Rooflight U-values and Part L, explained plainly

Glass is only part of a rooflight. Around it sits a frame, and between the panes runs a spacer bar that holds them apart. Heat escapes through all three, and the edge of a sealed unit gives up more than its middle does.

A centre-pane U-value measures the glass alone, taken well away from the edges. It flatters the product, because the frame and the spacer are left out. A whole-unit U-value takes in the glass, the frame and the spacer together, weighted by how much of the unit each one fills. For a roof window it is often written Uw.

The two can sit a long way apart. Excellent glass in a poor frame gives a handsome centre-pane figure and a duller whole-unit one. A small unit shows the gap more than a large one, because the frame makes up a bigger share of what you see. So a leaflet that quotes only the centre-pane number tells you less than it seems to.

It also explains why the regulations are written around the entire unit. What matters to the room is how much warmth leaves through the whole opening, not through the best square of glass in it.

Approved Document L draws a line between two products. Think of a roof window as the unit that follows the slope, the way the familiar pitched-roof units do. A rooflight stands out of the plane, up on a kerb or upstand. The two have different yardsticks, and the figures are never interchangeable.

In England, a new or replacement roof window in an existing home should have a U-value no worse than 1.4 W/m²K, or a Window Energy Rating of Band B or better. Both routes are open, so a data sheet that gives one without the other is not necessarily a problem. Whenever a figure is quoted for a roof window, ask whether the other half of the pair exists too.

Two companion points sit in the same document. New and replacement roof windows should be draught-proofed along with meeting the limit. A replacement should also be no worse than the unit it replaces, so a better old unit is not swapped for a poorer new one.

The document is guidance. It describes one accepted way of meeting the Building Regulations, which is why the wording says should, and why the local building control team or registered building control approver has the last word on a job.

A rooflight in an existing English home is held to 2.2 W/m²K: the document says the U-value of a new or replacement one should be no worse than that. A flat rooflight and a roof lantern both come under that figure, since a lantern is a glazed unit raised on a kerb and so falls inside the document's definition of a rooflight. No band is offered as an alternative route for a rooflight.

The area the figure is judged over catches people out. A rooflight's U-value is based on its whole outer developed surface area, which is often larger than the hole in the roof. Deep sides on a lantern, or a tall upstand, add surface that the opening on a drawing does not show.

That is where the kerb comes in. Where the kerb comes as one assembly from the same supplier, the 2.2 limit covers the kerb too. An upstand built on site is treated separately: it should be no worse than 0.35 W/m²K.

Our flat roof rooflights page and the lantern pages deal with how the kerb is chosen. For the number itself, what counts is that the kerb is named in the same breath as the glass.

A Window Energy Rating is a banded label rather than a single U-value, and Approved Document L offers it as a route for windows and roof windows, not for rooflights. It is built to take in more than heat loss alone, because a window also lets in useful warmth from the sun and can leak air around its edges. The label gives a band letter, and Band B is the minimum Approved Document L accepts as an alternative for a roof window in an existing home.

The band system has one attraction: it rewards a unit that does several jobs reasonably well. A roof window with a middling U-value but good solar performance can earn its band where a bare comparison of numbers would sell it short. For the same reason, a band letter is no use for comparing a roof window with a flat rooflight, because only the roof window has that route open to it.

When both a band and a whole-unit U-value are printed for a roof window, they are two ways to the same check, and either can carry the product. If a maker gives only the band on the data sheet, the whole-unit U-value is worth requesting as well, so that the two can be read together at survey.

Fitting a new window, roof window or rooflight counts as building work under the Building Regulations, because it is a controlled fitting. Swapping out an entire rooflight counts as a controlled fitting as well. Building Regulations approval will generally be needed for a new rooflight, and replacing roof windows usually calls for it too.

One exception catches people out. Replacing only the glass, or a window in its existing frame, is not providing a controlled fitting, so the energy limits do not apply to that work. Take a sealed unit swapped inside a sound frame: that is a different job from a new unit and new frame, and the paperwork follows the difference.

On glazed area, Approved Document L gives a guide: where a window or roof window is enlarged or a new one created, the total area of windows, roof windows, rooflights and doors should not exceed 25% of the home's total floor area, unless compensating energy-efficiency measures are taken. Extensions have their own, separate test, so the whole-home figure is not applied to them.

For the unit you settle on, where the work is notifiable, we handle the Building Regulations notification with the council's building control team or a registered building control approver. Full detail on approvals sits in skylight building regulations.

Data sheets are written for specifiers, and a homeowner can find them hard going. A few habits turn them into something useful.

Start with which glazing the figure belongs to. A range can offer several specifications of one frame, each with its own number, and a brochure may print the best of them in large type. Next, look at the size. Some makers quote a figure for one reference size, and a smaller or larger unit can land differently. After that comes the subject of the figure: centre-pane, or whole unit. If the sheet uses a symbol rather than a word, the maker's notes should say what it stands for.

For a rooflight, check whether the kerb is inside the figure, since the 2.2 limit counts it when it is supplied as one assembly. For a roof window, check that the band and the U-value belong to the same specification. Quotes that skip these questions are quotes that cannot be compared.

Numbers matter less than how a room feels. A unit that holds heat back keeps its inner pane warmer, which spares you the chilly patch overhead, and the damp air from cooking and washing is less likely to settle on the glass. The link between a warm surface and condensation is covered in condensation on rooflights.

A lower figure does not settle everything. A south-facing extension may need solar control glass more than a marginal gain in warmth, and whether a third pane earns its weight is a separate question, taken up in triple glazed rooflights. Better glazing also moves the price, which the cost answer explains.

Our fitting comes with the 10-year workmanship guarantee, in writing. At handover you are talked through the controls, the choice of blinds and the cleaning of the outer glass, because an efficient unit is only as good as the habits it is used with: a vent opened in a steamy kitchen, a blind drawn against a hot afternoon.