The short answer
A modern double glazed skylight loses more heat than the same area of insulated roof, because glass is a thinner barrier than a roof build-up, but far less than an old single glazed unit or a tired sealed unit. The loss you notice most in a room is often cold air sinking off the glass, and that is something the glazing choice can change.
Glass against the roof around it
A pitched roof is built up from tiles or slates, battens, a membrane, rafters filled with insulation and a plasterboard ceiling. Glass is a single layer, or two or three layers with a gap, so a rooflight will always let heat out faster per square metre than the insulated roof beside it. That is physics rather than a fault in any particular product.
What matters is how much faster, and how big the opening is. A small roof window in a large roof makes a small difference to the total heat a house loses. A big glazed lantern over an open plan extension makes a larger one, and it is worth choosing its glass with that in mind.
Heat leaves a rooflight through the glass itself, through the frame, and at the edge of the sealed unit where the panes meet the spacer. A good unit deals with all three, and a quote should tell you what the glass, the frame and the spacer are for each option.
Why older units feel cold
A single glazed skylight has no gap to slow heat down, so its inner face is cold on a winter morning and the air next to it cools and drops into the room. That is the draught-like feeling under an old rooflight, even where nothing is actually leaking.
An early sealed unit can feel almost as cold if the seal has begun to fail. Once the gas has gone and moisture has got between the panes, the unit loses much of what it was made to do. Our answer on why a rooflight mists between the panes explains how to spot it.
Frames matter too. An old metal frame with no thermal break carries cold straight through to the inside face, which is why condensation tends to form along the frame first. A modern frame has a break in it, or is made from a material that conducts less.
What a modern unit gives you
Current glass is a sealed unit with a low emissivity coating on one pane, which sends heat back into the room, and a gas fill between panes that slows heat crossing the gap. A warm-edge spacer keeps the glass edge warmer than the old aluminium spacers did. Triple glazing adds another pane and another gap.
Part L of the Building Regulations, in Table 4.2 of its 2021 edition, fixes the figures that new and replacement units in existing dwellings are fitted to: rooflights have to reach 2.2 W/m²K, whereas roof windows are measured against ordinary windows, so they need 1.4 W/m²K, or else a Window Energy Rating of Band B. Our guide to rooflight U-values explains what those figures measure.
| Item | Old single glazed | Modern double glazed | Triple glazed |
|---|---|---|---|
| Heat through the glass | Highest | Much lower | Lowest |
| Inner face on a cold morning | Feels cold | Much warmer | Warmer still |
| Weight on the frame | Lightest | Middle | Heaviest |
| Light let in | Most | Slightly less | Least of the three |
| Cost | Not offered new | Lower of the two | Higher |
Triple glazing is the better insulator, but it is heavier, it lets in a little less light, and it costs more. On a small roof window the gain is modest. On a large flat rooflight or a lantern that is the main light source for a room, it can be a sensible step up. Our comparison of double and triple glazed rooflights sets the two side by side.
Heat loss in winter, heat gain in summer
A rooflight that is good at keeping heat in is not automatically good at keeping summer sun out. Glass that faces south or west can warm a room a great deal on a bright afternoon, and the insulating layers do little about that.
If the room is a loft bedroom or a kitchen extension that overheats, the glass can carry a solar control coating, or the opening can have an external blind. Both are covered in our answer on solar control glass for a rooflight, and each has a cost in light or in money that we set out in the quote.
- Glass, frame and spacer named for each option
- Part L limit met by each unit quoted
- Overheating considered for south and west roofs
What we look at on the survey
On the free survey visit we take in the room itself, the direction the roof faces and the size of the opening. We also note what the existing unit is, because a replacement can often reuse the opening and the kerb, and the improvement from old glass to new is usually the biggest single gain.
Next, the written quote lists several suitable units for your roof, with makers other than just one included where the roof permits, and with the glass, frame and lead time shown for every unit. Where the work replaces a whole rooflight, we handle the Building Control notification. A new sealed glass unit alone does not need notifying.
Once you decide to go ahead, book a free survey, and your roof's options will be priced one beside another. If you are weighing up what a replacement involves, our guide to rooflights and the Building Regulations covers the rules.
Related questions
Will a new skylight make my room colder than the roof did?
Around the glass the room will always be a little cooler than under solid roof, but a modern double glazed unit is much closer to the insulated roof than an old single glazed one. The cold spot people notice under an old unit is much reduced.
Is a bigger rooflight a worse choice for heating bills?
A bigger opening loses more heat in total simply because there is more glass. Choosing better glass and a frame with a thermal break reduces the difference, and the daylight may let you switch lights off for more of the day.
Does a skylight lose more heat than a roof window?
The two are measured against different limits in Part L because they are built differently, so the comparison depends on the specific units. We quote the figures for the options on your roof so you can compare them like for like.
Can I add an extra pane to my existing rooflight?
Usually the sealed glass unit is replaced as a whole, since the frame is built to take a particular thickness. We check the frame at the survey, then say whether a new unit will fit it or whether the whole rooflight should change.