The short answer
They can, but they do not have to. A skylight faces the sky, so it takes in more summer sun than a wall window of the same size, and a south or west facing one in a poorly ventilated loft room can overheat. Shading on the outside, the right glazing, a way to let hot air leave at the top and good roof insulation all bring the temperature back down.
Why a skylight takes in more sun than a wall window
Sunlight reaches a room as heat as well as light, and the glass angle decides how much. A vertical window in a wall sees the sun low in the winter and at a glancing angle for much of a summer day. A roof window is tilted towards the sky, so at midday in summer the sun is almost square to it. More energy gets through the glass and lands on floors, furniture and the people in the room.
Three things then decide whether a room becomes uncomfortable. The first is orientation. A window on a south or west facing slope receives the strong sun of the afternoon and evening, exactly when a loft room has already stored heat. A north facing slope gets little direct sun in summer and rarely overheats. The second is size: a big window lets in more than a small one, and two together more again. The third is what the heat can do once it is inside. Loft rooms sit directly under the roof, with hot air rising and nowhere to go, so a window that would be fine in a ground-floor room can tip a bedroom over the edge.
None of this makes a skylight a mistake. Position, glass and shading deserve thought on day one, not a fan bought in July.
Shading that works from the outside
The most effective shading stops the sun before it reaches the glass. Once sunlight has passed through a pane, it has turned into heat inside the room and an internal blind can only slow it down a little. A blind that sits on the outside of the window, whether a roller blind or an awning fitted to the sash, intercepts that energy first. Roof window makers sell external blinds made for their own frames.
Internal blinds still help, particularly pleated ones with a reflective face towards the glass. A dark blind is the weakest option because it absorbs heat and releases it into the room.
An external blind needs a control you can reach, so think about that when you choose where the window goes. When replacing a window, check the maker still supplies a blind to suit the sash.
Glazing that keeps heat out as well as in
Modern glazing is not all the same. Roof windows are built from double or triple glazed units with coatings on the glass, and the coating decides how much solar energy gets through. A low-emissivity coating mostly reduces heat leaving a room in winter; a solar control coating, sometimes called a sun-reflecting or tinted finish, cuts the energy let in during summer. Some makers offer a pane with both. A dark tint cuts daylight too, so ask to see a sample.
Look for the solar gain figure on the datasheet, usually shown as a g-value, where a lower number lets in less solar energy. Compare it with the U-value, which describes heat loss rather than heat gain. For a replacement window in an existing home, Approved Document L (2021) gives a limit of 1.4 W/m²K for roof windows, or Window Energy Rating Band B; the standard is concerned with heat loss, so a window can meet it without having particularly good summer performance. Choose by both numbers.
Our answer on condensation and heat loss around roof windows covers the winter side of the same glass.
Letting hot air out through the top
Heat builds up under a roof because warm air rises and has nowhere to go. Giving it somewhere to go is often the lowest-cost improvement. Opening a roof window on one slope lets some air out, but the best effect comes from a gap at a low level to draw cooler air in, such as a ground-floor window or the vent position on a lower roof window, and an opening at the top to let hot air leave. That stack effect works without a fan and gets stronger as the height difference grows.
Two roof windows on one slope, one near the eaves and one near the ridge, can cool a room better than one large one. Electric and solar-powered windows can open as the room warms, and some close again on a rain sensor.
Think about security before leaving a window open in an empty house. Where you can put an opening is also limited by the timbers; see whether rafters need cutting to fit a roof window.
Roof insulation and flat rooflights
Heat does not only arrive through the glass. A roof that is poorly insulated lets the sun's heat conduct through the covering and into the room beneath it, and a loft room with thin insulation will run hot even with no window at all. The same insulation that holds heat in during winter slows it in summer, so for a loft room, settle the insulation and the glazing together. VAT: zero-rated insulation installs run until 31 March 2027, and your quote shows how VAT applies.
Flat rooflights over an extension or a kitchen are a special case. They face straight up and take the full midday sun. A big fixed unit over a south facing extension can overheat unless it has solar control glass, a blind and venting. The kerb that carries the unit also matters; see our answer on rooflight upstands and kerbs for why its height and insulation are part of the same system.
Orientation and room use are fixed by the house, so a roofer cannot cure everything. We can inspect the roof from the loft, photograph it and tell you what a change involves, before any work starts. Our roof window and skylight page lists what we fit; ring 01453 933613 or use the enquiry form.