Hold a hand near an old single-glazed rooflight on a January night and the cold coming off it is easy to feel. Hold it near a modern sealed unit and the difference is obvious. The U-value is the number that puts a size on that difference, which is why every sheet of specifications for a roof window or lantern leads with it.
A U-value also invites mistakes. Two sheets can print two different figures for what looks like the same glass, and a low figure on a brochure does not always mean the unit in your ceiling will perform. The wider picture of choosing glass for heat and light sits on our energy-efficient rooflights page.
A U-value is a rate, so lower is warmer
The unit is watts per square metre per kelvin, written W/m²K. It records how many watts of heat flow through one square metre of a construction when the air on one side is one degree warmer than the air on the other. A kelvin step is the same size as a degree Celsius step, so you can read it as degrees.
That makes the maths easy to do by hand. Multiply the U-value by the area of the unit and by the temperature difference, and the answer is the heat flow in watts. A one square metre rooflight at 2.2 W/m²K, with the room twenty degrees warmer than outside, passes about 44 watts. At 1.4 W/m²K the same area passes about 28. A bigger unit multiplies both numbers in proportion.
Notice what the figure leaves out. It says nothing about daylight, solar heat, noise or how the unit looks. It also describes a steady state, with no wind and no sun, which is why two units with the same figure can feel different on a windy night. The value is a comparison tool, and a good one, as long as the two figures you compare were measured in the same way.
Reading the unit
W/m²K means watts of heat lost per square metre of the unit for each degree the room is warmer than outdoors. A lower number means a warmer room.
Centre-pane, glass-only and whole-unit figures
Look through a few data sheets and three different U-values appear, sometimes on the same page. They are not interchangeable.
The centre-pane figure
This describes a patch of glass in the middle of the pane, away from the edges. It is the most flattering number, because it leaves out the metal or plastic bar that holds the panes apart and the frame that holds them in place. Glass makers publish it because it is a clean description of the coating and the gas fill.
The glazing figure
Some sheets give a figure for the whole sealed glass unit, edge spacer included. It is a fairer number than the centre-pane one, but still stops at the glass.
The whole-unit figure
This covers the glass, the spacer and the frame together, and it is the one that tells you what the room will feel. It is also the figure the Building Regulations apply. A sealed unit with an excellent coating can sit in a frame that gives it away, and the whole-unit figure shows it.
Size matters here as well. A frame is a fixed width around every unit, so on a small roof window it takes a larger share of the area than on a wide lantern. A catalogue figure belongs to a stated test size, and a different size can carry a different number. We ask for the figure for the exact size we are fitting.
| Figure | What it covers | What it leaves out |
|---|---|---|
| Centre-pane | The middle of the glass only | Edge spacer, frame, kerb |
| Glazing | The whole sealed glass unit with its spacer | The frame |
| Whole unit | Glass, spacer and frame together | The roof build-up around the unit |
The limits that apply to an existing home
Approved Document L, the energy volume of the Building Regulations, sets the highest U-value allowed when glazing is added to or replaced in a house that already stands. In the 2021 edition, a new or replacement roof window takes the window limit of 1.4 W/m²K, or a Window Energy Rating of band B at least. A rooflight or roof lantern takes 2.2 W/m²K.
The document separates the two by how the glass sits. A rooflight is a glazed unit installed out of plane with the surface of the roof, on a kerb or upstand. A roof window is installed in the same orientation as the surrounding roof, in plane with it. A lantern is a rooflight. So the product you choose decides which number applies.
Both limits are ceilings, not targets. A unit that comes in under its limit passes, and a unit that comes in over it does not, however good the glass looks. Where the work is notifiable, we handle the Building Control notification, and the council or a registered building control approver checks it. Our guide to Part L and rooflights takes the document apart section by section, and the page on skylights and Building Regulations covers the notification route.
The Planning Portal publishes the national guidance on these rules at planningportal.co.uk, and the energy standards themselves are on gov.uk.
| Product | Limit for an existing dwelling | How it sits |
|---|---|---|
| Roof window | 1.4 W/m²K, or Window Energy Rating band B | In plane with the surrounding roof |
| Rooflight | 2.2 W/m²K | Out of plane, on a kerb or upstand |
| Roof lantern | 2.2 W/m²K | A rooflight, standing on an upstand |
What moves the number, part by part
Each part of a unit adds its own share to the whole-unit figure, and some are cheap to improve while others are not.
