What U-value should a rooflight have?
No worse than 2.2 for a replacement. Any decent modern unit is around 1.2 to 1.4.
Part of our Energy Efficient Rooflights guide
For a replacement rooflight in an existing dwelling the current standard expects a whole unit U-value no worse than 2.2.
That is not demanding. A modern double glazed unit with a low emissivity coating, argon fill and a warm edge spacer lands around 1.2 to 1.4, comfortably inside it.
Compare the right number
The trap in comparing quotations. Centre pane U-value measures the glass away from the edges and ignores the spacer and frame. Whole unit includes all of it and is what the regulations use.
Whole unit is always worse, often by 0.4 or more. A quotation showing 1.0 and another showing 1.4 may be describing the same product measured two ways.
Ask for whole unit figures, or you are comparing nothing.
Where it matters more
New extensions. The rooflight forms part of the whole extension calculation, so a poor one forces better performance in the walls or roof to compensate. Getting the glass right is cheaper than compensating elsewhere.
Large glazed areas. The absolute loss scales with area, so on a big lantern the U-value is worth more attention than on a small utility rooflight.
Where it matters less than people think
Chasing from 1.2 to 0.8 with triple glazing. Real, modest, expensive, heavier, and it transmits less light.
The number nobody quotes
The kerb. There is no U-value on a quotation for the upstand, and an uninsulated one frequently loses more heat than the glass. Ask whether it is insulated.
The regulated number
For a replacement rooflight in an existing dwelling, Part L expects a whole unit U-value no worse than 2.2 watts per square metre kelvin. That is the compliance threshold rather than a target.
What you should actually expect from a current unit is 1.2 to 1.4. Any decent modern double glazed rooflight with a low emissivity coating, argon fill and a warm edge spacer lands in that range, which clears the requirement with a wide margin.
Which number you are being quoted
This is where quotation comparison goes wrong, because two figures describe the same product.
Centre pane measures the glass away from the edges. It ignores the spacer bar and the frame, which is exactly where heat escapes. It is always the better figure and it is what marketing tends to quote.
Whole unit includes glass, spacer and frame weighted by area. It is what the building experiences and what the regulations use. It is always worse, frequently by 0.4 or more, and on a small unit with a lot of frame relative to glass the gap is wider still.
A rooflight advertised at 1.0 and another at 1.4 may be identical performance described two ways. Ask both installers for whole unit figures, or you are comparing nothing.
Where the heat actually leaves
On a typical installation, not through the middle of the glass.
The spacer bar. A traditional aluminium spacer conducts heat extremely well, bridging the two panes around the whole perimeter. The visible result is condensation forming as a band around the edge of the glass on cold mornings while the centre stays clear, and it is regularly mistaken for a failed unit. A warm edge spacer in stainless or structural foam runs several degrees warmer and that edge misting largely disappears.
The kerb, on a flat roof. The larger loss, and it appears on no specification. A 1200 by 1800 rooflight has six metres of perimeter, and an uninsulated timber kerb punching through the roof insulation presents close to a square metre of poorly performing envelope at a U-value several times worse than the glass sitting on it.
Insulating the outer face of the kerb is a sheet of rigid board and an hour. It is invisible once the covering is dressed over it, which is precisely why it gets skipped.
Should you go lower than 1.2
Usually not. Triple glazing takes the whole unit figure to around 0.8, which is real and modest. It is heavier, which affects the actuator on an opening unit and sometimes the structure. It transmits noticeably less daylight, which works against the reason you fitted a rooflight. And it costs substantially more.
The same money spent on solar control glass where it faces the sun, a properly insulated kerb and a sealed vapour control layer does more for how the room feels than the third pane will.
Where the number matters most
On an extension, where the rooflights form part of the thermal calculation for the whole structure and a poor unit forces better performance somewhere else in the build. Getting the glass right first is cheaper than compensating for it in the walls.
The number to write on the quotation
Whole unit, with the model and glazing specification alongside it. Ask for it in writing rather than accepting a figure quoted verbally, because centre pane and whole unit are frequently used interchangeably in conversation and they are not the same thing.
Anything at or below 1.4 whole unit is a good modern rooflight. The regulated threshold for a replacement is 2.2.
Ask us
Send the specification you have been quoted and we will tell you whether the figure is centre pane or whole unit. It is the single most common reason two rooflight quotations look further apart than they are.
Where the edge condensation comes from
A traditional aluminium spacer bar conducts heat around the whole perimeter of every pane, so the edge of the glass runs several degrees colder than the centre. The result is a band of condensation around the edge on cold mornings while the middle stays clear, and it is regularly mistaken for a failed unit when it is simply a cold spacer.
One last point
Anything at or below 1.4 whole unit is a good modern rooflight. The regulated threshold for a replacement is 2.2, so the question is rarely whether a unit complies and usually whether you are comparing like with like.
More on Energy Efficient Rooflights
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