Extension Unbearably Hot in Summer: Is the Rooflight to Blame?
If your extension is unbearable in summer and it has a large clear rooflight, the rooflight is very often the main cause, because a sky-facing rooflight collects solar heat all day and clear glass lets roughly two thirds of it into the room. The quickest relief this week is to purge the hot air, open the rooflight and a low window on the shaded side, ventilate overnight, and shade the glass during peak sun. The permanent fix is a solar control unit at a g-value around 0.30 with an opening vent, which cuts the heat entering the room to about a third while keeping it bright. Diagnose it by the glazing, the orientation and the area.
A hot extension is genuinely miserable, so here is what to do now, then how to tell if the rooflight is the culprit, and what a replacement specification would actually change.
Quick relief this week
- Purge the hot air. Open the rooflight if it opens, and open a window or door low down on the shaded side of the house. Hot air leaves at the top, cooler air comes in low. This cross-purge is the most effective free thing you can do.
- Ventilate overnight and early morning, then close up before the sun is on the roof, holding the cool in through the day.
- Shade the glass at peak sun. An internal blind cuts glare and takes the edge off, even though it only modestly reduces heat. An external shade helps more where you can rig one safely.
- Move the air. A fan makes a hot room feel several degrees cooler even without lowering the temperature.
- Reduce added heat. Cook and use the oven in the cool of the day, and turn off unnecessary heat sources under the rooflight.
None of these fix the cause, but they make a hot spell bearable while you plan the real change. More detail is on how to stop a room overheating under a rooflight.
Is the rooflight to blame?
Diagnose it on three things. If they point at the rooflight, it is almost certainly the main cause:
| Factor | Points to the rooflight if… |
|---|---|
| Glazing | It is clear glass with a high g-value, roughly 0.60 to 0.72 |
| Orientation and pitch | It is flat, or faces south or west, so it sees the sun all day |
| Glazed area | The rooflight is large relative to the room, dominating the ceiling |
| Timing of the heat | The room heats up through the day and peaks in the afternoon sun |
A large clear rooflight, flat or south-to-west facing, over a modest room, that bakes from midday, is the classic case, and it is the most common overheating job we survey in Suffolk. A flat rooflight is the worst offender because it sees the whole sky dome whatever way the house faces, so orientation does not save it. The mechanism is on controlling solar gain through rooflights. If the rooflight is small or north-facing and the room still bakes, look also at wall glazing, west-facing bifolds and a poorly ventilated space, because the rooflight may not be the whole story.
Why a rooflight overheats more than a window. A wall window sees the sun at an oblique angle for part of the day. A rooflight faces the open sky and collects from sunrise to sunset, and a flat one is close to square-on to a high summer sun for hours. So the same clear glass overheats a room far more in the ceiling than in the wall. That is why the fix has to be in the rooflight glass, not in the curtains.
What a replacement specification would change
The permanent fix is a new unit specified from the compass:
- Solar control glass at a g-value around 0.28 to 0.38, which rejects heat at the outer pane before it enters the room, cutting the solar heat to roughly a third while keeping light transmission above 60 per cent so the room stays bright. Covered on solar control glazing for rooflights.
- An opening vent so the hot air that collects at ceiling level has a route out. This wants planning before the ceiling closes, because retrofitting a vent means removing the unit. Powered options with rain sensors are on our electric skylight installation page.
- An internal blind for glare and night privacy, once the glass and vent are doing the heat work.
Note that a solar control double unit beats clear triple on a hot aspect, so the answer to overheating is the coating, not more panes. Retrofit solar film is not the answer either, because it works on the wrong side of the glass and can crack a pane on a rooflight that already runs hot. The comparison of glass against blinds is on solar control glass or rooflight blinds.
Why is my extension so hot with a rooflight?
Because a rooflight faces the open sky and collects solar heat all day, and clear glass lets roughly two thirds of that heat into the room. A large, clear, flat or south-to-west facing rooflight over a modest room is the classic overheating case. A flat rooflight is worst because it sees the whole sky dome whatever way the house faces. The fix is solar control glass at a g-value around 0.30 plus an opening vent, which cuts the heat entering the room to about a third.
What can I do now if my rooflight room is too hot?
Purge the hot air: open the rooflight and a low window on the shaded side, so hot air leaves at the top and cooler air comes in low. Ventilate overnight and close up before the sun is on the roof. Shade the glass with a blind at peak sun, run a fan, and avoid adding heat from the oven in the afternoon. These make a hot spell bearable. The permanent fix is a solar control unit with an opening vent, specified from the orientation.
Get the rooflight specified to cool the room
We take the orientation at survey, confirm whether the rooflight is the cause, and specify solar control glass and a vent to keep the extension usable through summer. One fixed price with the g-value written on it, a 10-year workmanship guarantee, and installers with 25 years on rooflights.
Request a fixed quote, or start from energy efficient skylight installation.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.