Why does my skylight make the room too hot?

A wall window sees the sun for part of the day and at a low angle for much of the year. A roof window tilts towards the sky, so it faces the high summer sun for much of the day. The glass lets sunlight in, the floor, furniture and ceiling absorb it, and the room gives it back as heat that cannot pass out through the glass again. Anyone who has sat in a parked car on a bright afternoon knows the effect.

The room itself matters as much as the window. A loft bedroom or an upstairs landing sits directly under the roof covering, so the slates or tiles warm the space from above while the window warms it from the side. Warm air rises, collects against the sloping ceiling and stays there unless something lets it out. A room on the north slope may never feel the problem. A south or west slope without shade often feels it from late morning onwards.

Where you put the shade makes a bigger difference than what the shade is made of. An internal blind stops light, but the heat has already come through the glass by then and is trapped between pane and blind. An external blind, awning or shutter fitted outside the window stops a large share of that heat before it enters. Roof window manufacturers make external roller blinds and awnings for their own windows, usually with a fabric designed to reflect heat, and many can be driven by a small motor or a solar-powered controller.

If outside fitting is not possible, a reflective-faced internal blind is better than a plain one, and a pleated blind that sits close to the glass in a tight frame beats a loose roller that lets air circulate behind it. Fabric colour counts too: pale and reflective on the glass side keeps more heat out than dark. Blinds with side channels that hold the fabric close to the frame leave less of a gap for warm air to leak round, and they also darken a bedroom for sleeping, which is often why people buy one at all.

Our page on the types of rooflight shows which units take which accessories, which is worth checking before you buy a blind.

A roof window that opens is a ventilator as well as a light source. Hot air gathers at the top of the room, so opening a high-level window lets it leave, and a second opening lower down, such as a wall window or the door to the landing, lets cooler air in to replace it. Two openings at different heights move far more air than one on its own. Some roof windows have a small ventilation flap in the top of the frame that stays open with the sash closed, which suits nights and times when the room is empty.

Electric windows can open at a set temperature and close when a rain sensor is triggered, which solves the worry of leaving a window open and going out. If the concern about leaving a window open overnight is safety rather than weather, our page on roof window security covers restrictors and locking positions that allow a gap without inviting anyone in.

Older roof windows often have plain double glazing with no heat-reflecting coating. Newer units come with solar-control coatings or tinted laminated panes that cut the sun's heat while keeping the room bright. They do darken the view slightly, and they usually do not stop the room needing shade on the hottest slopes, but they help. If the existing window is sound and the room only overheats on a few weeks of the year, shading is the cheaper step.

If a window is replaced, it counts as a replacement element, and Approved Document L Table 4.2 caps the heat loss of a roof window at 1.4 W/m²K, and a Band B rating is the other way to comply. That standard is about heat lost in winter, not heat gained in summer, so ask for the solar-control version of the glass if summer comfort is the goal. A window that suits both seasons usually costs a little more than one that only meets the minimum, and the written quote will say which specification is priced.

Finally, look at the roof around the window. Thin or missing insulation in the rafters lets the roof covering pass heat straight into the room. Adding insulation can change a loft room more than any window upgrade, but the depth that fits depends on the rafters and on keeping the ventilation gaps the roof needs.

We look at which way the slope faces, how big the window is compared with the room, what shades or opening parts it already has, and what the roof around it contains. The photographs from the survey show the underside and the outside of the unit, so you can see what the sun is striking. The written quote that follows describes the smallest change that properly tackles the heat, which is often a blind and a ventilation habit rather than a new window.

Where a replacement does make sense, the existing opening usually allows a like-for-like unit without altering the rafters, and the work comes with our 10-year workmanship guarantee in writing. For more on windows in general, see our skylights and roof windows service, or send us a photo of the room and the slope and we will say what we would try first.