Do flat rooflights make a room hot?

Orientation is the thing everyone forgets with flat glazing. A window in a wall faces a compass direction, so for part of the day the sun is off to one side and only a slice of its energy lands on the glass. A flat rooflight has no compass direction at all. It faces the sky, and in June and July the midday sun sits high overhead, which means its rays hit the glass close to head on. That is the angle at which glazing lets through the most solar energy.

The season makes it worse. In deep summer, when you least want extra heat, the sun climbs highest and pours the most energy onto horizontal glass. In winter, when a bit of free warmth would be welcome, it stays low and much of its energy skims off the surface. So a flat rooflight collects heat at exactly the wrong time of year unless you specify it to do otherwise. This is the trade-off at the centre of every flat roof skylight installation: the flat plane is brilliant for even, glare-free daylight and awkward for summer heat, and the glass has to settle the argument.

The single figure that governs how hot a flat rooflight makes a room is the G-value, sometimes called the solar factor. It is the share of the sun’s energy that gets through the glass and ends up as heat inside. It runs from 0 to 1. Clear double glazing sits around 0.6, so roughly sixty per cent of the solar energy reaching it comes into the room. Solar-control double glazing brings that down to around 0.3 or lower, blocking most of the heat while still letting daylight through.

Put numbers on it and the difference is easy to feel. On a bright summer day the sun delivers something close to 800 watts to each square metre of well-oriented glass. A square metre of clear glazing at 0.6 lets through around 480 watts of that, which is a decent portable heater left running over your head. The same square metre at 0.28 passes roughly 220 watts. The room stays bright either way. Only one of them turns into a greenhouse by mid-afternoon.

The catch is that Building Regulations set a ceiling on the U-value of a new or replacement rooflight, which is a winter heat-loss figure, but they do not cap the G-value. A flat rooflight can be fully compliant on paper and still overheat the room below it, because the number that controls summer heat is the one nobody is obliged to check. If you want a cool room, you have to ask for a low G-value by name. On a flat roof, treat that as the default rather than an upgrade.

Glass area multiplies whatever the G-value lets through. One modest rooflight at a sensible specification will not trouble a room. Three large ones, or a four metre lantern over an open-plan kitchen, are a different proposition, because each square metre adds its own share of heat and they all pour into the same space. As a working rule, if the glazed roof area climbs past about a sixth of the floor area of the room below, drop the glass one band lower than you otherwise would. Big glass punishes a lazy specification.

The room matters as much as the roof. Flat rooflights tend to go over the deepest, most enclosed rooms in a house, which are often kitchens, and a kitchen already makes its own heat from the oven, the hob and the fridge. It usually has fewer walls open to the outside for that heat to escape through. So the room that most needs top light is frequently the one least able to shed a warm afternoon, which is why a badly glazed flat rooflight over a kitchen extension is the version people notice first. Getting rid of the heat also helps: a rooflight that opens at the highest point of the room lets the hottest air out, and pairing one opening unit with low G-value glass does more than pushing either on its own.

Southend-on-Sea faces south across the Thames Estuary, which is unusual for an English seaside city and is the reason flat glazing needs handling carefully here. A great many of the 1930s semis across Westcliff, Thorpe Bay and out towards Hadleigh were built with the garden at the back, and the back of the house faces south. Add a rear kitchen extension with a flat roof, put a couple of rooflights in it to bring light into a deep room, and you have built a south-facing solar collector over the one room that already generates its own heat.

The estuary adds to it. There is an open southern horizon with no terrace or tree line in the way, so the glass gathers sun from mid-morning until it swings west, and light bouncing off the water lifts the total again on a clear day. That combination of orientation, open aspect and reflected light is exactly what our energy-efficient skylight installation work is built around. The good news is that none of it forces you to accept a hot room. It just means the glass specification earns its keep, whether the rooflight is brand new or a replacement for a clear unit that has been overheating for years.

The order of priority is simple. Specify solar-control glass with a G-value around 0.3 or lower and treat that as standard on any flat roof, since orientation gives you no help up there. Ask for the glass specification sheet and read the G-value, the light transmittance and the U-value together, rather than trusting the phrase “solar control”, which gets printed on units at 0.5. Keep the glazed area sensible for the room, and drop a band lower if it runs past a sixth of the floor. Add an opening unit so hot air can leave at the top, and if you can shade the glass externally, that beats any coating because it stops the heat before it reaches the glass at all.

If you would rather hand that over, tell us the roof size and the room it covers and we will come back with a glass specification and a price aimed at daylight without the heat. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the opening in it.