Roof window condensation in a loft room

Warm air holds water vapour. Cold air holds much less. When warm, moist room air touches a surface that is colder than a certain temperature, the air right against that surface can no longer hold all its moisture, and the surplus comes out as liquid on the glass. That threshold temperature is the dew point, and it depends entirely on how warm and how humid the room air is.

A roof window loses the race for three reasons that a vertical window does not. First, it faces the open sky. On a clear night the glass radiates its heat straight up into space, and the outer pane can fall several degrees below the outdoor air temperature, which drags the inner pane down with it. A wall window faces a hedge, a fence, the house opposite, all of which are warmer than the night sky and radiate some warmth back. Second, warm air rises and collects at the highest point in the room, which in a loft is the sloping ceiling where the roof window sits. The most humid, warmest air in the house ends up pressed against the coldest glass in the house. Third, a loft room is often the room where moisture is generated and least well ventilated, which is the next section.

So the glass is colder and the air against it is wetter than anywhere else in the building. That is not a fault in the window. It is geometry and physics doing exactly what they always do, and it is why the same double-glazed unit can sit bone dry in a downstairs wall and weep every winter morning in a loft ceiling.

People underestimate how much water they release into a room without ever spilling any. A sleeping adult breathes out and perspires around 40 grams of water an hour. Two people through an eight-hour night put the best part of half a litre of water into the air of a bedroom. Drying a single load of washing indoors adds a litre or more. A shower in a loft ensuite, a mug of tea, a set of damp towels on a rail, a houseplant on the sill: all of it goes into the air as vapour and stays there until something removes it.

A loft conversion tends to trap that moisture better than the rest of the house. It is at the top, so warm humid air migrates up to it from the floors below and sits there. It often has a single roof window and no other opening, so there is nowhere for the moist air to go. Many are used as bedrooms, which are occupied through the coldest, longest part of the night with the door shut. Add an ensuite, which is common in a loft, and you have a shower dumping steam into a small sealed space at the top of the house. The result is high relative humidity against the coldest glazing in the building, night after night through the winter.

None of this is unusual for south Essex housing stock. The region has an enormous belt of 1930s semis, and the loft conversion into that steep original roof is one of the most common ways of adding a bedroom to one. Victorian and Edwardian terraces along the cliff carry similar conversions. In almost all of them the new room is the highest, most airtight and most intensively occupied space in the house, which is the exact recipe for a roof window that mists up.

Two numbers decide whether you get condensation: the dew point of the room air, and the temperature of the inner surface of the glass. If the glass surface is warmer than the dew point, the window stays dry. If it is colder, water forms. Everything you can do about condensation is really about moving one of those two numbers.

Here is the dew point of a room held at 20 degrees, at different humidities. Relative humidity is the figure a cheap hygrometer reads out.

Now the other number. The inner surface of a roof window does not sit at room temperature: it sits somewhere between the room and the outside, depending on how good the glazing is. With the room at 20 degrees and the roof surface near freezing on a cold night, the inner glass surface runs roughly as follows for different glazing standards.

Put the two tables side by side and the whole problem resolves. A room at 20 degrees and 60 per cent humidity has a dew point of 12 degrees. Single glazing at 7 degrees is well below that, so it streams. Modern double glazing at 16 degrees is comfortably above it, so it stays dry. The answer is either to warm the glass, by fitting a better unit, or to lower the dew point, by getting the humidity down. Usually you want a bit of both.

Three different things get called condensation, and they need different responses, so it is worth telling them apart.

Surface condensation on the room-side face of the glass is the ordinary kind described above. You can wipe it off. It is a humidity and glass-temperature problem, and the rest of this guide is about it.

Condensation only around the perimeter of the pane, a clear band an inch or two wide right at the edges with a dry centre, is a cold-edge effect. The metal spacer bar that holds the two panes apart runs colder than the middle of the glass, so the edge of the pane is the first place to drop below dew point. Older units use an aluminium spacer, which conducts heat readily and makes a pronounced cold bridge. A warm-edge spacer, made from stainless steel or a structural polymer, lifts the edge temperature by two or three degrees and is standard on a good modern unit. If your condensation is a neat frame around a dry middle, the spacer is telling you the unit is dated.

Misting or fogging between the two panes, where the moisture is inside the sealed glass unit and you cannot wipe it off from either side, is different again. It means the edge seal has failed and the cavity has lost its gas fill and taken in damp air. That unit is finished as a piece of insulation, and the answer is a replacement glazing unit or a replacement window, which is straightforward installation work. It is worth knowing that a misted unit is not a leak in your roof and does not mean water is getting into the building. The water was sealed inside the glass and got there through a microscopic failure in the perimeter seal.

You cannot warm a loft ceiling to the point where humidity stops mattering, so the other lever, getting the moist air out, is the one that does the heavy lifting. The aim is to remove water vapour faster than the room produces it, which drops the dew point below the glass temperature and keeps the window dry.

