Specifying U-Values and g-Values for Rooflights
When you specify a rooflight, insist on the whole-unit U-value, the size it applies to, and the plane it was declared in, all in writing. The whole-unit figure includes the glass, the spacer at the edge of the sealed unit and the frame, so it is the real performance. The centre-pane figure describes only the glass in the middle of the pane and always reads better, because it ignores the spacer and frame entirely. On top of that, the declared figure changes with the size of the unit and the angle it is installed at, so two quotes for the same product can show different numbers and both be honest. If you compare centre-pane against whole unit, or a large size against a small one, you are not comparing like with like.
This is the single most useful thing to understand before you take rooflight quotes, because it is where cheap units hide and where good ones prove themselves.
Centre-pane against whole unit
The centre-pane U-value, sometimes written Ug, describes the glass alone, measured in the middle of the pane well away from the edges. It is the flattering number. It ignores the spacer bar around the perimeter of the sealed unit, and it ignores the frame entirely.
The whole-unit U-value, written Uw or given as Urc for a rooflight, includes the glass, the spacer, the frame and the way they interact. It is the number Building Control cares about, and it is the only one worth comparing between quotes. The gap between them is not trivial. A sealed unit with a centre-pane figure of 1.0 can land at a whole-unit figure of 1.4 or 1.5 once the frame and spacer are included, and worse than that on a small unit.
The trap in one line. If a quote gives you a single U-value with no qualifier, it is very often the centre-pane glass figure, because that is the one that reads best. Ask which it is. On a good quote it says whole unit, in writing, next to the size. If the installer cannot tell you, they have not read the datasheet, and the rest of their specification is unlikely to be better thought through.
Why unit size changes the answer
Frame is worse than glass, thermally. Every rooflight is therefore a weighted average of a good bit, the glass, and a bad bit, the frame and edge, and the ratio between them depends on size.
- A 600 x 600mm rooflight is mostly frame and edge by proportion, so the whole-unit figure is dragged toward the frame’s performance.
- A 2.0 x 1.0m rooflight is mostly glass, so the whole-unit figure sits close to the good centre-pane number.
The consequence is that the same product, in the same specification, declares a materially worse whole-unit U-value in a small size than in a large one. Manufacturers usually publish the figure for a reference size, often 1.2 x 1.2m or 1.0 x 1.0m. If you are buying a 600 x 900mm unit for a bathroom, the real figure is worse than the brochure. If you are buying a 2.0 x 1.0m unit for a side return, it is better. This is also why a warm-edge spacer matters more on small units, where the perimeter is a large share of the area. The full calculation is on how rooflight U-values are calculated.
Why installation angle changes the declared figure
This is the part almost nobody explains. Heat loss through a sealed unit depends partly on convection in the gas-filled cavity between the panes, and convection behaves differently depending on the angle of that cavity.
In a vertical window the gas in the cavity circulates in a fairly stable loop. Tilt the unit toward horizontal and the warm gas at the lower pane is now directly under the cold upper pane, which drives stronger convection, moves more heat, and raises the U-value. A sealed unit lying nearly flat loses more heat than the identical unit standing upright.
So rooflight U-values are declared against a stated angle, and the convention differs between standards. Rooflights in England have historically been assessed in the vertical plane for Part L purposes, a deliberate simplification that lets a rooflight be compared against a window. The EN 1873 and EN 14351 product routes declare figures at the installed pitch, often a shallow angle for a flat rooflight. A manufacturer can honestly publish 1.0 for a unit that, at the 5-degree pitch it will actually sit at, behaves more like 1.3. Neither number is a lie. They answer different questions.
| Variable | Flattering end | Honest comparison |
|---|---|---|
| What is included | Centre-pane, glass only | Whole unit: glass, spacer and frame |
| Unit size | Large reference size, mostly glass | The actual size you are buying |
| Declared angle | Vertical plane, less convection | The plane relevant to your job, stated |
g-value, and specifying it alongside
The g-value belongs on the same specification. It is the proportion of solar energy that passes through the glazing, from 0 to 1, and it decides whether the room overheats in summer. Clear glass sits at 0.60 to 0.72, solar control at 0.28 to 0.38. You specify it from orientation, not from the catalogue, and it is covered in full on solar control glazing for rooflights and defined on what g-value means on a rooflight. A complete rooflight specification carries the whole-unit U-value, the g-value and the light transmission together, because optimising one without the others gives you a warm room that bakes or a cool room that is dark.
What to ask a supplier to confirm in writing
Copy this and send it with any rooflight enquiry:
- The whole-unit U-value, not centre-pane, for the exact size I am buying.
- The plane it was declared in, vertical or the installed pitch, so I can compare quotes on the same basis.
- The g-value and light transmission for that unit.
- The spacer type, warm-edge or aluminium.
- The gas fill, air, argon or krypton, and the cavity width.
- Whether the frame is thermally broken.
A supplier who can answer all six in writing has read the datasheet. One who gives you a single unqualified number has not. On our quotes those figures appear next to the size as a matter of course, because three quotes with no common basis is not a comparison, it is a guess.
Why do two quotes give different U-values for the same rooflight?
Almost always because one is quoting centre-pane and the other whole unit, or because they are quoting different sizes, or because one figure is declared in the vertical plane and the other at the installed pitch. Centre-pane describes the glass in the middle of the pane and ignores the spacer and frame, so it always reads better. Ask every quote for the whole-unit figure, the reference size and the plane. If nobody can tell you, nobody has read the datasheet.
What is the difference between U-value and g-value?
U-value measures heat loss, the rate at which heat escapes through the unit, in W/m²K, and lower is better for keeping the room warm. g-value measures solar gain, the proportion of the sun’s energy that passes through the glass, from 0 to 1, and lower keeps the room cooler in summer. A complete rooflight specification states both, plus the light transmission, because you want low heat loss and controlled solar gain without making the room dark.
Get the numbers written on the quote
We put the whole-unit U-value, the size it applies to, the plane, the g-value, the light transmission, the spacer and the gas fill on every quote. One fixed price, a 10-year workmanship guarantee, and installers with 25 years on rooflights.
Request a fixed quote, read meeting Part L with replacement rooflights, or start from energy efficient skylight installation.
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