Will a rooflight make my room too hot?

Why a roof slope gains more than a wall

A vertical window looks at a street, a fence or the house next door. Half of what it sees is ground, and much of the sky is blocked by whatever stands in front of it. A rooflight looks at the sky and little else. That is why it delivers far more daylight than a window of the same area, and far more heat with it.

A second effect catches people out in summer. At the height of the year the sun sits high at midday, so a south-facing wall window is close to edge-on and partly shades itself. A shallow roof slope presents nearly its whole area to the strongest sun of the year, at the hottest part of the day, into the warmest air the house has.

None of that argues against putting glass in a roof. It argues against assuming a rooflight behaves like the window downstairs.

The compass does most of the deciding

If one variable settles whether a room overheats, it is which way the slope faces. Two identical houses on the same street can want different glass because one rear roof runs north and the other west.

North. No direct sun through most of the year. Soft, even light all day, and overheating is rarely the complaint. Here the specification can chase daylight and winter performance without much penalty.

South. The largest total gain, but it arrives predictably around midday, and a steep slope sheds high summer sun much as a south wall does. A shallow slope does the opposite, and produces the complaints.

West. The hardest, and consistently underestimated. Sun arrives low and late, straight into the glass, at the end of a day in which the fabric has absorbed all it can hold. The peak lands in the evening, when the room is occupied.

East. A similar quantity of energy, but in the morning, into a building that has cooled overnight and can soak it up. A flat rooflight faces every direction at once, so its question is total exposure rather than orientation.

The solar factor, in plain terms

Two figures describe a glazed unit thermally and they pull against each other. The U-value describes how readily heat passes out through the unit, so a lower figure means less warmth lost in winter. The G-value, or solar factor, describes how much of the sun’s energy striking the glass becomes heat in the room, so a lower figure means less warmth gained in summer.

Solar gain is not a defect. In January it is heat the building does not pay for. In July, on a slope the sun reaches, the same property decides whether the room is usable after lunch. Specify for the winter number alone and the room bakes; specify for the summer number alone and the room is dim.

Solar control glass is the lever, and it costs less daylight than people expect, because the coatings reject far more of the invisible heat in sunlight than of the visible. Its limitation is that it cannot be switched off. The energy efficient skylight installation page covers the physics in full.

Size is the mistake people actually make

The instinct is to go as large as the roof will take, on the reasoning that more glass means more light. Up to a point that holds. Past it the gain stops being something you would notice, because the eye adapts to brightness rather than measuring it. Solar heat does not adapt. It scales with area in a straight line.

What matters is the size of the unit relative to the room beneath it, not the size of the slope. The same rooflight over a generous open-plan kitchen and over a small bedroom is two different propositions: the energy arriving is identical, and in the second case it arrives into a fraction of the air, floor and furniture available to absorb it. Our Didcot page works that through on modern estate housing.

Distribution beats area. Two moderate units set apart light a ceiling more evenly than one large one and put less peak load into the space. Moving part of the glazed area from the roof into the rear wall changes summer behaviour considerably while barely changing how bright the room looks.

The cheapest cooling in the house

Ventilation at the highest point of a room is the most effective remedy available, and it is cheap when planned in and expensive when wanted afterwards.

Warm air rises and gathers at the ceiling. An extract fan at head height and an open door at floor level move it around rather than remove it. An opening rooflight takes it out of the building where it has already collected, and where the room has doors along one side the two turn the space into a stack: cool air in low, warm air out high.

The decision has to be made before the unit is ordered, because a non-opening rooflight cannot be made openable later without taking it out of the roof. Above comfortable reach, an electric actuator with a rain sensor earns its place, and it needs a power supply planned into the works.

A room that can be purged in the evening starts each day even. A sealed one starts each day warmer than the last.

Shade outside the glass beats a blind behind it

There are two ways to keep solar heat out of a room and they are not equally good. You can reject it at the glass, or stop it reaching the glass at all.

An internal blind sits behind the glazing, so the energy is already inside the room before the blind meets it. The blind absorbs it and radiates much of it back into the space. Internal blinds are genuinely good for glare, privacy and blackout, and much weaker than people assume at temperature.

An external awning intercepts the sun before it touches the glass, so the heat is absorbed outdoors and carried off by the air. For temperature it is decisively better, and because it retracts it offers seasonal control a permanent coating cannot: full daylight in February, shade in August.

The reasons to think twice are practical. It is a moving component living outdoors, and on a designated elevation an awning may be unacceptable in appearance, throwing the load back onto the glass. Better to establish that before the unit is chosen, which is why we ask which way the slope faces first.