01Short answer
Yes, glass in the roof loses more than glass in the wall
Pane for pane, a rooflight gives up more warmth than an ordinary window of the same make, and there are two reasons why.
Take a rooflight and a wall window with exactly the same glass. Same coating, same gas, same spacer. The one in the roof will still lose more heat. That is not a fault in the product. It is physics, and it is why a good rooflight needs a bit more care than a good window does.
The first reason is the angle. Warm air inside a room rises and presses against whatever sits above it. Heat leaves a sloping or flat pane more easily than an upright one, because the air in the sealed gap between the panes circulates differently when the glass lies down. That is why U-values (how fast heat passes through, in watts per square metre for each degree of temperature gap; lower is warmer) are quoted higher for glass in the horizontal position than for the same glass standing vertically.
The second reason is what the glass looks at. A wall window faces a garden, a fence and the house next door. A rooflight faces the cold night sky, which is a far colder thing to swap heat with. On a clear winter night the glass loses warmth by radiation straight upwards, and there is nothing in the way to send any back.
| Position | What it faces | Effect on heat loss |
|---|---|---|
| Wall window, upright | Garden, street, neighbours | Lowest for this glass |
| Roof window, sloping | Open sky, at an angle | Higher than the wall window |
| Flat rooflight, horizontal | Straight up at the sky | Highest quoted U-value |
02The numbers
How big is the gap, and does it matter?
The gap is real, but a modern unit keeps it small enough that you are very unlikely to feel it.
Good double glazing in a wall window can reach a U-value of around 1.2 to 1.4 W/m²K. The same sort of glazing in a rooflight is usually quoted a little higher. Manufacturers publish both figures, so you can read them off the data sheet. Always check which one you are looking at, because a number measured in the vertical position can flatter a product that is going into a roof.
Now set that against the roof around it. A well insulated pitched or flat roof sits at roughly 0.15 to 0.18 W/m²K. Even a first-rate rooflight is many times that, so you are swapping a very good barrier for a merely good one. That is true of wall windows too, but the rooflight carries the extra penalty from the angle and the sky.
Does it matter? For one or two well placed openings, usually not enough to worry about. Building Regulations accept this: the current Part L limit for a replacement rooflight is 2.2 W/m²K, which is a more relaxed figure than the limit for wall windows. Our guide to rooflight U-values explains how to read the labels.
Heat loss in the roof
More from each square metre of glass, and a clear view of a cold night sky. Worth specifying well, and worth sizing with care.
Heat loss in the wall
Less per square metre, with neighbours, fences and trees to bounce some warmth back. Easier to live with at any size.
03What helps
What keeps the loss small
You cannot change the angle or the sky, so the work is done in the build.
Low-e glass has a thin coating that bounces heat back into the room, and argon in the cavity slows its escape across the gap. Triple glazing adds a third pane and a second cavity, which is worth weighing up on a large lantern or a roof that gets no winter sun. We compare the two on our page about double and triple glazed rooflights.
The edge of the unit is where a surprising amount goes missing. A warm-edge spacer, the bar that holds the panes apart, keeps the rim of the glass warmer than a plain aluminium bar would. Warm-edge spacers are part of every unit we fit. Past the glass, the frame and the kerb (the raised collar a flat roof rooflight sits on) should be insulated and joined to the roof insulation with no gap. Left out, those two details can undo good glass.
Last, think about the size. If one square metre of glazing does the job of daylight for a landing, there is no gain in fitting two. We say so at the survey. The full reasoning behind our choices is on the warm specification page.
- Low-e coated glass, with an argon filled cavity
- A warm-edge spacer around the whole unit
- A frame that does not carry cold straight through
- An insulated kerb or upstand with no gap to the roof insulation
- An opening sized for the room, not for the brochure
04In practice
What it means for your own roof
Think about the room first, and the glass second.
A bright room that is a little cooler at night is a fair trade for some people and a bad one for others. If it is a kitchen-diner where the heating runs all evening, you will want a better unit and probably a blind. If it is a hallway you pass through, a standard good double glazed unit will be fine.
A fitted thermal blind helps more than people expect, because it traps a layer of still air against the glass at night. We cover the idea on our blinds for warmth page. And if you are looking at a whole lantern, the longer discussion is in are roof lanterns cold in winter.
We will not pretend a skylight is as warm as solid roof. It is not. What we aim for is a room where you cannot feel the difference sitting under it. Book a free survey and written quote through our quote page and we will look at your roof and tell you straight.
Will a rooflight make my room colder in winter?
Is a skylight colder than a velux-style roof window?
No, they are the same idea. A roof window is a skylight that opens and sits in the slope of a pitched roof. Heat loss depends on the glass, the frame and the fitting, and the slope matters more than the brand.
Why is the U-value higher for horizontal glass?
Because heat moves through a lying pane more easily. Air in the cavity circulates differently when the glass is flat, and the pane faces the cold sky, so the same unit tests worse in the horizontal position.
Will a rooflight make my room cold at night?
A well specified one should not, not noticeably. Old single glazed or plastic dome units are the ones that do. Our energy efficient rooflights hub and our page on what U-value a rooflight should have gives the figures to look for.
Does a rooflight need Building Control approval?
Changing the entire rooflight, frame and glass together is a controlled fitting under Part L, so it is notifiable, and we send the notification in. See do rooflights need Building Regulations for the detail.
