The short answer
It can, when the glass faces the afternoon sun and the heat has nowhere to go. Skylight overheating in summer comes from three things working together: strong sun on the glass, clear glass with no solar control, and hot air trapped under the ceiling. Each has a fix, and we choose the combination for your roof once we have seen which way it faces.
- Free survey, fixed written quote
- 10-year workmanship guarantee
Why a roof gets more sun than a wall
In June the sun climbs high, and a roof slope or a flat roof meets it far more squarely than a wall does. Glass in the roof therefore collects more heat for its size than a window of the same area beside it. The heat then rises, and it gathers in the top of the room, where the rooflight is.
A loft study under a south-facing slope shows it well. The slates or tiles outside warm up, the air under the rafters warms with them, and a clear roof window adds direct sun on the desk. Compare that with a north-facing slope over the same room: the light is soft and even, and the glass adds little heat.
Aspect decides more than size.
| Slope faces | Heat risk | What we look at |
|---|---|---|
| North | Low | Light quality and winter heat loss |
| East | Morning sun, gone by midday | Glare on bedrooms and a lived-in kitchen |
| South | Strong through the day | Solar control glass and a vent |
| West | Late-afternoon sun into the evening | Solar control and an outside blind |
Glass that holds the sun back
Clear double glazing lets much of the sun's energy through as heat. Solar control glass carries a coating that reflects part of it away while still passing daylight. On a data sheet the figure to read is the g-value, the proportion of the sun's energy that reaches the room as heat. A lower g-value means less heat gain.
The trade is some brightness. A strongly coated pane can look slightly tinted from inside, and a loft room that already gets little light may not want the strongest coating. A south-facing garden room with a lantern over it usually does. We weigh it room by room, because a landing and a kitchen want different answers.
Colour of the room matters too
Dark floors and worktops under a rooflight soak up the sun and give the heat back for hours. A pale floor reflects more of it back out through the glass. It is a small point, but it explains why two identical rooflights can feel different in neighbouring houses.
The energy side of the same glass is covered in our guide to rooflight U-values explained.
Giving the hot air a way out
Hot air collects at the highest point, so a rooflight that opens turns the problem round. With the sash raised and a low window or door open elsewhere, the warm air leaves at the top and cooler air comes in underneath. A converted garage with a flat rooflight over the old door end can clear its heat in this way.
A fixed unit has no such route, so the warm air stays under the glass until something else moves it. If the room is hot already, an electric opening rooflight can be set with a rain sensor so it closes itself in a shower, and it lets you vent the room while you are out. Roof windows with a top-hung or centre-pivot sash can open by hand and suit loft bedrooms.
Blinds and where they sit
A blind on the room side stops glare but the sun has already passed through the glass by then, so much of the heat is in the room. An external blind or a shading screen stops it at the pane. We can price a blind in the same quote as the unit.
Worth deciding before the quote
If the room faces south or west, say so when you ask for a survey. Glass, vent and blind are cheaper to choose together than to add one at a time.
What we check at the survey
We note which way the roof faces, what overshadows it and where the heat would collect, and we measure the opening. The glass, the vent and the blind are then priced on one fixed written quote after a free survey. If a new opening is notifiable we handle the Building Control notification.
Roof lanterns over flat extensions need the same care, and the main roof lantern section is at roof lanterns. For the whole picture of glass, frames and the Part L limits, go back to energy-efficient rooflights, or book a survey.
