What a U-value actually measures
A rooflight U-value is the rate at which heat passes through one square metre of the unit for every degree of temperature difference between the room and the outside air. It is written in W/m²K, and a lower number means less heat escapes.
The idea is simple once the units are unpacked. Take a square metre of glazing, keep the room at 20°C and the air above the roof at 0°C, and the U-value tells you how many watts will flow through that square metre while the 20 degree gap holds. A skylight with a U-value of 1.3 W/m²K loses 1.3 watts per square metre for each degree, so 26 watts per square metre in that example. An old single-glazed unit rated somewhere near 5.5 to 6 W/m²K would lose well over four times as much through the same area.
Three things are worth holding on to from the start. First, a U-value is a steady-state figure: it describes heat flow when conditions are stable, not what happens when the sun comes out or the wind picks up. Second, it covers conduction, convection and long-wave radiation through the assembly, but not the sunshine coming in. Solar gain is a separate property, the g-value, and a unit can have an excellent U-value and still overheat a loft room in July. Third, a U-value is only as honest as the area and conditions it was calculated for, which is where most of the confusion around rooflight figures begins.
For comparison, a new roof built to current standards in an extension sits at around 0.15 to 0.18 W/m²K. Even a very good rooflight is several times less insulating than the roof around it. That is not a reason to avoid roof glazing, because the daylight has real value, but it is why the number matters and why it pays to read it properly. The energy efficient rooflights hub sets out the wider picture; this guide stays with the number itself.
Centre-pane, frame and whole-unit figures
A rooflight datasheet can carry three or four different U-values, and only one of them describes the unit you will actually fit. The glass figure is almost always the lowest and the most prominently printed.
The centre-pane value, written Ug, describes the sealed glass unit away from its edges. It is calculated for the glass build-up alone: the panes, the coatings, the gas in the cavity and the cavity width. It is useful for comparing glass options, and it is the number glass suppliers quote, but it ignores everything around the glass.
The frame value, Uf, describes the frame section on its own. Aluminium frames without a thermal break conduct heat very readily, while a well-designed thermally broken aluminium profile, a timber frame or a PVC frame performs far better. Our guide to thermally broken rooflight frames explains how the break works.
Between the two sits the edge of the glass, where the spacer bar holds the panes apart. This junction is described by a linear thermal transmittance, the psi value (Ψ), measured in W/mK because it applies per metre of edge rather than per square metre. A traditional aluminium spacer gives a noticeably higher psi value than a warm edge spacer.
The whole-unit value combines all three, weighted by area and edge length. For windows it is usually written Uw; for rooflights you will also see Ur or Ud. This is the figure that describes the product as installed, and it is the one Building Control is interested in.
| Figure | What it covers | Units |
|---|---|---|
| Ug | Glass centre only | W/m²K |
| Uf | Frame section only | W/m²K |
| Psi (Ψ) | Glass edge and spacer | W/mK |
| Uw / Ur | Whole rooflight | W/m²K |
Because the frame and edge nearly always perform worse than the glass centre, the whole-unit value is higher than Ug. How much higher depends on size. A small skylight has a lot of frame and edge relative to its glass, so its whole-unit figure drifts further from the glass figure than a large one does. A 600 mm square unit and a 1,500 mm square unit using identical glass can have quite different whole-unit U-values, and a brochure that quotes one value across a whole range is usually quoting a reference size or the glass alone.
Why the angle changes the number
Glass loses more heat lying flat than standing upright, so a rooflight's U-value measured horizontally is higher than the same unit measured vertically. Current guidance asks for the horizontal figure, which is why modern datasheets can look worse than older ones.
The reason is convection inside the cavity. In a vertical double-glazed unit, the gas between the panes circulates slowly in a single tall loop. Tip the unit flat, with warm glass underneath and cold glass above, and the warm gas rises straight across the narrow gap to the cold pane, gives up its heat, sinks, and repeats. That churning transfers heat far more efficiently than the lazy vertical loop, and the centre-pane U-value rises as a result.
The effect is not small. As a rough guide, a low-E argon-filled double-glazed unit that calculates at around 1.1 W/m²K vertically can come out somewhere near 1.5 W/m²K when assessed horizontally. Triple glazing shows the same pattern, often moving from around 0.6 vertically to somewhere in the region of 0.9 or 1.0 horizontally, depending on the cavities and gas. The exact shift depends on the build-up, but the direction is always the same.
