01Verdict
They can be, if the glass and the build-up are right
A slim frame and a period look do not mean a poor U-value, but you have to ask for the numbers.
Are conservation rooflights energy efficient? A good one is, and it can match the rooflights fitted to a new extension. The shape of the frame is not what decides it. What decides it is the sealed glass unit, the edge spacer, the thermal break in the frame, and how the whole thing is joined to the roof insulation around it.
The measure to ask about is the U-value: the rate heat passes through the glazing, in watts per square metre for each degree of difference between inside and out. Lower is warmer. Good double glazed conservation rooflights commonly quote somewhere around 1.4 W/m²K or better, and triple glazed or vacuum-sealed units go lower. For comparison, the Building Regulations limit for a replacement rooflight is 2.2 W/m²K, so a modern conservation unit has plenty of headroom. Our guide to rooflight U-values explained shows what the numbers mean in practice.
- 2.2W/m²K, the current limit for a replacement rooflight
- 1.4W/m²K or better, a good double glazed conservation unit
- 0.15 to 0.18W/m²K, a well insulated roof, for comparison
02Slim frame
What a thin profile does to heat loss
Less frame means more of the opening is glass, which has pluses and minuses.
A conservation rooflight has a narrower frame than a box-style roof window. That gives more daylight for the same hole in the roof, which is the point. It also means the frame is a smaller share of the area, and in a well made unit the frame is thermally broken: it has a layer of low-conductivity material between its inner and outer metal, so cold does not run straight through. Poor ones skip that, and a bare metal frame in a heated room will bead with condensation in winter.
The glass is the other half. A sealed unit with a low-e coating (a thin, invisible layer that sends warmth back into the room) and an argon-filled gap is the sensible baseline. The spacer bar at the edge should be warm-edge, a composite rather than plain aluminium, and we fit warm-edge glazing as standard. Some makers push slimmer double glazed units to keep the frame thin, so it is worth checking that the narrower build-up has not cost you a worse U-value.
What to ask a maker or fitter
- U-value
- For the whole rooflight, in writing
- Glazing
- Double or triple, low-e, argon filled
- Spacer
- Warm-edge, not bare aluminium
- Frame
- Thermally broken metal
- Glazing bars
- Applied outside, or inside the sealed unit
03Glazing bars
Bars, opening and other choices that affect warmth
The details that make a rooflight look right can move the thermal numbers a little, so weigh them up.
Glazing bars give the traditional look. Bars stuck onto the glass, or fitted between the panes, do not break the sealed unit and have almost no effect on heat. Separate small panes with real bars between them mean more edges, and every edge is a place where heat can escape, so the overall figure is a touch worse. Our page on glazing bars and sizes covers how the looks and the performance trade off.
Opening is another question. A rooflight that opens has seals that have to hold shut, and a hinge and catch to add. It can be efficient when closed, and it lets you ventilate the room on a warm day. Can conservation rooflights open explains the options, and a fixed unit is simplest if you only want light.
| Choice | Heat effect | Worth it when |
|---|---|---|
| Triple glazed unit | Lowest U-value, heavier | The room is heated and you want it snug |
| Applied glazing bars | Almost none | You want the period look |
| Separate panes with bars | Slightly worse | The council asks for it |
| Opening version | Seals need to be good | You want ventilation |
04Fitting
The join to the roof is where it is won or lost
A top-quality unit set into a gap in the insulation will feel colder than a plain one fitted well.
A rooflight cuts a hole in the roof insulation, and the insulation has to meet the frame all the way round without a gap. Inside, the lining and a vapour control layer keep warm, damp room air from reaching cold timber. We handle the whole fitting with one team, and the reasoning is set out on our warm specification page.
On the rules, replacing a whole rooflight is a controlled fitting under Part L and is notifiable to Building Control, which we handle where the work is notifiable. Whether your house needs permission at all is a separate question, covered by our planning permission guide. For a listed building you always need listed building consent, and in some conservation areas an Article 4 direction may remove permitted development, so check with the council.
Our main conservation rooflight installation page shows how we go about the job, with a free survey and written quote to start.
Do you lose warmth by choosing the slim heritage look?
Is a conservation rooflight colder than a normal one?
No, not if it is built to a good specification. Thermal break, low-e glass, argon and a warm-edge spacer are available in both. The frame shape is not the deciding factor.
What U-value should I look for?
Anything at or better than the Building Regulations limit of 2.2 W/m²K for a replacement. We would aim lower, and our page on what U-value a rooflight should have explains why.
Will it save on my heating?
Replacing an old, single glazed or failing skylight with a modern unit should cut heat loss at that opening. Adding a new opening adds a little loss, which daylight partly offsets.
Does a listed building have to settle for worse efficiency?
Not necessarily. Listed building consent controls how it looks, and many conservation-style units are also well insulated. Ask the council what they will accept first.
