Condensation between the glass panes: how to check, and what it means

How to tell that condensation between glass panes is inside the unit

Condensation between glass panes is the one kind you cannot reach, and that is the test. Wipe the inside face with a dry cloth. Go outside and wipe the outside face. If both faces come up dry and the mist is still there, the water is inside the sealed cavity and the edge seal of that unit has failed. Nothing else puts water in that position.

Two further checks take a minute each, and they tell you what the glass is as well as whether it has gone.

The torch test

In a dark room, hold a torch or a phone light at a shallow angle to the glass and look at the line of reflections. A double glazed unit gives four reflections, one from each glass surface. If one of those four is a slightly different colour, usually a little pink or green, there is a low-emissivity coating on that face, which tells you the unit is reasonably modern and worth replacing like for like. Two reflections only means a single sheet, so you have secondary or single glazing and a different problem.

The fingernail test

Run a fingernail across the mark. Scratches, paint flecks, cement splashes and limescale run-off from a leaking gutter all sit on an outer surface and can be felt. Nothing inside a cavity can be felt from outside. This matters, because builders’ marks and coating damage get mistaken for a failed unit every week, and a window you can clean is a window you do not need to buy glass for.

One more thing to rule out. Condensation on the outside of the glass on cold clear mornings looks alarming, but it is a sign of a good unit: the outer pane is staying cold because heat is not leaking out through it. It wipes off, and nobody should be quoting you for it.

Why it appears at dawn and clears by lunchtime

A failed unit mists when the cavity faces of the glass fall below the dew point of the air trapped in the cavity. Both of those figures move through the day, which is why the problem looks intermittent.

Overnight the outer pane radiates heat to a cold sky and the cavity cools. By dawn the glass surfaces are at their coldest and the moisture in the cavity air condenses onto them, starting in the bottom corners because that is where the glass is coldest and where any standing water already sits. As the sun comes round and warms the glass, the water evaporates back into the cavity air and the mist vanishes. Nothing has been cured. The water has simply changed state and is still in there.

That daily cycle is why somebody looking at two in the afternoon on a sunny day can honestly say they see nothing wrong. Photograph the window at eight in the morning and keep the picture. It saves an argument with a surveyor, a landlord or a managing agent.

The same mechanism explains the pattern across a house. North-facing windows mist first and stay misted longest, because they get no early sun to clear them. Windows behind closed blinds and heavy lined curtains also stay misted, because the cavity is shaded from the warmth that would dry it. So the windows that look worst are not necessarily the ones that failed first.

Why spring and autumn are worse than the middle of winter

Most people expect the worst of this in January, and are then surprised that their windows look clearer in a cold snap than they did in October.

Two things drive that. The first is the size of the daily temperature swing. Clear nights in spring and autumn give big falls between afternoon and dawn, which is exactly what takes the glass below the dew point. A settled cold week in midwinter, often under cloud, has a smaller swing. The second is absolute humidity: air in October is carrying far more water than air in January, so each time a failed cavity breathes it takes in more moisture.

A failed unit also breathes with a pressure cycle, not only a temperature one, so a windy, wet autumn week loads the cavity with water far faster than a still, dry cold one. A unit that looked acceptable in February and dreadful in April has not deteriorated. It has had a wetter month.

Does condensation between glass panes mean the house has a damp problem?

Short answer, no. This is worth being clear about, because people spend real money in the wrong place on the back of it.

Condensation between glass panes is water that came in through a failed factory seal from the outside. It has nothing to do with the humidity in your rooms, your ventilation, drying laundry indoors, or any damp in the structure. A house with perfect ventilation and a dehumidifier in every room will still show misting in a unit whose seal has gone.

Water inside the glass is a failed component. Water on the glass is a conversation about the house. They look similar from the sofa and they cost very different amounts to put right.

