Energy Efficient Rooflights

U-value

U-value is measured in watts per square metre per degree, and lower is better. A single glazed rooflight from the 1970s might be around 5.0. A modern double glazed unit with a low emissivity coating and argon fill is around 1.2 to 1.4. Triple glazing gets to around 0.8.

The important detail is that a whole unit U-value and a centre pane U-value are different numbers, and the marketing usually quotes the better one. Centre pane ignores the frame and the spacer, which are exactly where the heat actually escapes. When you are comparing quotations, compare whole unit figures or you are comparing nothing.

Part L, and what it asks for

For a replacement rooflight in an existing dwelling, the current standard expects a whole unit U-value no worse than 2.2. That is not difficult to meet with any decent modern unit, which is why nobody talks about it much.

It matters in two situations. On a like for like replacement of a very old unit, the improvement is dramatic and worth quantifying for the customer. And on a new opening in an extension, the rooflight forms part of the whole extension calculation, so a poor rooflight forces better performance somewhere else in the build.

g-value, and the Essex summer

The g-value is the proportion of solar energy that gets through. A clear double glazed unit is around 0.7, meaning seventy per cent of the sun’s energy arrives in the room. Solar control glass brings that to around 0.4 or lower.

Overhead glass gets the most sun of any glass in the building, because in summer the sun is high and shining almost straight down onto it. A south facing lantern over a kitchen with clear glass is a greenhouse from about eleven in the morning.

This is the specification most often dropped to save money and most often regretted, because the only remedy afterwards is replacing the glass.

Which way the slope faces

• South. Solar control is not optional. Consider external shading as well on a large area.
• West. Similar, and often worse in practice, because late afternoon sun is low and hits at an angle that internal blinds struggle with.
• East. Early morning gain. Fine in a kitchen, difficult in a bedroom without blackout.
• North. Even, cool, glare free light all day. This is the best light in the building and it needs no solar control at all. Specify for U-value alone and take the money you saved elsewhere.

Where heat actually leaves

Not usually through the glass. Through the spacer bar at the edge of the unit, and through the kerb. A warm edge spacer replaces the traditional aluminium bar with a low conductivity material, which raises the temperature of the glass at the perimeter and is the difference between a unit that mists at the edges on a cold morning and one that does not.

The kerb matters more. An uninsulated timber upstand is a cold bridge running the whole perimeter of the rooflight, and it will lose more heat than the glass does. Insulating the outer face of the kerb is cheap, invisible and routinely skipped.

Is triple glazing worth it

In a rooflight, usually not. It gets the U-value from around 1.2 to around 0.8, which is real but modest. It is heavier, which affects the actuator on an opening unit and sometimes the structure. It costs substantially more.

The same money spent on solar control glass, an insulated kerb and a properly sealed vapour control layer will make more difference to how the room feels than the third pane will. We will fit triple glazing where someone wants it, and we will say this first.

Where the heat actually leaves

Quotations compare U-values for the glass. On a typical installation the glass is not where most of the loss is.

Comparing quotations honestly

The most common source of confusion is that two different numbers describe the same product.

Centre pane U-value measures the glass away from the edges and ignores the spacer and the frame, which are exactly where heat escapes. It is always the better figure and it is what marketing tends to quote.

Whole unit includes glass, spacer and frame weighted by area. It is what the building experiences and what Building Regulations use.

Whole unit is always worse, frequently by 0.4 or more, and on a small unit with a lot of frame relative to glass the gap is wider still. A rooflight advertised at 1.0 and another at 1.4 may well be the same performance described two ways. Ask for whole unit figures, or you are comparing nothing.

Specify by orientation, not by habit

A quotation frequently arrives with the same glass on every unit regardless of which way each faces. That wastes money in both directions at once.

South. The most sun, through the middle of the day when it is strongest. Solar control is not optional. On a large area consider external shading as well.

West. Often worse in practice than south, because late afternoon sun is low and arrives when the building has already been warming all day. Low angle sun is also where internal blinds struggle most.

East. Morning gain, shorter and cooler. Fine in a kitchen. Difficult in a bedroom, where the issue is light rather than heat and blackout blinds are the answer.

North. No direct sun at all. Even, cool, glare free light all day, and the best light in the building for a work surface or a studio. No solar control at all: specify for U-value alone and spend the saving elsewhere. Fitting solar control on a north facing rooflight makes a good room slightly darker and delivers nothing.

Part L, and what it actually asks

For a replacement rooflight in an existing dwelling the standard expects a whole unit U-value no worse than 2.2. Any decent modern unit lands around 1.2 to 1.4, which clears it comfortably, and that is why it is rarely discussed.

It matters in two situations. On a like for like replacement of a very old unit, the improvement is dramatic. And on a new opening in an extension, the rooflight forms part of the whole extension calculation, so a poor rooflight forces better performance somewhere else in the build. Getting the glass right first is cheaper than compensating for it in the walls.

There is also a limit on total glazed area as a proportion of floor area, so a heavily glazed extension may need a calculation rather than an assumption.

What we would not sell you

Triple glazing, in most cases. It takes the U-value from about 1.2 to about 0.8, which is real and modest. It is heavier, which affects the actuator on an opening unit and sometimes the structure. It transmits noticeably less daylight, which works against the reason you fitted a rooflight. And it costs substantially more.

The same money spent on solar control glass, an insulated kerb and a properly sealed vapour control layer will do more for how the room feels than the third pane will. We will fit triple glazing where somebody wants it, and we will say this first.

Overheating, and the rule most people have not heard of

Since June 2022 there has been a separate part of the Building Regulations dealing with overheating in new dwellings. Part O exists because the previous generation of highly insulated, highly glazed homes turned out to be uncomfortable in summer, and rooflights are directly in its scope.

It applies to new dwellings, including new flats and some material change of use conversions. It does not apply to an extension to an existing house, which is where most of our work sits, and it does not apply to replacing a rooflight in an existing roof.

It is still worth understanding when it does not apply, because the physics does not care about the scope of the regulation. A south facing extension with a large glazed roof will overheat whether or not anyone is required to calculate it.

There are two routes to compliance. The simplified method sets limits on how much glazing may face which orientation and requires a minimum free opening area for purging heat. The dynamic thermal modelling route simulates the building through a design summer and demonstrates the result, which allows more glazing but costs a modelling exercise.

The part that catches people is that the simplified route limits glazing area by orientation, and a design with a lot of south or west facing rooflights can fail it outright regardless of how good the glass is. Solar control glazing helps the modelled route considerably; it does not exempt you from the simplified route’s area limits.

The practical advice is the same in a regulated project and an unregulated one. Decide orientation first, specify solar control where it faces the sun, and provide a way for hot air to leave at high level. Doing that at design stage costs almost nothing. Retrofitting shading and ventilation to a room that is unusable in July costs a great deal.