A rooflight whole-unit U-value is an area-weighted average of three parts: the glass in the middle, the edge of the sealed unit where the spacer sits, and the frame. Each part has its own heat-loss rate, and the whole-unit figure blends them in proportion to how much of the unit each one covers. The frame and the edge lose heat faster than the glass, so the more frame and edge a unit has relative to its glass, the worse the whole-unit figure. That is why the same glass gives a different U-value in a different frame, and a worse one in a smaller size.
Understanding the calculation is the difference between comparing quotes properly and being sold a flattering centre-pane number.
The three parts of the calculation
Every rooflight is divided into three zones for the calculation, each with its own U-value:
- The centre-pane (Ug). The glass away from the edges. The best-performing part, and the flattering figure quoted on its own.
- The edge of the glass. A band around the perimeter of the sealed unit where the spacer bar sits. The spacer conducts heat across the edge, so this band loses heat faster than the centre. A warm-edge spacer narrows the penalty here.
- The frame (Uf). The surround that holds the unit. Worse than glass, and much worse if it has no thermal break.
The whole-unit figure is the areas of these three zones multiplied by their U-values, added up, and divided by the total area, with a term for the extra heat loss along the glass edge. That is an area-weighted average. The key consequence is that changing the proportions changes the answer, even with identical glass.
A worked example
Take one sealed unit and put it in two sizes. The glass and spacer are the same. Only the proportion of frame changes.
| Unit size | Share that is glass | Share that is frame and edge | Whole-unit figure trends |
|---|---|---|---|
| 600 x 600mm | Smaller | Larger | Toward the frame, worse |
| 1.2 x 1.2m | Moderate | Moderate | The published reference figure |
| 2.0 x 1.0m | Larger | Smaller | Toward the glass, better |
Illustrative. The exact figures come from the manufacturer’s calculation for each size.
Read across and the point is clear. A small unit is mostly frame and edge by proportion, so its whole-unit figure is dragged toward the frame’s poor performance. A large unit is mostly glass, so it sits close to the good centre-pane number. This is why manufacturers publish for a reference size, usually 1.2 x 1.2m, and why a 600 x 900mm bathroom unit performs worse than the brochure while a 2.0 x 1.0m side-return unit performs better. The full specification guidance is on specifying U-values and g-values for rooflights.
Why the frame and spacer matter so much
The frame and the edge are the parts people forget, and they are where a cheap unit hides. A rooflight can carry decent glass and still perform badly because the frame has no thermal break and the spacer is aluminium, giving the heat a way round the glass. Two components decide it:
- The frame’s thermal break. A thermally broken frame has an insulating barrier through the profile, so the inner face stays warm. Without it the frame is a heat exchanger. See what a thermally broken rooflight is.
- The spacer. A warm-edge spacer of stainless steel, foam or composite conducts far less than aluminium, improving the whole-unit figure by roughly 0.1 to 0.2 W/m²K and, more usefully, raising the edge glass temperature where condensation forms first.
Why angle also changes it. On top of the geometry, the declared figure depends on the angle the unit sits at, because convection in the cavity is stronger when the unit is near horizontal. So the same unit is declared differently in the vertical plane, used for Part L, and at the installed pitch. When you compare U-values, compare the same size and the same plane, or you are not comparing like with like.
What this means when you take quotes
Because the calculation blends glass, edge and frame over a specific size at a specific angle, a single unqualified U-value tells you almost nothing. Ask every quote for the whole-unit figure, the exact size, and the plane. A quote that only offers a centre-pane glass number is quoting the one part of the calculation that flatters the unit and leaving out the two that do not. This is exactly why two honest quotes for the same product can show different numbers, explained on the specification page, and why the Part L figure is a whole-unit one, covered on when Part L applies to replacing a rooflight.
Why does the same rooflight glass give a different U-value in a different frame?
Because the whole-unit U-value is an area-weighted average of the glass, the edge and the frame, and the frame has its own heat-loss rate. Put the same glass in a poorer frame, or in a smaller unit where the frame is a bigger share of the area, and the average shifts toward the frame’s performance, so the whole-unit figure gets worse. The glass has not changed. Its share of the total, and the quality of the surround, have.
Is the centre-pane U-value the one Building Control wants?
No. Building Control wants the whole-unit U-value, because that is the real performance of the thing installed in your roof. The centre-pane figure describes only the glass in the middle of the pane and ignores the spacer and the frame, so it always reads better than the unit actually performs. Always confirm the whole-unit figure for your exact size, declared in the relevant plane, when you compare quotes or evidence compliance.
Get the real figure for your size
We put the whole-unit U-value for your exact size and plane on the quote, alongside the g-value and the spacer type, so the calculation works in your favour rather than against it. A 10-year workmanship guarantee, and installers with 25 years on rooflights.
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