For years many rooflight figures were quoted as though the glass stood upright, because that is how window glass is normally tested and the standard calculation defaults to it. The result was that a flat skylight could appear to perform as well as a vertical window when, in its real position on a flat roof, it did not. The 2021 edition of Approved Document L for England addressed this by asking for rooflight U-values to be based on the unit in the horizontal plane. A product whose figure looks slightly worse than the one on an older brochure may simply be telling the truth about how it will sit on your roof.
Pitched roof windows sit somewhere between the two cases. A window in a 40 degree slope behaves partly like a vertical unit and partly like a flat one, and manufacturers have historically declared roof window values in different ways. When comparing a roof window with a flat rooflight, check which angle each figure refers to before comparing the numbers at all.
- Outer toughened pane: heat-strengthened to take hail, foot traffic during maintenance and thermal stress.
- Solar-control coating (optional) on the inside face of the outer pane, cutting heat gain on south and west roofs.
- Argon-filled cavity: a denser gas than air, so less heat crosses the gap.
- Warm-edge spacer: a low-conductivity edge that keeps the units’ edges warmer and cuts condensation at the frame.
- Low-E coating: a microscopically thin metal layer on a cavity face that reflects room heat back inside.
- Inner laminated pane: two sheets bonded to an interlayer, so if it ever breaks, the glass stays in place overhead.
- Triple glazing only: a third pane and a second argon cavity, usually with a second low-E coating. Warmer, heavier, and it lets in slightly less light.
What the regulations ask for, in U-value terms
In England, Part L sets a limiting whole-unit U-value of 2.2 W/m²K for rooflights, assessed horizontally, and uses 1.7 W/m²K for the notional dwelling in new-build calculations. Most good modern units sit comfortably inside the limit.
The limiting value is the worst figure an individual rooflight can have when it is installed as new or as a replacement in an existing home. It is a backstop, not a target: meeting 2.2 W/m²K only confirms the unit is not unreasonably poor. The 1.7 W/m²K figure belongs to the notional dwelling, the reference building used in energy calculations for new homes, and it indicates the level designers generally expect to work from.
Two practical points follow. The figure the regulations care about is the whole-unit value, not Ug, so a glass specification alone does not demonstrate compliance. And because the assessment is horizontal, a supplier should be able to state the horizontal whole-unit figure for the size you are fitting, or at least for a representative size. Our separate guide to Part L requirements for rooflights covers how the rules apply to extensions, new homes and replacements in more detail, and whether a like-for-like swap is caught is answered in does a replacement rooflight have to meet Part L.
Regulations in Wales and Scotland are set separately and have their own values. For homes in Cambridge and the surrounding villages, the English Approved Document is the one that applies.
Reading a rooflight datasheet without being misled
A datasheet is reliable when you know which questions to put to it. Six checks separate a figure you can use from one that only looks good.
- Which U-value is it? If the headline number is labelled Ug, or simply "glass", it is the centre-pane figure. Look for Uw, Ur or "whole unit".
- Which angle? For a flat rooflight, the figure should say horizontal. If it says vertical, or says nothing, ask.
- Which size? Whole-unit values are calculated for a stated size. If your opening is much smaller than the reference size, expect a slightly higher figure.
- Is the upstand included? Some rooflights are supplied with an insulated kerb and quote a value for the complete assembly; others quote the rooflight alone and leave the upstand to the builder.
- Calculated or tested? Both are legitimate. A value calculated to the relevant BS EN ISO method or measured in a recognised test is what you want to see referenced.
- What spacer and gas? The same frame with a warm edge spacer and argon fill will perform better than with an aluminium spacer and air.
The Rooflight Association has published technical guidance on how rooflight U-values should be calculated and declared, and reputable manufacturers generally follow it. Asking a supplier to confirm the horizontal whole-unit value for your size is a reasonable request, and a clear answer is a good sign.
Turning a U-value into watts
Multiply the U-value by the area and by the temperature difference, and you have the heat flowing out through the rooflight at that moment. The sum is quick and makes the choice between two units much easier to judge.
Take a 1 by 2 metre flat skylight over a rear kitchen extension in a Romsey terrace. On a still January evening the kitchen is at 20°C and the air outside is at 0°C, a difference of 20 degrees.
- An older single-glazed unit at roughly 5.8 W/m²K: 5.8 × 2 × 20 = 232 watts.
- A modern double-glazed unit at 1.4 W/m²K horizontal: 1.4 × 2 × 20 = 56 watts.
- A triple-glazed unit at 0.9 W/m²K horizontal: 0.9 × 2 × 20 = 36 watts.
- The same 2 m² of new insulated roof at 0.15 W/m²K: 0.15 × 2 × 20 = 6 watts.
