Will a rooflight make the room too hot in summer?

The sun is high in summer, and a rooflight is glass that looks straight up at it. A wall window catches the sun at a low angle for part of the day. A pane in the roof takes the sun from a steep angle through the middle of the day, so more of its heat comes through the glass and into the room.

Two things then make it worse. Heat that enters through glass is held inside by the room's walls and ceiling, much as it is in a greenhouse. And warm air rises, so it collects under the roof where the glass is, unless something lets it out. A big pitched lantern over a small kitchen puts a large sheet of glass over a small patch of floor, which is the arrangement most likely to feel hot.

It does not happen everywhere. A rooflight on a north slope, one in the shade of a taller building, or a small unit over a landing, takes far less sun. Whether a given room is at risk comes down to four things: the direction the glass faces, how much of it there is, what shades it, and whether it opens.

Solar control glass has a coating that turns away part of the sun's heat, and makers give it a solar factor: the lower the number, the less sun heat passes through. On a south or west slope it is the single biggest help. The glass looks slightly tinted, and it admits a little less daylight than clear glass, which is a fair price for a room that would otherwise bake.

The direction of the roof matters for sloping glass such as a roof window or the sides of a lantern. A south-facing pitch takes the most sun, a west one takes the hot late-afternoon sun, and a north one takes little. A flat rooflight lies level and takes the sun from wherever it is in the sky. Each kind of glass is matched to the way a roof faces in the guide to choosing glass by the room.

Triple glazing is a separate decision. It reduces heat loss in winter, and it does some work against heat entering in summer, but solar control is the coating aimed at the sun.

Glass is only part of the answer, and three other measures work alongside it.

Shading outside the glass

A blind or awning on the outside of the unit stops sun heat before it reaches the pane. An inside blind catches the sun after it has passed through the glass, so the heat is already in the room by then. Inside blinds still cut glare and give privacy, which is useful in a bedroom or a home office. Roof window makers offer external blinds made for their own models, and each maker lists which blind suits which unit.

A vent at the top

Warm air collects under the glass, so a vent there lets it out while cooler air enters lower down, through a door or a window. Opening roof windows and electric rooflights do this job, and many electric models close themselves when a rain sensor gets wet, so a vent can stay open on a changeable day. A fixed rooflight cannot do this, which is the usual reason to choose an opening unit for a kitchen.

Trees and neighbours

A tree or a taller neighbour that shades the roof in the afternoon changes the picture for a slope that would otherwise face the sun straight on. We note what shades a roof when we look at it.

Overheating is one of the trade-offs to settle at the start, because glass area, position and glazing are all chosen together. A smaller unit placed above the stretch of floor that needs light lets in less sun than one sized to the whole ceiling. A lantern with a vent suits a kitchen that faces the sun better than a fixed one of the same size. A sun tunnel, with its roof dome and ceiling diffuser joined by a mirrored tube, brings daylight to an internal bathroom or a landing with very little glass in the roof, so it gains little heat from the sun.

The price of each unit depends on its size, its glazing and the roof it goes into (what drives the cost of a skylight explains how those choices move it), and the written quote sets it down once the survey is done. To talk through a south-facing room that gets hot, use the quote form. Replacing a rooflight is notifiable building work, and our team sends the Building Control notification.