Do Rooflights Make Rooms Hot in Summer?

Why a rooflight gets the sun

A wall window sees the sun low in the sky for part of the day. A rooflight points upward, so at midday in summer the sun is almost square on to it. Light passes through the glass, lands on the floor, furniture and people, and turns into heat. Warm air rises and collects under the roof, which is why a room with a rooflight can feel stuffy in the afternoon.

Aspect decides most of it

A north-facing rooflight gets gentle, even light and very little direct sun. East-facing ones warm up in the morning, when the house is usually cool. South and west facing units take the strongest sun, and the west gets it at the hottest part of the day. If you are choosing where to put a rooflight, which way the slope faces will change the summer heat in the room by about as much as how big you make the opening.

Trees, a neighbouring gable or a taller part of your own house can throw useful shade, and we note them when we look at the roof.

Glass that lets in light without so much heat

Solar control glass has a coating that reflects part of the sun's energy while still letting daylight through. Some have a slight tint, others look almost clear. The trade-off is that it can reduce the free warmth you would otherwise gain in winter, and it can change the colour of the light a little. The page on solar control glass for rooflights covers which rooms and slopes it suits.

It is worth saying that low-E coating, which keeps winter heat in, is a different thing. See what low-E glass is.

Shading and ventilation

Glass choice is only one lever. Practical options include:

• A blind on the inside, which helps with glare but lets some heat in first.
• An external blind or awning, which stops the sun before it reaches the glass and works better.
• An opening rooflight or roof window, so hot air can escape at the top of the room.
• A second opening lower down, which lets cooler air in and creates a draw.

A powered or rain-sensing opening unit makes the venting easier to use, because nobody has to climb to reach a handle.

Size and position

A large fixed rooflight over a small room will heat it faster than a modest one over a bigger space. Spreading the glass, for example with two smaller openings instead of one big one, can give the same daylight with less heat load. A rooflight that sits near the ridge also vents better than one low on the slope.

Rooms that suffer most

Some spaces are more exposed than others. A loft room with a roof window sits directly under the tiles and the sun's heat, and it also has the least air above it to absorb the warmth. A kitchen extension with a glazed roof gets the sun for hours and adds cooking heat on top. In a bedroom the problem is worst, because it is hard to sleep when the air will not cool. If you have a room like that, think about venting and shading first, and glass second.

It also helps to notice when the room feels hottest. If the heat peaks in the late afternoon, a west slope is the likely cause, and an external blind or a lower-transmission pane will do more than extra venting. If it peaks in the morning, look east instead. Tell us at the survey and we will point the advice at the right slope.

Night purging

Heat that builds up in the day can be flushed out after dark. Leave an opening rooflight ajar on a warm evening, with a door or window open elsewhere, and the cooler air moves through while the warm air leaves at the top. The house then starts the next morning cooler. It works best with a rain sensor or a securely closing vent, so a shower does not catch you out.

Fixed or opening

A fixed rooflight cannot help with ventilation at all, so it relies wholly on the glass and any shading. An opening unit changes the picture, because it lets the hottest air out at the highest point. If a room already runs warm, an opening or electric unit is often a better choice than a larger fixed one. A sun tunnel is another route: it brings daylight down a small tube and passes far less heat than a full pane.

What winter looks like in the same room

A good answer for summer should not make the room worse in winter. That is why the choice is made together: the glass type, the spacer, the frame and the opening. Our page on whether rooflights are cold in winter looks at the other half of the year.

Planning and fitting

Cutting a fresh opening needs Building Regulations sign-off for both the structure and the heat loss, and we organise the notification. On a house, a rooflight that rises no more than 150mm from the roof plane is generally allowed without a planning application, subject to other conditions, and the remaining conditions sit in our planning guide for skylights. At the survey we look at which way your roof faces and talk through shading before the quote is written. More on the range is on the energy efficient rooflights page.