The biggest saving by far comes from leaving single glazing behind. Going from a good double unit to triple saves another 20 watts in this example, which is worth having but is a smaller step, and it comes with more weight and slightly less light. Whether that trade suits your room is the subject of our guide to double vs triple glazed rooflights.
The same arithmetic is what a heating engineer uses when sizing a heat pump. Each rooflight enters the room-by-room heat loss calculation as its whole-unit U-value multiplied by its area and the design temperature difference, which is why an accurate horizontal figure matters if you are planning a heat pump alongside a new extension. More on that in do rooflights affect heat pump sizing.
A U-value does not tell you about comfort directly, but it hints at it. The inner pane of a low U-value unit stays closer to room temperature, so you feel less cold radiating down from it and there is less of a cold downdraught across a sofa or desk underneath. The inner glass surface is also less likely to reach dew point, which helps keep condensation away on the coldest nights. Across the flat, open ground of the Cambridge area, where a clear winter night can drop well below freezing with little shelter from surrounding hills, that inner-surface temperature makes a real difference to how the room feels at 7am.
Where the U-value stops
A U-value describes the rooflight, not the opening around it and not the light that comes through it. Three things sit outside the number and still decide how the finished room performs.
The first is the upstand and the junction with the roof. A flat rooflight usually sits on a kerb raised 150 mm or more above the roof surface. If that kerb is a timber frame with little insulation, it can lose a surprising amount of heat around the perimeter, and the cold spot often shows up as a line of condensation or mould on the inside of the lining. An insulated upstand, properly continuous with the roof insulation, closes that gap. The same applies to a roof window in a pitched roof: the insulation collar and lining around the frame matter as much as the unit's own figure. We cover the whole path heat takes in rooflight heat loss in winter.
The second is installation quality. A unit with an excellent rating, bedded on a gappy kerb or fitted with air leaks round the frame, will underperform its datasheet. The number assumes a correct installation; it does not guarantee one.
The third is the light and the sun. The coatings and extra panes that lower a U-value also change how much daylight and solar heat come through. A soft-coat low-E layer, explained in our guide to low-E coatings and argon fill, reflects long-wave heat back into the room and only slightly reduces visible light. Adding a third pane takes a little more light away. None of this appears in the U-value, which is why we always read it alongside the g-value and light transmission figures. On a south-facing flat roof at Trumpington Meadows, the g-value is likely to matter more for summer comfort than a small difference in U-value. On a north slope in a De Freville loft, which never sees direct winter sun, the U-value carries more of the weight.
How the numbers feed a light plan
We treat the U-value as one line in the written light plan that comes with every quote, set against orientation, room use and the light you want. It is rarely the deciding factor alone, but it is never left out.
For a tired unit on a Mill Road side return, the conversation usually starts with what the existing rooflight is. Wired glass, single-glazed domes and early double-glazed units with failed seals all sit well above today's limiting value, and replacing them brings the biggest thermal step change available on the roof. Our guide to upgrading single-glazed rooflights goes through that case.
For a new extension, we look at the slope and the sky it faces. A north-facing roof light gains almost nothing from winter sun, so a lower U-value earns its keep all year and a triple-glazed unit may well be worth the extra weight. A south or west exposure brings strong summer gain, so we tend to spend the budget on solar control glass first and a sensible whole-unit U-value second. East-facing glass over a breakfast table sits between the two. The reasoning behind each is set out on our light planning page.
Whatever is chosen, the light plan states the whole-unit U-value in the horizontal plane for the size being fitted, the glass build-up behind it, and why that combination suits the room. We install to current Building Regulations and handle the Building Control notification where the work is notifiable, so the figure on paper is the one that goes on the roof. If you would like us to look at your roof and your room together, book a light survey or message us on WhatsApp.
Questions people ask
What is a good U-value for a rooflight?
Anything within the 2.2 W/m²K limiting value meets the minimum in England, but a good modern double-glazed flat rooflight will typically sit somewhere between about 1.2 and 1.6 W/m²K measured horizontally for the whole unit. Triple-glazed units can go below 1.0. Compare like with like: whole unit, horizontal, similar size.
Why is the skylight U-value higher than the glass U-value?
The glass figure, Ug, covers only the centre of the sealed unit. The frame and the spacer at the glass edge conduct more heat than the glass centre, so once they are included the whole-unit skylight U-value rises. The smaller the unit, the bigger the gap between the two.
Is a lower U-value always better?
For keeping heat in, yes. For the room as a whole, not always: the extra panes and coatings that lower the figure add weight and trim a little daylight, and they do nothing for summer overheating. The right choice balances U-value against light transmission, g-value and the direction the rooflight faces.
