Rooflight U-Values Explained in Plain Terms

A U-value is a rate of heat loss. It is written in watts per square metre per degree of temperature difference, shortened to W/m²K. Take one square metre of the rooflight. Make the air inside one degree warmer than the air outside. The U-value is the number of watts that flow through that square metre to close the gap.

So the scale runs the way people expect once you know it is about loss. A high figure means the unit leaks heat quickly. A low figure means it holds heat in. A single sheet of ordinary glass has a high figure. A sealed double glazed unit with a coating and a gas fill has a much lower one. A good triple glazed unit with a warm frame is lower again.

Why it is a rate and not a grade

The number is not a score out of ten. It is a physical measurement, which is why two products with the same figure behave the same way in the same room. It also means you can do simple sums with it. Double the area of glass at the same U-value and you double the heat that leaves through it. This is why a large lantern over an open-plan kitchen matters more to the heating bill than a small roof window over a landing, even if the figures on the paper are identical.

What it does not measure

A U-value says nothing about how much daylight comes through, how much summer sun the glass lets in, or how well the unit seals against rain. Those have their own measures. The U-value is only about heat leaving a warm room in cold weather. Keep it in that box and it is easy to use.

A rooflight is not one material, so its U-value is not one simple property. Several parts each lose heat in their own way, and the final figure is a blend of them.

The glass

A sealed unit has two or three panes with a gap between. The gap is filled with a gas that conducts heat less well than air, commonly argon. One pane carries a thin low-emissivity coating, which is a transparent metal layer that reflects heat back into the room instead of letting it radiate out. Add the coating and the gas and the glass figure drops well below plain double glazing.

The spacer

The strip that holds the panes apart around the edge is called the spacer. An old metal spacer carries heat across the edge of the unit and cools the glass there, which is one reason a ring of condensation can appear around the rim. A warm edge spacer made of a less conductive material cuts that loss. If you want a fuller look at how that edge behaves in damp weather, our guide on stopping condensation on a skylight goes into it.

The frame

The frame is often the weak point. Aluminium conducts heat very well, so a rooflight frame in aluminium needs a thermal break, a strip of insulating material that stops the cold outside face from touching the warm inside face. Timber and uPVC are poorer conductors by nature. A roof lantern built in a thermally broken aluminium frame can be a very efficient unit, but only if that break is properly designed.

The whole-unit figure is a weighted average of these parts, based on how much of the unit each one makes up. That brings in the next point.

This is the trap that catches most comparisons. A supplier can quote the glass alone, taken at the centre of the pane where the unit performs best. That figure is real, but it flatters the product. The frame, the spacer and the edge of the glass all lose more heat than the middle of the pane, so the figure for the finished rooflight is higher.

When you compare two quotes, check which number each one is giving you. A glass figure on one and a whole-unit figure on the other is not a like-for-like comparison, and the product with the worse sounding number may be the better rooflight.

Why size changes the figure

Because the frame loses more heat than the glass, the share of frame in the unit changes the result. A small rooflight is mostly frame and edge, so its whole-unit figure is higher than that of a large rooflight built from the same parts. A big lantern has a larger proportion of glass in the middle of the panes, which helps the overall number. This is not a flaw in the figure. It is simply arithmetic, and it is why a published figure should say what size of unit it was calculated for.

The angle of the glass

Heat rises, and glass set into a sloping or flat roof behaves differently from a window set upright in a wall. Tilted glazing loses somewhat more heat than the same unit held vertical, because the air in the gap moves differently when it is laid down. Figures are often quoted for the upright position. For a rooflight, ask for the figure at the installed angle, so that you are comparing the thing as it will actually sit in your roof.

When you fit a new rooflight, or replace one, the work has to meet a heat loss limit. The rules for existing homes in England sit in Approved Document L, part of the 2021 Part L, and the limit depends on what the product is classed as.

• Roof windows, the kind set in a slope that you reach and open, follow the window rule. The whole unit must get down to 1.4 W/m²K, or else carry a Window Energy Rating of band B as the alternative.
• Rooflights cover lanterns and the fixed or flat roof units. Their ceiling is 2.2 W/m²K.

Those apply to new and replacement units in existing dwellings, and sit in Table 4.2 of the document. The rooflight limit is the more relaxed of the two, which reflects the way these larger units are built, but a good modern product is comfortably within it.

Replacing like for like

Swapping a tired rooflight for a new one in the same opening is called a like-for-like replacement, and it is still a controlled fitting. Whatever goes in must reach the figure above, which rules out putting back an old single glazed unit. A like-for-like swap may be self-certified by a competent person scheme. A brand new opening is another matter, since it alters the structure of the roof and so needs Building Regulations approval via the local authority or a registered building control approver. Skylark Skylights deals with that notification itself whenever the job calls for one.

Planning is separate

These are heat loss rules, and they are separate from planning. Putting a rooflight in a house is often permitted development, though conditions attach. The detail is in our planning guide and is worth reading in full.

With the idea in hand, a quote becomes easier to read. You are looking for five things, and any supplier worth using can give them to you plainly.

Ask which figure it is

Is the number the glass at the centre, or the whole unit? Is it for the size of rooflight you are buying, or a standard test size? Is it for the installed angle? If the answer is a vague "around" figure, ask again.

Ask what the unit is made of

The build-up behind a number tells you whether it is believable. Look for a low-emissivity coating, a gas fill, a warm edge spacer and, in aluminium, a thermal break. If those are named, the number has a reason to be good.

Do not chase the lowest number alone

A very low U-value is welcome, but it is only one side of a rooflight. A heavy triple glazed unit that cuts heat loss may also cut the winter sun reaching the room, and the weight changes what the roof and the frame must carry. Choosing between the two glazing types is its own question, and we deal with it in double or triple glazed rooflights.

Check the limit is met

Whatever the figure, it must sit inside the limit for the class of unit. If it does not, the supplier is quoting something you cannot legally fit.

The free survey is where all of this stops being abstract. The survey starts with the room and the place you want lit, then checks the roof over it. What follows is a fixed written quote that names the unit and its figure.

Numbers are easier to trust once you can connect them to the place you stand. A U-value shows up in the room in three ways.

The cold you feel from the glass

Glass that loses heat quickly is cold on its inner face. Sit at a desk under it on a winter morning and you feel a slight chill coming down, even though the air is warm. This is why the bed, the desk and the top of the stairs are worth thinking about when you pick a unit. A better figure keeps the inner pane warmer, so the cold does not drop onto your shoulders.

Condensation

A warmer inner pane resists condensation, because moisture in the room air condenses on the coldest surface it finds. A unit with a good edge and a good figure tends to stay clear in conditions where a poor one mists at the rim. Condensation can also be a sign of a failed seal, which is a different problem altogether. We cover the difference in our answer on whether skylights lose heat.

The heating bill

The bill follows the sums we did earlier. Heat lost is area multiplied by U-value multiplied by the temperature difference. A small, well made roof window over a landing adds very little. A big lantern over a kitchen island adds more, and is worth specifying carefully. Whatever the size, the glass in the roof is only one part of the house, so a good unit does not turn a leaky loft into an efficient one.

The thing to take away is simple. Look for the whole-unit figure, at your size, at the installed angle, inside the limit for its class, and backed by a build-up you can name. If you want that checked against your own roof, book the survey on the form, or send a WhatsApp to 07880 284133.