Whole-unit or centre-pane U-value: the figure worth comparing

A U-value says how readily heat passes through a surface. A lower figure means less heat lost. Every rooflight has one, and most brochures print at least two.

The trouble is that a rooflight is not a uniform slab. It is a patch of glass with a border. Heat leaves through the middle of the pane at one rate, through the edge of the glass at another, and through the frame at a third. Lab tests can measure each part separately, and the maker can then publish whichever number sells the product best.

The centre-pane figure is the glass at its finest: the coating, the gas fill and the gap at work, with no spacer bar, no frame and no corners in the way. The whole-unit figure is the honest one, because it adds those parts back in. It is what your room actually loses.

Think of a winter coat. The fabric on your chest may be superb. What matters is what happens at the collar, the cuffs and the zip.

Four elements sit inside the whole-unit number, and each one can drag it upwards.

• The glass in the middle, which is the centre-pane part.
• The glass edge, where the spacer bar sits between the panes. A cheap metal spacer conducts heat around the rim, and a warm edge spacer slows it.
• The frame, which can be aluminium, timber, PVC or composite. An aluminium frame without a thermal break carries cold straight through.
• The junction with the roof, which most published figures do not cover at all.

A small rooflight has more edge and more frame for its glass area than a big one. That is why a modest unit can have a lovely centre-pane figure and a disappointing whole-unit figure: the perimeter takes a bigger share of the total. Our guides on warm edge spacers and argon fill and thermally broken rooflight frames explain the two parts that change the result most.

Sales material often leads with the glass number. A few habits help you see past it.

First, look for the words. "Glass U-value", "Ug" and "centre-pane" mean the glass alone. "Uw", "whole unit", "overall" or "U-value of the complete rooflight" mean the unit as a whole. Where a sheet gives one figure and no label, ask which it is before doing anything else.

Second, check the size the figure was calculated for. Makers often quote a standard test size. The same product in a smaller format scores worse, for the reason given above, so a figure for a large unit tells you little about a small one.

Third, compare like with like. A centre-pane figure from one maker set beside a whole-unit figure from another is not a comparison at all. If one supplier publishes only the flattering number, that is a reason to ask for the other.

For the plain basics, our page on rooflight U-values explained starts from zero, and how rooflights are rated shows where the published numbers come from.

Building Regulations work with the performance of the complete element, not the glass in isolation.

Part L sets limits for new and replacement rooflights and roof windows, and those limits refer to the unit as a whole. The figure tested and calculated to the recognised European standards for the product is the one that counts, which is why makers publish it. A compliant centre-pane number on its own proves nothing about the finished window. We work from the whole-unit figure whenever we check a product against the current rules, and we install to current Building Regulations. Our page on Part L and rooflights goes into what the energy rules ask for, and whether a swap has to meet them is answered in does a replacement rooflight have to meet current U-value rules.

The glass makes both differences, so the question of which figure to read is also a question about sound.

A laminated acoustic pane can be part of a well-insulated unit, and the whole-unit number will reflect it, because the extra sheet is inside the build. We write the glass, the spacer, the frame and the seals into the quiet spec that comes with every quote, so you can see the parts behind any headline figure. In the loft bedrooms and kitchen extensions around Crawley, a room under a poorly built rooflight is cold at the edge of the ceiling and loud in a storm, and both faults often trace to the same thin pane and loose frame. Our page on acoustic laminated glass and U-value covers the overlap.

The full range of what we fit is on our energy efficient rooflights page.

Can a rooflight have a good centre-pane figure and still feel cold?

Yes. If the frame has no thermal break or the spacer conducts heat, the edge of the ceiling feels cold whatever the middle of the glass does. Our page on why a skylight makes the room feel cold looks at the causes.

Does triple glazing fix a poor whole-unit figure?

Not on its own. A third pane improves the centre of the glass, but a weak frame and spacer still set a floor under the result. See when the third pane earns its place.

Why does the same rooflight have different figures in different places?

The figure changes with size, frame material and calculation method. Ask for the number for your actual size, not a sample size.

Will a lower U-value cut my heating bill?

It helps, but it is one part of the picture with heating habits and the rest of the roof. We discuss it in will energy efficient rooflights lower my heating bills.

If you have a datasheet that does not say which figure it shows, get a quote and we will read it with you.