A rooflight loses heat far faster than the roof it sits in. A pitched or flat roof with modern insulation runs at roughly 0.15 to 0.18 W/m²K. An early double-glazed rooflight from the early 2000s runs at something like 2.8 to 3.2 W/m²K, and an old single-glazed dome is worse again. So each square metre of that glass is shedding heat fifteen to twenty times faster than the insulated roof around it. It sits at the highest point of the room, where warm air has already collected, which is exactly where you least want a weak spot.
That is the short version. The rest of this page explains where the number comes from, how single and early double glazing actually behave, and how the loss shows up in a Suffolk winter as cold draughts, misted glass and a heating bill that never quite makes sense.
Where the heat actually goes
U-value is the rate of heat loss through a building element, in watts per square metre for each degree of temperature difference between inside and out. Written W/m²K. Lower is better. To see why a rooflight dominates the heat loss of a room, put the elements side by side for a typical Suffolk extension.
| Element | Typical U-value (W/m²K) | Heat loss relative to insulated roof |
|---|---|---|
| Insulated warm roof | 0.15 to 0.18 | Baseline (1x) |
| Insulated cavity wall | 0.28 to 0.30 | About 2x |
| Modern solar-control rooflight | 1.2 to 1.4 | About 8x |
| Early 2000s double-glazed rooflight | 2.8 to 3.2 | About 18x |
| Old single-glazed dome or wired glass | 4.5 to 5.5 | About 30x |
Whole-element figures for illustration. Your survey confirms the real numbers for your build-up and the unit you are replacing.
Read down that table and the point makes itself. Even a good rooflight is the weakest square metre in the roof. An old one is a hole. On a 1.5 x 1.0m dated rooflight, one and a half square metres of ceiling is losing heat like thirty square metres of insulated roof. You cannot insulate your way out of it from the loft, because the weak element is the glass itself.
Why the position makes it worse than the number suggests
The U-value describes the material. It does not describe where the material sits. Warm air rises and collects under the ceiling, so the air pressing against a rooflight is the warmest air in the room. Heat loss is driven by temperature difference, and the difference across a ceiling rooflight on a January night is larger than the difference across a wall window in the same house. The rooflight is losing heat faster per square metre than its U-value alone implies, because it is working against the hottest air you have.
There is a radiant effect on top of that. On a clear night the sky radiates at a temperature far below the air, close to minus twenty degrees on a hard frost. A rooflight faces that cold sky squarely. A wall window faces a hedge, a fence or a neighbour’s warm wall. So the rooflight radiates heat to a colder surface than any window in the house.
How single and early double glazing perform
Single glazing and old domes
A single sheet of glass or a single-skin polycarbonate dome has no insulating cavity at all. The inner surface sits within a couple of degrees of the outside temperature. In a Suffolk winter that means the glass is often below the dew point of the room air, so it streams with condensation, and the cold surface drives a downdraught you can feel standing under it. These are the units where people describe a room as unusable from November to March. There is nothing to specify here. The unit is the problem.
Early double glazing
Early double-glazed rooflights, roughly the units fitted before about 2010, brought a sealed air-filled cavity and an aluminium spacer bar around the edge. Better than single, but three weaknesses remain:
- Air-filled cavity. Air conducts heat more readily than the argon used today, so the cavity does less work.
- Aluminium spacer. The metal spacer conducts heat straight around the perimeter of the unit, short-circuiting the glass. This is why you often see a band of condensation around the edge of a pane with clear glass in the middle. The whole-unit figure is dragged down by that edge.
- No thermal break in aluminium frames. A frame with no insulating barrier runs cold, condenses and can drip.
Put those together and an early double-glazed rooflight lands somewhere around 2.8 to 3.2 W/m²K whole unit. That is still roughly double the current Part L floor of 2.2 for a replacement, and about twice the heat loss of a modern solar-control unit at 1.2 to 1.4.
The frame and edge are half the story. Heat leaves a rooflight through three routes: the glass, the spacer at the edge of the sealed unit, and the frame. A cheap unit can carry decent glass and still perform badly because the spacer and frame are giving the heat a way round. When you compare quotes, compare the whole-unit U-value, not the centre-pane glass figure.
How the loss shows up in the house
You rarely see heat loss as a number. You feel it, and you see its side effects. The common signs, in the order people usually notice them:
- A cold downdraught under the rooflight. The cold inner surface chills the air against it, that air sinks, and you feel a draught even though nothing is leaking. Stand under an old rooflight in January and you can feel it on the back of your neck.
- Condensation on the glass and frame. When the inner surface drops below the dew point of the room, water forms on it. Morning misting that clears by midday is the cold unit doing its worst overnight.
- A room you stop using in winter. The extension that was the reason for the whole project becomes the room with the door shut from November.
- A heating bill that does not add up. The rooflight is a steady drain that runs day and night through the heating season. It is rarely the single line you can point at, but on a large or old unit it is a real share of the room’s heat demand.
Two of those signs, the draught and the condensation, are worth separating out, because a cold rooflight and a leaking perimeter feel similar from underneath and need completely different work. A cold unit needs replacing. A leaking perimeter needs sealing. And a thermally bridged upstand can produce mould around the reveal while the unit itself is fine. Getting the diagnosis right at survey is what stops you paying for the wrong fix.
What actually reduces the loss
The measures that work, in order of impact, are a better unit, an insulated upstand, and an airtight, vapour-controlled perimeter. Blinds and films come a long way down. We set that out in full on how you reduce heat loss through rooflights, and we are honest on whether new rooflights pay for themselves: comfort and damp prevention lead, fuel saving contributes rather than leads. If you want to know whether your own units are the problem, the tests for a rooflight losing heat are simple enough to do yourself before you spend anything.
How much heat does an old rooflight really lose compared with the roof?
An insulated roof runs at roughly 0.15 to 0.18 W/m²K. An early double-glazed rooflight runs at about 2.8 to 3.2, and an old single-glazed dome worse again. So each square metre of old rooflight sheds heat roughly fifteen to thirty times faster than the roof around it. Because it also sits at the warmest, highest point of the room and faces the cold sky, the real-world loss is worse than the bare number suggests.
Is a misting rooflight losing heat or leaking?
Usually losing heat. Condensation on the room side of the glass means the inner surface is dropping below the dew point of the room air, which is a sign of a cold, poorly insulating unit rather than water getting in from outside. Water appearing after rain, or a tide mark on the reveal, points to a leak instead. The two need different work, so it is worth confirming which you have before you spend.
Find out what your rooflights are costing you
We survey the unit, the upstand and the perimeter, tell you the whole-unit U-value of what is up there now against what a replacement would achieve, and give you one fixed price with the numbers written on it. The installers doing the work have 25 years fitting rooflights, and every installation carries a 10-year workmanship guarantee.
Request a fixed quote for a rooflight replacement in Ipswich, Suffolk or north Essex, or read the wider picture on energy efficient skylight installation first.
Thinking about this job? We survey across Ipswich, Suffolk and north Essex and give you a fixed price before any work starts, with a 10-year workmanship guarantee.