What causes condensation on a rooflight?

Warm air, cold surface, and the point between them

Air always carries water vapour, and how much it can carry depends on how warm it is. Warm air holds more. Cool the same air down and its capacity falls, and cool it far enough and it reaches the temperature at which it can no longer hold what it already has. That temperature is the dew point. Past it, the surplus stops being vapour and becomes water on whatever surface did the cooling.

Nothing in that sequence involves rain, and nothing in it involves a roof failing. The same thing happens on a cold bottle out of the fridge, on a bathroom mirror and on the inside of a car windscreen. The rooflight is doing exactly what the bottle does.

Two variables decide whether you see it: how much moisture is in the room air, and how cold the coldest surface in that room is. Both are what a specification and a ventilation strategy actually act on.

Why the rooflight shows it first

Water does not settle out evenly across a room. It settles on the coldest surface available, and in a room with glass in the roof that surface is very likely to be the glass. A rooflight is the thermally weakest element in most roofs, and it sits at the top of the room, which is where the warmest and wettest air in the house has already collected.

It is also the only part of the roof facing straight up. On a clear night a surface pointed at the open sky loses heat faster than one facing sideways, so the inner face of a rooflight can sit colder than the walls around it even where the unit is well specified. That is why the first cold, clear, still morning of the autumn is when people notice it, often in a room that behaved perfectly all summer.

So water on a rooflight is not evidence that the rooflight is worse than the rest of the house. It is evidence that it is the place where a whole-house moisture load becomes visible.

The band around the opening

The glass is only part of it. Look closely at where the water actually sits and it is usually worst along the bottom of the pane where it meets the frame, and on the plastered reveal running around the opening. Those surfaces are colder than the middle of the glass, and they are colder for a structural reason.

The reveal is the lining running from the frame out to the finished ceiling. Structurally it is timber, and timber conducts heat far better than insulation does. Where insulation is stopped short of the frame, or the head of the opening is left thin because it was awkward to reach, a conductive path runs from outside to inside right around the perimeter. What you see is a neat dark band round the opening, sometimes with mould in the corners, while the ceiling a hand’s width away stays dry.

This is why the reveal detail matters as much as the glazing specification. A well specified unit set into a poorly insulated opening will still produce water, because the coldest surface in the room was never the glass.

Where the moisture in the room comes from

Every household generates water vapour continuously. Cooking, showering, washing, drying clothes indoors and simply breathing all put moisture into the air, which is why kitchens and bathrooms show condensation before other rooms do. A loft conversion containing a shower room, or a rear kitchen with glass in the roof, carries a heavier load than a spare bedroom ever will.

The other half of it is what has been done to the building. Older houses handled moisture badly but constantly, by leaking air through gaps in floors, around windows and up through the roof. Every sensible improvement since, from replacement glazing to draught sealing to loft insulation, has reduced that air movement. Unless ventilation was deliberately put back, the moisture that used to drift out now stays in.

So the newly sealed, newly insulated, newly extended house is often the one showing condensation for the first time, and the rooflight installed during that work gets the blame for a change that happened around it.

Ventilation is the actual remedy

Two things reduce condensation: making the cold surface warmer, and making the air drier. The first is a specification question settled before anything is ordered, through the glazing build-up, a warm-edge spacer, and an unbroken line of insulation carried tight to the frame on all four sides with the vapour control layer sealed to it rather than tucked near it.

The second is ventilation, and it does most of the work. Damp air has to leave the building, and it leaves best from high up, because warm air is already trying to go that way. An opening rooflight is an efficient extract point for that reason alone. Where a unit sits above easy reach, an electric opener means it actually gets used, and a unit nobody can reach is a unit nobody opens.

Extract in the kitchen and the bathroom, trickle ventilation and somewhere to dry washing that vents outside all belong in the same conversation. Where a unit is genuinely under-specified for the room it serves, the answer is a replacement installation specified properly.

Cold mornings or wet weather

Separating condensation from water coming in matters, because the two have entirely different answers. There are honest ways to tell them apart without opening anything up.

Which side of the glass. Condensation forms on the room face of the inner pane and it wipes off. Moisture sitting between the two panes of a sealed unit is a different thing again: that unit has reached the end of its life and is replaced. Water on the outer face is simply weather.

What it follows. Condensation follows temperature. It is worst on cold, clear, still mornings and often absent in mild wet weather. Ingress follows rain and wind direction rather than the thermometer, tends to show at one corner or down one side, and marks the plaster around it.

Our page on whether rooflights leak sets that distinction out in more detail, and the Wolvercote page covers what changes where the ground itself is damp. We install new rooflights and replacements across Oxford and up to twenty five miles by road, we handle the Building Control notification, and our workmanship carries a ten-year guarantee.