The kind that does tell you something about the house is the kind on the room side of the glass, the kind that wipes off. That one is warm moist indoor air meeting a cold surface, and the cures are ventilation, heating and, where the glass is the coldest surface in the room, better glazing. If you have both kinds, deal with them separately. Replacing sealed units will not noticeably reduce the condensation you can wipe off, and we will tell you that rather than let you expect it and be disappointed.

What a failed unit does affect is the window’s insulation, and that has one knock-on. A window that has lost its gas fill runs colder on the room side, so the wipeable condensation on that particular window may genuinely increase, and with it the risk of mould on the frame and the plastered reveal. So the two are linked at one end. The water inside the glass is still not evidence of a damp house.

What it is costing you while you leave it

A sealed unit keeps weather out, reduces heat loss, and in most modern units takes a little noise off. Only the heat loss is affected when the seal fails, but it is affected immediately and invisibly.

The gas fill escapes through the same breach the water comes in through, so the cavity goes from argon to air, and air moves heat more readily. Then the low-emissivity coating, which does most of the real work in a modern unit by reflecting warmth back into the room, starts to degrade where it is repeatedly wetted. Once that coating has clouded it has stopped reflecting, and the window is performing much like plain glass in a frame.

Replacement glazing has to meet the thermal requirement in Approved Document L, and a current unit with a soft-coat low-emissivity pane, a warm-edge spacer and an argon fill does that comfortably. A failed thirty-year-old unit with a plain aluminium spacer and no coating never did. So replacing a failed unit in an older window is usually an upgrade rather than merely a repair, which is one of the few genuinely cheerful facts in this subject.

What is not worth doing is replacing the sound units elsewhere in the house at the same time for the sake of consistency. The gain is small, the cost is not, and the units you have are still doing their job. Replace what has failed.

The point where waiting starts to cost you more

There is a real deadline here and it is not a sales one.

While the cavity faces are simply wet, the only cost of waiting is performance. Once water has sat against the glass through enough cycles it leaves mineral deposits, and eventually it etches the surface. Etched glass cannot be cleaned, polished or recovered. That does not change what you pay for the new unit, because the whole unit is replaced either way. What it changes is your option to do nothing: a lightly misted unit still looks acceptable from across a room, and a stained one does not, so you lose the ability to postpone.

It matters more on units that are hard to match. Patterned glass no longer in production, genuine leaded panels and sealed-in Georgian bars in unusual widths all get harder to match as the years pass, because patterns get discontinued. If a failed unit is one of those, bring it forward, because the glass you need to match against is coming off a shrinking list.

Otherwise, condensation between glass panes is not an emergency. It is not a safety issue, it will not leak into the room, and it cannot get wetter than wet. Anybody telling you it must be dealt with this week is selling, not surveying.

Which windows to deal with now, and which can wait

Grouping the work is the practical decision for most households, and it is the one place a little patience saves money. A fitter already on site with the beads off is a much cheaper way to replace a second unit than a separate visit a month later, so go round every window once, properly, before ordering anything.

Do it on a cold morning before the sun has been on the glass. Open each window, look along the bottom corners, and note the room, which pane it is in a bay or a French door, and whether the mist clears. Include anything only slightly hazy: those will be obvious next year, and adding them to the same order costs a fraction of calling somebody back.

Deal first with anything in a door or a panel beside a door, because those are critical locations where the replacement must be safety glass under Approved Document K, and it is better to find an ordinary float pane in a door now than later. Then the rooms you actually look out of. A bedroom window behind closed curtains can usually wait until you have another reason for somebody to come.

Then call us before you order anything. The survey is free across Greater London, we measure each unit to the rebate ourselves and we quote in writing with every unit listed by room and size. We make the units at 6 Atlas Road in Wembley, which is why we can match odd thicknesses and old patterns instead of talking you into a standard unit, and why we would rather make glass for the frames you have than sell you new windows. Monday to Friday, 8am to 5pm, on 020 4634 6045.