at the edges, which is where it usually starts.
- Is there any point cleaning a window with a failed unit?
- Often yes, because the two outer faces are still yours to keep clean and they are most of what you touch and look through. We will just say plainly which panes are going to look better afterwards and which one is not, so it is not a surprise at the walk-round.
Last updated