The answer
Skylights are energy efficient in a relative sense: a modern unit still loses more heat per square metre than the insulated roof around it, because glass cannot match insulation. One that meets Part L loses far less than an old single-glazed unit, though, and its daylight can reduce the use of electric light in a room with no window.
What a skylight lets out compared with the roof
Heat leaves a house through every surface, and glass is the weakest one in a roof. An insulated roof is thick and slows heat down at every layer. A pane of glass is thin, so a skylight or rooflight passes more heat per square metre than the slates or tiles with their insulation beneath. That is true of the best units on sale as well as the cheapest.
What changes between one unit and another is how much more. The measure is the U-value, the rate of heat flow through the unit, and the Building Regulations set a ceiling on it. For a house in England, Part L gives 1.4 W/m²K for a roof window, or Window Energy Rating band B, and 2.2 W/m²K for a rooflight, roof lanterns included. A unit fitted today has to sit at or under the figure for its type. Because a rooflight fitted before these ceilings existed may sit well above them, swapping it for a current unit keeps real heat in. The way those W/m²K figures are measured is covered in what a rooflight U-value means.
A skylight also covers a small part of the roof. One opening over a landing leaves the remainder of the slope fully insulated, so the loss through the glass stays limited to that opening.
What the daylight gives back
An internal landing, a stairwell or a bathroom with no outside wall is lit by a bulb whatever the weather is doing outside. A skylight above it puts daylight into the room for as long as it is light, and the switch stays off for part of the day. That is the other side of the energy account, and it is easy to leave out.
How much it is worth in money terms depends on the room: how often it is used, what the light fittings are, and how the household lives. What can be said is that a rooflight over a room without a window to the outside does a job the bulb would otherwise do, for as long as the sky stays bright enough to read by.
Sun heat is a smaller part of the story. In winter the sun sits low, and a glazed roof catches only a modest amount of its warmth. In summer the sun is high and the same glass can let in more heat than the room wants, so rooflight overheating has an answer of its own.
Which glass to ask for
How efficient a unit is comes down to three details of the glass, and it is worth asking about each of them on any quote.
- A low-emissivity coating with a gas fill. The coating reflects warmth back into the room and the argon between the panes slows heat crossing the gap.
- A warm-edge spacer. The bar between the panes is made from a material that conducts little heat, which keeps the rim of the glass warm.
- A thermal break in the frame. Where the frame is aluminium, an insulating strip stops the metal from carrying the cold through.
Triple glazing and solar control coatings go further for particular rooms. A triple glazed rooflight on a north-facing slope holds more warmth in, and solar control glass on a south-facing slope keeps more sun heat out. The guide to choosing glass by the room sets each against the direction the roof faces and the way the room is used.
Where a skylight costs more heat than it earns
Some positions are poor for energy. A very large unit over a room that is already bright adds heat loss and little daylight, because the extra glass lights floor that was lit anyway. An old single-glazed rooflight that is left in place loses heat steadily for as long as it stays. A unit fitted over a cold, uninsulated kerb can have a cold ring round the glass, which is where condensation and mould start.
The remedy in each case is in the specification and the fitting. The size is chosen from the room and the roof, the glass is chosen for the direction it faces, and the kerb and reveal are insulated so the frame sits in a warm surround instead of a cold one. Forming a new opening is structural work, and replacing an existing roof window or rooflight is notifiable under Part L, so our team handles the Building Control side whichever job it is.
If a landing or a back room of your house stays gloomy through the day, call us on 0113 436 0654 and we will talk through which unit and glass would suit the roof above it. We cover Leeds and the towns within 30 road miles of it.