What U-value should a rooflight have?
For a rooflight going into a house in Essex, the figure to aim for is a whole unit U-value of around 1.4 W/m²K or better on a double glazed unit, and somewhere near 1.0 if the make-up is triple. Above about 1.6 the unit becomes the surface you notice on a cold morning. Below about 1.0 you are paying for a number that the rest of the roof cannot make use of.
That is the short answer. The rest of this page is about which figure the number refers to, what Building Control actually checks, and how far past the minimum it is worth spending. The wider specification sits on the energy efficient rooflights page.
The figure has to include the frame
Glass makers quote a centre pane value, measured away from the edges of the sealed unit where nothing is conducting except the glass, the coating and the gas. It is a real number and it is not the one to buy on. The whole unit figure adds the frame, the spacer bar and the cold edge of the pane, and it is always worse.
The gap between the two is routinely three or four tenths of a watt. A unit advertised at 1.0 at the centre of the pane can land at 1.4 or 1.5 as a finished rooflight. When a supplier quotes a single number with no qualifier, ask which of the two it is before comparing it with anything.
The ceiling a replacement has to clear
Approved Document L sets a limiting whole unit U-value for rooflights in existing dwellings, and a replacement has to satisfy it. Every mainstream double glazed unit currently sold clears that limit without effort, which is why the Regulations rarely decide the specification on a domestic job.
What they do decide is the paperwork. Fitting a new rooflight or swapping an old one is notifiable work, so the declared figure has to be evidenced against a completion certificate. Sunspire handles the notification as part of the installation, which means the glazing specification is recorded at sign-off rather than reconstructed from memory when a buyer’s solicitor asks for it eight years later.
What good looks like, unit by unit
The type of rooflight moves the achievable figure as much as the glass does, because frame depth and the ratio of frame to glass change with the format.
| Rooflight type | Sensible target | What limits it |
|---|---|---|
| Flush flat rooflight, double glazed | 1.3 to 1.5 W/m²K | Horizontal cavity, slim frame section |
| Flush flat rooflight, triple glazed | 0.9 to 1.1 W/m²K | Weight, and the kerb carrying it |
| Roof window in a pitched slope | 1.1 to 1.3 W/m²K | Timber or polyurethane frame helps |
| Roof lantern, thermally broken | 1.2 to 1.6 W/m²K | Bar count and the break depth |
| Old twin skin polycarbonate dome | 2.8 to 3.2 W/m²K | Included for the comparison |
The last row is the reason a straight swap of a nineteen eighties dome is the single biggest thermal step available on an Essex roof.
How far past the minimum is worth paying
Improvement runs into diminishing returns quickly. Going from 2.0 to 1.4 removes roughly a third of the loss through that opening. Going from 1.4 to 1.0 removes about another third of what is left, but the absolute saving is much smaller because the starting number is already low.
The area involved is what settles it. A single roof window is perhaps a square metre of a house losing heat through a hundred. Chasing the last two tenths there returns very little. On four square metres of lantern over a kitchen that is occupied all evening in January, the same two tenths is felt as a temperature at head height rather than seen on a bill.
The angle the figure was declared at
The same rooflight can carry two honest U-values, because glazing is assessed at different angles and a cavity lying near horizontal convects more freely than one standing upright. The horizontal declaration is always the less flattering of the two.
A unit stated at 1.2 in the vertical plane may be nearer 1.6 lying flat. Comparing one supplier’s vertical figure against another’s horizontal figure is the most common way a rooflight comparison goes wrong, and it is fixed by one question on the phone.
The loss the U-value never counted
A U-value describes the unit and stops at its outer edge. It says nothing about the kerb the unit sits on, the trimmer around a pitched opening, or the junction between the frame and the ceiling.
On a small rooflight in a well insulated roof, the perimeter can lose as much heat as the glass. A timber upstand standing 150mm proud of an insulated deck is a cold bridge running right around the opening unless the insulation line is carried up its outer face. That detail costs very little during installation. It is one of the four invisible details that the ten-year workmanship guarantee is built around, and it never appears on a glass data sheet.
Matching the target to the house
The right number depends on what it is competing against. In a solid wall Victorian terrace in Chelmsford or Colchester, the fabric loses heat everywhere and an excellent rooflight is a small share of the total, so a good double glazed unit at 1.4 with a properly insulated kerb is the sensible spend.
In an interwar semi that has been cavity filled and loft insulated, the rooflight becomes the weakest element in the roof and lifting it pays. On a tight trussed estate house at Beaulieu Park or Great Notley, the air leakage detail around the opening matters more than the last tenth of a watt, because the rest of the envelope is already sealed.
Getting the number written down
A performance figure only exists if it is on the order before anything is made. The line to insist on names the whole unit U-value in W/m²K, the plane it is declared in, the glass make-up pane by pane, the cavity width and gas fill, the spacer type, and whether a metal frame is thermally broken.
Then check the delivery against it. Sealed units carry an etched or printed reference on the spacer bar inside the cavity, and it should match what was ordered. Reading it takes a minute while the unit is still on the ground.
Tell us about your roof
We start with the building, the covering and the slope. Then we tell you what will suit it, and what it will cost, as a fixed written number.
- Surveyed before it is priced
- 10-year workmanship guarantee
- Building Control notification handled
- New installations and replacements