Dew point in plain terms
Air carries water as an invisible gas, and warm air can carry more of it than cold air. Cool a parcel of air far enough and it can no longer hold all that water; the temperature where that happens is its dew point. Below it, the spare water comes out as droplets on whatever surface did the cooling.
A cold can of drink fresh from the fridge mists over on a summer day for exactly this reason. A conservatory roof on a winter evening is the same can, laid flat over your head, with a kettle, a dog, a drying rack and two people breathing into the air below it.
Why the U-value decides how cold the ceiling gets
A U-value is a leak rate for warmth: watts lost through one square metre with the room one degree warmer than the garden. A big figure lets heat stream out, so the inner face is dragged down towards the outdoor temperature. A low number means the roof holds the heat back, and the inner face stays nearer to the warmth of the room.
That is why the same room can stream with water under one roof and stay dry under another. The closer the ceiling stays to room temperature, the further it sits above the dew point of the air, and the less water it collects.
| U-value | Inner face on a cold night | |
|---|---|---|
| Old 16 mm polycarbonate | about 2.3 to 2.8 | Very cold, condensation likely |
| Old double glazed roof units | about 2.8 | Very cold, condensation likely |
| 25 mm polycarbonate | about 1.6 to 2.4 | Cold, condensation common |
| Modern low-e roof glass | about 1.2 to 1.6 | Cool, some condensation in damp rooms |
| 75 mm insulated panel roof | about 0.43 | Much nearer room temperature |
Water you cannot wipe away
Not every bit of moisture is on the room side. Polycarbonate sheets are sealed at the ends with tape and caps, and when those fail, damp air gets into the flutes and condenses inside them, out of reach. That is where the grey film and the green streaks come from, and no amount of cleaning removes them.
A glass roof unit that fogs up inside, where no cloth can reach, tells a similar story: the seal round the unit has gone, and only a new unit cures it. Both are signs worth reading alongside the others on our roof replacement signs page.
Cutting condensation without a new roof
Two levers work on any roof: less water in the air, and steadier warmth. Neither costs much.
Steps that help under any roof
- Ventilate a little, oftenOpen the trickle vents or a window on the latch for part of the day, even in winter.
- Keep washing out of thereA rack of wet clothes puts litres of water into the air, and the coldest surface in the room collects it.
- Close the inner doorStop the steamy air from the kitchen and bathroom drifting into the coldest room in the house.
- Keep the heat gentle and steadyA room that swings from cold to hot and back gets more condensation than one held at a mild level.
- Run a dehumidifierWorth it if the room is used for plants, pets or drying.
Where a warmer ceiling helps, and where it stops
A new roof takes away the biggest chilled face the room has. With a ceiling at about 0.43 in place of a sheet several times leakier for heat, the inner face of the roof keeps much closer to room temperature, which is the physics that keeps it above the dew point on most nights.
The glass walls stay glass, though, and a cold morning can still mist the windows in a damp room, so the steps above still earn their place. If you are weighing a new roof against linings fixed under the old one, our comparison of the two covers it, and our page on winter damp looks at the room as a whole. Guide prices are on the cost page.
