Do roof lanterns make a room hot?

01Why roof glass gathers more heat than a wall window

A lantern looks up at the sky, and in summer that is where the sun is. Roof glass meets the high June sun almost square on, while a wall window meets it at a glancing angle and turns much of it away.

Cambridge sits at about 52 degrees north. At midday in late June the sun stands around 61 degrees above the horizon; in late December it reaches only about 14 degrees. The glazed top and hips of a lantern are pitched at a shallow angle, typically somewhere around 20 to 25 degrees, so for most of a summer day the sun shines almost straight down through them. A vertical window facing south catches the same sun steeply from above and receives far less energy per square metre. In winter the position reverses: the low sun slides across the lantern and strikes the wall glass more directly. A roof lantern therefore collects the most solar heat in exactly the months you least need it.

What happens next is the part people feel. Sunlight passes through the glass and lands on the floor, the worktop, the table and the sofa. Those surfaces warm up and give off heat of their own as long-wave infrared, which glass does not let back out. The energy stays in the room, the air above the warm surfaces rises, and the hottest layer collects in the lantern void just under the ridge. That is the one advantage a lantern has over a flat rooflight: its height puts the hottest air above your head. It only helps if that air has a way out.

02How much heat a lantern lets in

On a clear June midday, a mid-sized lantern with clear glass lets in about as much heat as a small electric heater left running. Solar control glass cuts that by roughly half, without making the room noticeably darker.

The sums are simple enough to do yourself. Bright summer sunshine falling on a flat surface in southern England carries in the region of 800 watts per square metre around solar noon, which in Cambridge is about 1pm in summer time. When the sun is high, a lantern intercepts roughly its own footprint of that sunshine. The glass then passes a share of it, set by its g-value (the fraction of the sun's energy that ends up inside). Take a lantern of 2 metres by 1.5 metres, a common size for a kitchen extension, which is 3 square metres of footprint.

These are illustrative round figures for a cloudless midday, not measurements of any particular product. Cloud cuts them sharply, and the hours either side of midday bring less. The point is the scale: 1.4 kW arriving steadily from late morning to mid-afternoon is a lot of heat for one room, and it arrives on top of an oven, a hob, a dishwasher and the people cooking. The same lantern with neutral solar control glass looks almost identical from inside and brings in a little over half the heat.

03When the heat turns into a problem

Heat through the glass only becomes overheating when it arrives faster than the room can lose it. A few conditions tip a lantern from pleasant to stifling, and they tend to stack up in the same extensions.

• Clear glass on an open roof. Cambridge is flat, with big skies and few hills or tall buildings to shade a single-storey roof. A lantern on a new extension in a young garden at Trumpington Meadows sees the sun from breakfast to dusk. A lantern in the side return of a Romsey terrace, between tall gault brick walls, is shaded in the morning and evening and meets only the high midday sun.
• No opening vent. A fixed lantern traps the hot layer under the ridge, and it spreads down into the room as the afternoon goes on.
• Too much glass for the floor below. A lantern sized for drama rather than for the room lets in more heat than the space can absorb. The roof lantern size guide explains how to size one against the room.
• South or west-facing doors underneath. The lantern's heat peaks around midday; west-facing bifolds then take the low sun from mid-afternoon into the evening, so the room is warming for eight hours or more.
• An airtight modern house. Homes at Eddington and Great Kneighton are built to hold heat, which serves them well in January. In a heatwave the same airtightness means the day's heat has little way out overnight unless something is opened.

The floor matters too. Dark stone or porcelain in full sun soaks up heat through the day and gives it back through the evening, which is pleasant in April and oppressive in a July heatwave. A paler floor reflects more of the light back up into the room, where it brightens the walls instead of warming a slab.

04Glass that turns the heat back at the roof

The most effective choice is made before the lantern is ordered: the g-value of the glass. Heat stopped at the outer pane never reaches the room, and no blind or fan can match that.

A solar control coating sits on the inner face of the outer pane. It reflects a large share of the sun's invisible near-infrared energy while letting most of the visible light through, which is why neutral versions keep the colour of the room true. Tinted solar control glass, in blue or grey, cuts more heat and more glare, but the room below takes on a slightly cool cast and loses some daylight. It suits a large lantern over an exposed south or west-facing extension; over a dining table where you want warm evening light, neutral glass is the kinder choice.

Two other coatings are often confused with solar control. A low-E coating mainly holds warmth in during winter; on clear glass it does little to stop summer gain. A self-cleaning outer coating helps rain wash dust away and has no meaningful effect on heat. Triple glazing lowers heat loss in winter, as the answer on whether roof lanterns make a room cold explains, but it only trims the g-value a little unless one of its panes carries a solar control coating. The full comparison, pane by pane, is in our guide to roof lantern glass options.

Orientation can change the answer. On an extension shaded by the house for much of the day, clear low-E glass may be right because every bit of daylight counts, and the answer on north-facing extensions covers that case. Blinds come after the glass: once the sun's energy is inside the pane, an internal blind can soften glare and slow the heat, but it cannot send it back out. Read do roof lanterns need blinds? before assuming you need them.

05Letting the warm air out

Glass limits how much heat comes in; ventilation decides how long it stays. A lantern's height makes it a natural chimney, provided one of its panes can open.

Warm air under the ridge may sit 3 metres or more above the floor. Open a vent near the top of the lantern and a window or garden door at floor level, and the difference in temperature and height sets up a steady draught: cooler air comes in low, hot air leaves high. This stack effect works even on a still day when there is no breeze to help, and it works better the taller the lantern and the larger the vent. One opening pane in a hip clears the heat from a modest kitchen; a long lantern over a large open-plan room benefits from two, ideally on opposite sides. Our answer on whether roof lanterns can open covers manual and electric vents.

Electric vents are the practical choice on anything above head height. With a rain sensor they close themselves at the first drops, which means the vent can stay open through a warm night and flush out the heat stored in the floor and walls, so the room starts the next day cool.

Glass, vents and the lantern's position are all set out in the written light plan that comes with every quote from us, with the reason for each choice. The guide to controlling heat and glare under a roof lantern ranks the options by effect, how we plan the light shows the method, and our roof lantern installation page covers the whole job. To have the heat worked out for your own roof, book a light survey.

06Questions people also ask

Is a roof lantern hotter than a flat rooflight?

For the same opening, a lantern carries more glass, because its sloping hips add area, so it catches a little more sun early and late in the day. Its height works in its favour, though: the hottest air collects in the void above head height, where a vent can release it. A flat rooflight puts its hot layer right against the ceiling. The comparison of a roof lantern and a flat rooflight goes into both.

Can hot weather damage a roof lantern?

A well-made glass lantern is designed for British summers. Its outer pane is toughened, which resists the thermal stress that can crack ordinary glass when part of a pane sits in sun and part in shade, and the aluminium frame and its seals allow for expansion. Older lanterns with polycarbonate sheets are a different matter: years of sun can yellow and craze the sheets, which is a common reason to replace one with glass.

Does Building Control check an extension for overheating?

No. Part O of the Building Regulations, which deals with overheating, applies to new dwellings, not to extensions of an existing home. Building Control will check the lantern's thermal performance under Part L, where the limiting U-value for a rooflight is 2.2 W/m²K, and its safety glazing under Part K. Summer comfort is left to the choices you and your installer make.

Will a lantern overheat a north-facing extension?

It is far less likely. The house itself stands between the lantern and the midday sun for much of the year, and the garden doors face away from it. In high summer the sun clears the roofline for part of the day, so an opening vent is still worth having, but clear low-E glass is often enough and keeps all the daylight.