Question · Energy Efficient Rooflights
Do Skylights Make a House Colder?
The short answer
Not a modern, well-fitted one. Any opening in a roof loses more heat than the insulated roof around it, so a rooflight is always the weakest spot in the ceiling, but double glazing, a warm edge and a properly insulated frame keep that loss small. The cold rooflights people remember are old single-glazed units, or new ones fitted with gaps around the frame.
Where rooflight heat loss comes from
A roof is mostly insulation: a thick blanket of material between the warm rooms and the cold air. A rooflight replaces a square of that blanket with glass, and glass always lets heat through faster than mineral wool or foam does. That is the plain truth behind the question, and it is why Building Regulations set a limit for how much heat a unit may lose.
Heat leaves a rooflight in three ways. It passes straight through the glass. It leaks along the frame and the spacer bar that holds the panes apart. And it escapes through gaps where the frame meets the roof, if the insulation was not packed in properly. Each one can be improved separately, which is why two rooflights of the same size can feel very different on a January morning.
| Where heat escapes | What reduces it |
|---|---|
| Through the glass | Double or triple glazing, a coated pane, argon between the panes |
| Along the edge | A warm edge spacer instead of a metal bar |
| Round the frame | An insulated collar and sealed, packed joints |
Why old skylights feel cold
The skylight in a house built before the 1990s is usually a single pane in a thin frame. On a cold night the glass surface chills the air next to it, that cold air sinks into the room, and you feel it as a draught even when nothing is open. The glass also runs cold enough for moisture to settle on it, which is why old units drip in winter.
On a Victorian terrace in Bristol the original roof light is often a cast-iron-framed pane, charming and very cold. Replacing it is a case where the answer to the question flips: the new unit lets out less heat than the old one, even though it is still a hole in the roof. The rooflight U-values page unpacks the number behind that, and the condensation guide covers the dripping.
It helps to know what you are comparing against. If the old unit has a rattling sash, a perished rubber seal or timber that has started to soften, a good share of the cold you feel is air moving through those gaps, not heat conducted through glass. A like-for-like swap fixes both at once.
What current rules expect
For an existing home in England, Part L of the Building Regulations draws the line. Roof windows face the ordinary window test (1.4 W/m²K, with Band B as the alternative), while a plain rooflight has a little more latitude at 2.2 W/m²K. These figures describe heat flow, so smaller is better, and a unit inside them is nowhere near a hole in the thermal envelope.
That second line matters. Glass lets sunshine in as well as letting heat out, so a south-facing roof window on a cold, clear day contributes warmth. A north slope gives less of that, and the heat loss counts for more. Orientation is therefore part of the answer, not an extra.
Seasons pull in different directions too. In July the same glass that helped in January can add heat, and for that side of the story the solar control glass answer is the one to read.
Choices that keep a room warm
Four decisions do most of the work. The first is the glazing: a sealed double unit with a low-emissivity coating, which is a thin invisible layer that bounces heat back into the room. The second is the edge, where a warm edge spacer stops the rim of the glass being the coldest spot. The third is the frame and its surround: an insulated collar and well-packed gaps. The fourth is whether to step up to triple glazing, which suits a north slope or a bedroom better than a sunny kitchen.
Opening units are worth a mention here. A tilted sash closes onto a seal, and a good seal matters more than people expect. If you want fresh air as well as warmth, our fresh air rooflights page shows the range, and each quote marks where a vent bar or a full opening sash would fit. Our energy efficient rooflights hub brings the glazing choices together.
Where installation decides the result
A good unit fitted badly will still feel cold. The usual causes are gaps in the insulation round the frame, a flashing that lets a draught past, and a plasterboard lining that stops short of the glass. We pack the space between frame and rafters, seal it from inside, and line the shaft so there is no cold route into the room.
Changing the entire unit, sash and frame, is notifiable building work: it must satisfy those limits, and we tell Building Control. Dropping a fresh sealed pane into a frame that is still sound generally needs no notification, and also brings the glazing up to date. Whichever route suits, the survey costs nothing, the quote lists every item and the fitting sits under a 10-year workmanship guarantee. Our costs page shows what drives the price.
Cold rooms, warm rooms and the roof above them
Will a new roof window make my loft bedroom colder in winter?
A current sealed double-glazed unit usually loses less heat than the old window it replaces, so the room should be no colder and often feels warmer near the glass. A north-facing slope loses a little more than a south one because it gets less winter sun. Triple glazing is the option to ask about for that slope.
Is it the glass or the fitting that makes a skylight draughty?
Both can. An old single pane chills the air next to it, which you feel as a draught. A new unit with gaps around the frame lets cold air in directly. A neat fit with packed insulation and a sealed lining removes the second cause.
Tell us about the room
A few details is all we need. We'll come out, advise and give you a written quote.
No pressure, no obligation.