Rooflight U-Values and Part L Explained

Part L of the Building Regulations deals with saving energy in buildings. Its 2021 edition, in Table 4.2, lists what a new or replacement element must achieve when the house already exists. Only two rows of that table concern a roof.

The two numbers differ because the rules treat the products differently. A roof window is thought of as a window that happens to sit in a slope, so the window limit follows it. A rooflight is the glazed unit on a flat roof or the glazed part of a lantern, and its allowance is looser.

We choose each unit so that it clears the bar for its own category. The written quote names the unit, so you can hold it against the table above and see for yourself.

People often assume the stricter figure always applies, because both products are glass in a roof. It is the product type that decides, and there is a simple way to sort them.

A roof window in a Victorian terrace loft is an opening sash with its own flashing, and the window limit follows it. A flat roof rooflight over a newer estate extension falls under the rooflight limit. A roof lantern counts as a rooflight here too.

A conservation unit in a Georgian slate roof is a roof window in practice, because it sits in the slope. If you are unsure how yours is classed, the survey settles it before the quote is written. Our Building Regulations guide goes through the categories in more detail.

This is where a datasheet can mislead without meaning to. Glass performs best in its middle. At the edges it meets a spacer bar and a frame, and heat finds an easier path through both. A centre-pane figure ignores that and looks flattering. A whole-unit figure includes the edges and the frame, and it is the honest one.

Part L judges the finished window or rooflight, so a compliant unit is one whose whole-unit figure meets the limit. If a brochure shows only a centre-pane number, ask the maker or the installer which figure the compliance claim rests on.

The same logic explains why two units with identical glass can score differently. A frame that conducts heat well, or a spacer made of metal, drags the whole-unit figure up. A warm edge spacer is one of the cheaper ways to pull it back down, and it is worth reading about before you compare quotes.

A U-value is the end result of several choices, and knowing them helps you judge whether a figure is believable.

1. Number of panes. Two panes trap a layer of gas or air between them. A third pane adds a second layer, which is the subject of our page on double or triple glazed rooflights.
2. Coating. A thin low-emissivity layer on one pane reflects heat back into the room rather than letting it radiate out.
3. Gas fill. The gap between panes is often filled with a gas that conducts heat more slowly than air.
4. Spacer. The bar that holds the panes apart is a weak point, and its material affects the edge loss.
5. Frame. Timber, painted timber, polyurethane-coated timber and aluminium each conduct heat differently.

None of these needs a calculation from you. They are the reasons to ask for the whole-unit figure and the glazing specification together, rather than one on its own.

The rules shift depending on how much you replace, and it affects both cost and paperwork.

Swapping an entire rooflight or roof window, frame and glass in one go, counts as notifiable building work. It is handled through Building Control, and whatever goes in must reach the Part L figure for its type. We make the notification for you.

Replacing only the sealed glass unit into a frame that stays put does not usually need notifying. That is why a sealed unit that has misted is often swapped on its own. The frame has to be sound for this to make sense, and an old frame can limit what a new glass unit achieves, since heat still passes through it. Our service on misted and failed rooflights explains how we decide between the two routes.

A good quote lets you check the energy claim without a degree in physics. Look for these lines on any quote you receive, ours or anyone else's.

• The type of unit, named as a roof window or a rooflight, so you can match it to its limit.
• A whole-unit U-value, or a Window Energy Rating for a roof window.
• The glazing build-up: double or triple, coating, gas fill and spacer type.
• The frame material and finish.
• A note on whether the work is notifiable and who handles it.

If any of those are missing, ask. A figure with nothing behind it is hard to compare. Our quote is free, itemised and carries no obligation, and it sets the opening and fixed versions next to each other, so the energy figure can be weighed against the air it gives you. The costs page sets out the typical UK ranges for energy-efficient options.

The figures only mean something once they meet a real roof. Three common Bristol and Bath set-ups show how the same rules land differently.

A Victorian terrace loft in Bedminster or Bishopston. The rear slope usually takes a roof window, and the 1.4 figure applies. Because the room is used for sleeping, the inner surface temperature matters as much as the headline number, since a cold pane in a bedroom is where mist forms first. A coated double unit with a warm edge spacer often meets the limit comfortably, and a triple unit is an upgrade where the room is exposed.

A flat-roofed kitchen extension on a 1930s semi. A flat rooflight or lantern is the likely answer, and the 2.2 figure applies. There is more latitude here, so the sensible question becomes how much better than the limit you want to go. A large glazed area loses more heat than a small one, so the bigger the unit, the more a better figure pays back.

In Clifton or Bath, a Georgian roof. A conservation rooflight sits in the slope, so it works to the roof window figure. These units are slim, which helps the look but leaves less room for deep glazing, so the specification is worth discussing early. If the building is listed, listed building consent comes into it as well, which is a council matter.

Here is a plain comparison of the usual glazing routes. It shows direction, not exact figures, because the real number depends on the maker, the frame and the spacer.

The middle row suits many loft and extension roofs, because it reaches the limit and keeps the unit light enough for a standard frame. The top row is rarely offered where the floor-to-ceiling comfort is a priority. The bottom row adds weight, which can limit the opening types available and affects how a sash feels to lift.

That last point matters for us because we are in the business of opening rooflights. Heavier glazing can slow a manual sash or ask more of an electric motor, so the specification and the opening type are decided together at the survey, with the energy efficient rooflights options laid out beside each other on the quote.

It is tempting to chase the lowest figure on the list. That is sound within reason, but a U-value is one measure of comfort, not the whole of it.

In summer a loft room can overheat from the sun long before heat loss matters. A south-facing slope is where solar control glass earns its place, and a unit that opens helps warm air leave. In winter, condensation depends on how warm the inner surface stays and how much moisture the room produces, which our page on how to stop condensation on rooflights covers.

The question some homeowners put is whether a rooflight makes the house colder at all, and that answer looks at it directly. For the wider picture of cost against benefit, see whether an energy efficient rooflight is worth it. All of it sits under our energy efficient rooflights service, which pulls the options together.