A current double glazed rooflight with a low emissivity coating, an argon fill and a warm edge spacer typically lands somewhere around 1.2 to 1.4 W/m2K for the whole unit. The limiting standard for a replacement rooflight is 2.2, so the regulation is comfortably met and it is not what should decide the specification.
What a U-value measures
The rate at which heat passes through a square metre of the unit for each degree of temperature difference across it.
A lower number is better. It is a rate rather than a total, so it says nothing about how much heat a rooflight loses until it is multiplied by the area and by how cold it is outside.
Glass, frame, and the whole unit
Three different numbers, and quotes are not always clear about which is being given.
The centre pane figure describes the glass alone and is the best of the three. The frame is worse. The whole unit value combines them weighted by area and is the number that matters.
A quote citing a centre pane figure is quoting the flattering one, and it is worth asking which it is.
Why the frame matters so much
Because on a rooflight the frame is a large proportion of a small unit.
These are modest openings, so the perimeter is long relative to the glass area, and a bulky frame with a poor thermal break drags the whole unit figure well away from the centre pane one.
Which is one reason a slim framed unit can outperform a bulkier one carrying better glass.
The plane the value is quoted in
The detail almost nobody mentions and it changes the number.
A rooflight U-value can be declared in the horizontal plane or converted to the vertical plane, and the two are not the same figure for the same product.
Building regulations expect a particular basis, and comparing a horizontal figure from one supplier with a vertical figure from another is comparing nothing. It is worth asking which is quoted.
What the limiting standard is
For a replacement rooflight the limiting value is 2.2 W/m2K, which any current product beats comfortably.
For a rooflight in a new extension the standard is set alongside the rest of the fabric, and the detail is on do rooflights need to meet building regulations for heat loss.
Why the regulation is not the target
Because it is a floor rather than an aim, and every current unit clears it.
Which means asking whether a product complies is not a useful question. Asking what the whole unit figure actually is, and on what basis, is.
What makes the number good
Four components, and each is on its own page.
A low emissivity coating, which does most of the work. An inert gas fill. A warm edge spacer. And a frame with a genuine thermal break.
What the spacer does is on what is a warm edge spacer; the gas fill is on does argon leak out of a sealed unit.
Where triple glazing sits
Better on paper and not always better in practice on a roof.
A triple glazed unit will show a lower figure and it is heavier, thicker and more expensive, and on a roof those matter more than on a wall.
The decision is on is triple glazing worth it in a rooflight.
The number that is not on the label
Installation, and it is frequently worth more than the difference between two products.
A good unit fitted with insulation stopping short of the frame, and no continuity of the vapour control layer, loses heat around itself rather than through itself.
That loss does not appear in any U-value and it is real.
What a cold bridge at the reveal costs
More than most people would guess, because it runs the whole perimeter.
On a modest unit that perimeter is several metres of continuous path around the insulation, and it is also what produces water forming along a frame on winter mornings.
Insulation carried right up to the frame is the remedy, and it is done at installation or not at all.
Whether a lower number is always better
Not quite, and the exception is worth knowing.
Improving a U-value generally means more coating and more glazing layers, both of which reduce the light and the useful winter solar gain coming through.
On a north facing unit that trade is fine. On a south facing one the balance is different, and that is on the best glass for a south facing rooflight.
How much difference it makes in practice
Honestly, on one domestic rooflight, not much.
The difference between a good unit and an excellent one, over a heating season, on a single opening, is a small number against what the walls and the roof around it are doing.
It is worth specifying well and it is not worth agonising over.
Where it does matter
Two cases where the figure earns real attention.
A large glazed area, a lantern or several units, where the total area is no longer trivial. And a room that is difficult to heat already, where a poor unit is felt as a cold surface rather than measured as a loss.
What to ask a supplier
Three questions rather than a datasheet. What the whole unit U-value is, in which plane it is declared, and what the frame contributes. Any supplier who cannot answer all three is quoting a centre pane figure.
The other number on the same sheet
Worth noticing, because on a roof it frequently matters more.
The g-value describes how much solar energy the glass admits, and on a south facing roof it decides whether a room is comfortable in July.
What it is is on what is a g-value.
How the figure is built on the glazing
We quote whole unit figures on the basis the regulation uses, and we spend the effort on the insulation and sealing detail at the frame, because that is where the loss on a domestic rooflight actually is.
The wider picture is on the energy efficient rooflights page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.