Are skylights energy efficient?

People ask whether skylights are efficient as if there is one answer. There are two, and they pull in opposite directions.

The winter question is how fast the unit lets heat escape on a cold night. That is the U-value, measured in watts per square metre per degree, and lower is better. A single-glazed rooflight from the 1980s might sit around 5. A current double-glazed unit runs from roughly 1.6 down to about 0.8 depending on the build-up, the gas fill and the coating. Triple glazing goes lower again. On that measure alone, a new rooflight is far better than the one it replaces, and better than most of the wall it sits above was fifty years ago.

The summer question is how much of the sun’s heat the glass lets through. That is the G-value, a figure between 0 and 1, and here lower is better too but for the opposite reason. Clear double glazing sits around 0.6, so it passes about sixty per cent of the solar energy landing on it straight into the room. Solar-control glass brings that down to roughly 0.3. Building Regulations set a ceiling on the U-value under Part L when you fit a new or replacement rooflight, but they set no equivalent limit on the G-value. That gap is why a skylight can pass every thermal check on paper and still make a room unusable on a bright afternoon.

Heat rises, so the roof is where a house loses it fastest, and a poor rooflight is a weak point in the one surface you most want sealed. A sealed double-glazed unit with a warm-edge spacer and a low-emissivity coating changes that. The coating reflects heat back into the room instead of letting it radiate out through the glass, and an argon fill between the panes slows the transfer further.

There is a second winter effect that works in your favour. Glass facing the low winter sun collects free warmth during the day, and on a north-facing roof, where there is no summer overheating risk to worry about, you actually want a higher G-value so the room banks that gain. This is the part most people miss: the glass that is right for a north-facing bedroom is the wrong glass for a south-facing kitchen, and specifying the same unit for both is where efficiency quietly leaks away.

Condensation is worth a mention here, because it reads as an efficiency problem and often is one. A cold single pane runs with water on winter mornings; a warm inner pane on a properly specified unit stays clear, so you are not wiping the frame or running a dehumidifier to fight damp the glazing created.

This is the part of the question that matters most in Southend, and it is the reason this business exists. The city faces south across the Thames Estuary, and the southern horizon over the water is open, so a south-facing roof collects sun from mid-morning until the sun swings west with nothing in the way to shade it. Reflected light off the estuary adds to the load on a bright day.

Put clear glass in that roof and the room behaves like a greenhouse. On a clear July day the sun delivers roughly 800 watts to every square metre of well-oriented glass. A one-and-a-half square metre rooflight at a G-value of 0.6 pushes something like 700 watts of that into the room, which is a small fan heater running all afternoon over a kitchen that already makes its own heat from cooking. Solar-control glass at 0.28 cuts the same opening to around 340 watts. The room stays bright and stops being somewhere you avoid by two in the afternoon.

Glare is the other half of the summer problem, and it is separate from heat. Low sun and reflected estuary light wash out screens and make a room uncomfortable to sit in even when the temperature is bearable. Choosing the glass by orientation is the fix for both at once, and it is the whole basis of our energy-efficient skylight installation work. A flat roof skylight, by the way, points at the sky rather than at any compass direction, so it takes the high summer sun almost square on and needs treating as south-facing whichever way the house is turned.

The unit is most of the answer, but not all of it.

• Size against the room. A large lantern over a small kitchen amplifies whatever the glass does, good or bad. If the glazed roof area is more than about a sixth of the floor area below, drop the glass a band lower than orientation alone suggests.
• The fit. A good unit set badly loses its advantage at the edges. The upstand needs insulating and the perimeter needs sealing, or heat finds the gap around the frame that the glazing figure never counts.
• Ventilation. An opening rooflight lets hot air escape at the highest point of the room, which does real cooling work that no coating can. Pairing a fixed low G-value pane with one opening unit often beats pushing the glass specification harder.
• Shading. External shading stops heat before it reaches the glass and outperforms any coating. Internal blinds help with glare but do little for heat once it is already through the pane.

Start by working out which way your roof faces and whether it is pitched or flat, then ask for the actual glass specification sheet rather than the phrase “energy efficient”, which is printed on units at every G-value there is. The sheet should give you the U-value, the G-value and the light transmittance for the same build-up, so you can judge winter loss, summer gain and daylight together instead of trusting one word to cover all three.

If the roof faces south or the room already runs hot, treat the G-value as the number that matters and check it is around 0.3 or below. If you would rather hand that over, tell us the orientation and the rough opening size and we will come back with a specification and a price. Ring 01702 898232 or request a quote, and we will look at the whole roof rather than just the hole in it.