Damp answer
Why is there condensation on my flat rooflight?
Moisture on rooflight glass comes from three different causes in three different places. Which side of the glass it is on, and whether it clears by midday, tells you what is going on.
The answer
Water forms on glass when damp air meets a surface colder than its dew point. Inside the room it points to humidity and a cold frame or upstand. On the outside, in the morning, it shows the pane is insulating well. Moisture trapped between the panes points to a failed sealed unit, which has to be replaced.
Why does my rooflight run wet on the room side?
Warm, damp room air meets a cold surface. Cooking, washing and breathing add moisture, and a cold frame or uninsulated upstand gives it somewhere to land.
Air carries water vapour, and warm air carries more than cold. When room air touches a surface colder than its dew point, some of that vapour turns to droplets. A rooflight is the coldest large surface in many rooms, since it is glass in the roof and the warm air rises straight to it.
Where the damp comes from
A kitchen with a hob and a kettle, a shower room, a house drying laundry on racks and a room with no extractor all load the air. A sealed, draught-proofed extension holds that moisture longer than an older room that leaks air.
Why it forms on the frame
The glass itself is usually the warmer part of a good double or triple glazed unit. The coldest spot is often the edge, where the frame or the upstand lining conducts heat out. A bare, uninsulated rim chills the inside face round the opening, and droplets form there first. Our guide to flat rooflight upstands explains why the rim insulation matters.
Is moisture on the outer pane a fault?
No. On a clear, still night modern glass keeps the room heat in, so the outer pane gets cold enough for dew to form outside. It clears as the sun or air warms the glass.
This surprises people who have just paid for better glazing. Heat escaping through poor glass used to warm the outer pane slightly, and the dew stayed away. A well-insulated unit keeps the heat in the room, so the outside surface follows the temperature of the night sky and can fall below the dew point of the air around it.
What it looks like
A film of tiny droplets, or a mist, on the outer face on a still morning. It happens in spring and autumn, when nights are clear and the air is damp. It fades through the morning and leaves no trace. Rain, wind or cloud cover will often stop it forming at all.
Why it is not a problem
It is a sign of a thermal performance working as designed. Nothing is wrong with the seals, and nothing needs doing. If the same moisture is visible at midday on the inside of the outer pane, that points to the next case.
What if it is trapped between the two panes?
Then the sealed glazing unit has lost its seal. The gap cannot be dried from outside, and the unit is replaced, either as a new glass unit or as part of a new rooflight.
A double glazed unit has a sealed gap between the panes, kept dry with a desiccant in the spacer. When the edge seal fails, moist air gets in and the desiccant is overwhelmed. Droplets or a permanent cloudy film then form on the faces inside the gap, and no amount of wiping touches them.
What to do about it
The unit has to be replaced. Sometimes the glass alone can be swapped into the existing frame. Sometimes the frame, upstand or age of the rooflight point to a whole new rooflight. Weighing one against the other is done in our piece on why rooflights mist up.
Telling the three apart
Wipe the inside face and the outside face. If the film wipes off, it is on that surface. If it will not move, it is between the panes.
What reduces condensation on a flat rooflight?
Move moisture out of the room with an extractor or an opening unit, and warm the surround with an insulated upstand and lining. Better glazing helps, but the frame and the rim are the usual weak points.
Take the moisture out of the room first. An extractor over the hob or in the shower room is the dependable route. An opening rooflight lets damp air leave at the high point, as our guide to fixed and opening flat rooflights sets out.
Warm the surround
If condensation collects at the edges of the opening, the upstand and lining are probably uninsulated or poorly insulated. Fitting a new rooflight on a properly insulated rim fixes that at the source. The limiting U-value for a rooflight in an existing dwelling in England is 2.2 W/m²K under Part L, and a glazing unit with a warm edge spacer helps to keep the edge of the pane warm.
Plan it into the new work
When we survey a roof for a new unit, we read where the room moisture goes and how the rim will be insulated, and write it into the fixed quote. The wider choice of glass is on our flat roof rooflights page.
Wet glass, sorted by cause
Does triple glazing stop condensation?
The inner pane stays warmer, so damp forms on it less readily. It can bring more dew on the outer face on clear nights, because less heat escapes. Neither is a fault. Frame and rim insulation still decide what happens at the edges.
Is wet glass a sign of a leak?
Usually not. A leak shows as a drip, a stain or a damp patch that gets worse after rain, not a film that forms in the morning. If the water appears when it rains and not otherwise, have the covering around the rim looked at.
Will a trickle vent help?
A vent in the rooflight frame or in a nearby window gives damp air an exit, which cuts the build-up. It works best with an extractor in kitchens and bathrooms.