The coating on the glass is the single biggest lever. A low-emissivity layer is a very thin film on one surface of a pane. It sends warmth back towards the room while daylight passes through. Without it, a double-glazed unit is a modest improvement on single glazing. With it, the figure drops a long way.
The gap between the panes is the next. Argon conducts heat less readily than air, so a gas-filled gap does better than an air-filled one. The width of the gap is tuned by the maker, and wider is not always better, since once the gap passes a certain size the gas begins to circulate and carry heat across.
The spacer bar sets the temperature of the glass edge. Metal conducts heat around the rim, so the lower edge of a rooflight can run cold and mist first. A spacer made from a less conductive material narrows that cold band.
A third pane improves the figure again, at the cost of weight, which the frame and the roof structure have to carry. The frame itself matters too. An aluminium section with a thermal break, which is an insulating strip separating inner and outer faces, gives a warmer inner face than one without. uPVC conducts less to begin with. Finally, the kerb or lining around the unit, which sits outside the glass figure, decides whether warmth escapes at the junction.
Terms behind the number
- Low-e coating
- A very thin film on one pane that returns warmth to the room.
- Argon fill
- An inert gas in the gap between panes that conducts less heat than air.
- Warm-edge spacer
- A spacer bar made from a less conductive material than metal.
- Thermal break
- An insulating strip inside a metal frame that separates the warm face from the cold one.
- Window Energy Rating
- A letter band that rates a window on heat loss, solar gain and air leakage together.
A low U-value is half of the decision
Chasing the lowest figure can leave a room dim or hot. Two other numbers sit on a good data sheet, and they pull against the U-value in different ways.
The g-value, or solar factor, gives the share of the sun's energy that passes through the glass as heat. A low g-value keeps a south-facing loft bedroom from baking in July. A high one lets free warmth into a north-facing landing in March. The light transmission figure says how much daylight gets through, and a heavy coating or tint reduces it.
A bungalow hall with a single small rooflight is a good case to think through. It faces the road, gets no direct sun, and nobody sits there for long. A clear low-e unit that clears the 2.2 limit by a margin suits it, and a strong solar coating would just make the hall gloomier. A utility room under a southwest flat roof is the opposite case, where we would lean to a solar control pane and a vent.
That is why we choose the glass after looking at the roof's aspect and what shades it, and set the figures out together on the quote. Our short answer on whether skylights lose heat and the one on whether rooflights overheat a room look at each side of this in turn, and for material choices, our comparison of glass and polycarbonate for heat sets the two side by side.
A hall facing the road needs light more than solar control
Reading a data sheet or a quote line by line
When a sheet or quote lands on the doormat, go through it in a fixed order. The same check works on ours.
Start with what the number applies to. If the figure has no label, or the label says centre-pane, you cannot compare it with a whole-unit limit. Then look at the size it was measured for. A figure tested on a large unit may flatter a small one. Check the glass build-up written next to the figure, since a line such as 6.4 laminated inner, a 16 mm argon gap and a low-e coated pane tells you the thickness of each layer and what fills the gap. Last, check that the frame material and finish match what is written elsewhere on the quote.
If any of these is missing, ask for it before agreeing to the price.
Questions for any data sheet
- What does the figure cover? Centre-pane, glazing or whole unit, written on the sheet.
- Which size was it measured at? The figure should belong to the size you are buying.
- What is the build-up? Panes, coating, gas and spacer named beside it.
- What is the frame? Aluminium with a thermal break, uPVC or timber-lined.
- Does it meet the limit? 1.4 W/m²K (or Window Energy Rating band B) for a roof window, 2.2 W/m²K for a rooflight or lantern.
Comparing your old unit with a new one
A replacement has to meet the current limit, so it helps to know what you are replacing. Look at the edge of the glass for a stamped date or a maker's mark. Check whether the unit is double or single glazed, whether mist has formed between the panes and whether the frame is cold to the touch on a winter morning. A misted unit has a failed seal and may have lost its gas fill, so its real performance can fall below its original figure. Our page on misted rooflight replacement explains what fails.
The room tells us the rest. A loft study under a thin old roof window will warm up quickly once the unit changes, and condensation along the reveal often fades. A flat rooflight over an extension may need its kerb rebuilt as well, since a cold ring around good glass lets the heat out regardless of the figure.
Roof windows and lanterns are different products, so a pitched-roof option is on our roof windows page and the flat-roof ones are on roof lanterns. Our survey is free and the written quote is fixed, with the whole-unit figure for each unit named on it. A 10-year workmanship guarantee is given in writing. Questions before then go to us on WhatsApp, 07880 284133, or use the quote form.