• Use the trickle vent. Modern roof windows have a background ventilation flap built into the top of the sash. Left shut through the winter for warmth, it lets humidity build all night. Left open, it gives a slow continuous air change that carries moisture away. It is the single most effective free change most people can make.
• Open the window properly for a few minutes a day. A short, wide purge in the morning clears the wettest air the night has produced far faster than a window on the latch all day. On a high or out-of-reach ceiling, this is exactly what an electric opening roof window is for, since a vent nobody can reach is a vent nobody uses.
• Extract at the source. A loft ensuite or shower room needs a working extractor fan that runs on and clears the steam, not one switched off to save noise. Dry towels somewhere ventilated, not over a rail in a sealed bedroom.
• Do not dry washing in the room. A drying rack in a loft bedroom is a humidifier by another name, and it puts litres of water into the exact space with the coldest glass.
• Keep the room reasonably warm. A room that is heated a little holds its moisture as vapour and keeps the glass surface up. A cold room lets both numbers drift towards each other. Letting a bedroom go stone cold overnight and shutting the door is how a lot of loft windows end up wet by dawn.

Two identical windows can behave completely differently depending on how they were built into the roof, because condensation follows the cold spots, and cold spots are made or avoided at installation.

The critical detail is where the roof insulation meets the window frame. If the insulation layer runs right up to the frame and the internal lining is sealed against draughts, the reveal around the window stays warm and dry. If there is a gap, a cold bridge, or bare timber between the insulation and the frame, that strip of plasterboard runs cold, and you get condensation and eventually staining on the reveal below the glass even when the glass itself is fine. The plasterboard reveals are usually splayed, wider at the room side than at the glass, precisely so that warm room air washes across them and keeps them above dew point. A reveal cut square, boxing the window into a cold slot, invites condensation on the plaster.

Underneath all of it sits the vapour control layer, the sealed membrane on the warm side of the insulation whose job is to stop room moisture pushing up into the roof build-up where it would condense out of sight. A roof window has to be tied properly into that membrane, or the loft gains a hidden path for moist air to reach cold timber. This is why a competently fitted roof window installation is as much about the collar, the insulation and the air seal around the unit as it is about the glass in it. Getting a well-specified unit into a poorly detailed hole solves nothing. If you are converting a roof or replacing a tired window, the detailing around the frame is covered as standard on a proper pitched roof skylight installation, and it is the part that decides whether the reveals stay dry for the next decade.

Condensation and a genuine water leak look similar for the first few seconds and could not be more different underneath. Some quick tests tell them apart. Condensation appears on the room-side face of the glass, wipes off, is worst on cold clear mornings, and tracks the weather and the humidity. A leak appears from around the frame or the flashing, is worst during or straight after rain rather than on a dry frosty night, and often leaves a tide mark or a spreading stain that does not wipe away. If the water follows the rain rather than the cold, it is coming from outside, and that is a roofer’s job rather than anything to do with the glazing or the room air.

There is one more case that catches people out, and it is not a fault at all. On a clear autumn or winter dawn you may find dew on the outside of a roof window, on the outer pane, misting or even frosting it over. This happens on the best glazing, not the worst. A very good unit keeps almost all the room’s heat inside, so the outer pane gets no warmth from the house, radiates to the night sky, and falls below the outdoor dew point so the night air condenses on it. It clears within an hour of the sun reaching the glass. External condensation is a sign the window is performing, and on a south-facing loft slope it burns off fastest, because that plane catches the low winter sun early and warms first.

Take the two numbers, the dew point and the glass temperature, and move them apart. In order of cost and effort:

1. Put a cheap hygrometer in the room and watch it. If it sits above 60 per cent overnight, the moisture is your first target, and ventilation is cheaper than glazing.
2. Open the trickle vent, run the ensuite extractor, stop drying washing in the room, and keep a little background heat on. For a lot of rooms this alone takes the humidity below the point where the glass drips.
3. If the window is single glazed or early sealed-unit double glazing, the glass surface is simply too cold to win, whatever you do with the air. A replacement unit that brings the whole-window U-value down towards 1.2 lifts the inner surface into the high teens and takes it clear of any normal room dew point. This is also where you can pick up solar-control and low-emissivity glazing in one go, warmer glass in winter and controlled solar gain in summer, from the same specification.
4. If the misting is between the panes, the sealed unit has failed and the sensible answer is a replacement, since a fogged unit no longer insulates and will keep the glass cold.
5. If the reveals or the frame surround are staining while the glass itself is fine, the cold spot is in the build-up around the window, and the detailing at the frame is what needs putting right when the window is refitted.

We have been installing and replacing skylights and roof windows across south Essex for over fifteen years, and every installation is notified to Building Control and backed by a ten-year workmanship guarantee. If a loft window is streaming every winter and you want the glass warmed and the surround detailed so it stops, ask us for a survey and a specification with the whole-unit U-value written on it, so you can see exactly where the inner glass surface will sit against the dew point of your room.