What you need to know about rooflight U-values

Rooflight U-values – what you need to know

The thermal performance of rooflights, including the U-value they achieve, is an essential part of building fabric specification. Balancing outgoing heat loss with incoming daylight and solar energy all contributes to the wider sustainability goals of a project – including the overall thermal efficiency of the building, and the thermal comfort of its occupants.

Designers and specifiers therefore need to understand what quoted U-values mean, and the relevance of other performance measures such as g-values. This post concentrates on how rooflights allow solar energy in and limit heat transfer out. However, for projects where daylighting is just as important a consideration, factors such as light transmittance and reflectivity must also be taken into account.

Why can rooflights have two different U-values?

Anybody involved with building design and specification should be aware of U-values as a measure of thermal transmittance (the movement of heat energy) through building fabric from warm to cold.

The composition of roof glazing units – including overall size, relative areas of glazing and frame, and the thermal performance of the materials used – varies, so U-values quoted in a specification should be based on an actual product or a detailed calculation model.

However, performance specifications may not be clear about whether a whole-unit or centre pane value is required. Manufacturers themselves may not be clear about the type of U-value they are quoting, creating the potential for confusion.

In most circumstances, U-values should be for the whole unit, including glazing and frame. Whole-unit U-values can be improved through the use of a warm edge spacer. Traditionally, spacer bars are aluminium but, like any metal, aluminium has a high thermal conductivity. It acts as a thermal bridge, conducting heat from inside the building at the edge of the glass and bypassing features otherwise designed to improve the efficiency of the glazing.

‘Warm edge’ spacer bars use materials with a lower thermal conductivity to slow the rate of heat loss and create a more even surface temperature across the whole glass pane.

Centre pane U-values address the thermal performance of the glass only. They appear lower than whole-unit values because the cold bridging effect of the spacer and edge seal are not accounted for. Unfortunately, this means some manufacturers rely on quoting centre pane U-values when they should offer whole-unit U-values.

Specifiers can find themselves misled if a centre pane value is made to appear as though it is a better performing product. Centre pane values do have an application, though. They are useful for comparing one glass against another when being used in the same frame, as well as in conservation projects where traditional frame designs offer no meaningful thermal performance.

Why should you consider rooflight g-values alongside U-values?

Compared to the total surface area of the building fabric, the U-value heat loss through relatively small areas of roof glazing is more than offset by the contribution of solar gains and the reduced use of artificial lighting.

The measure of infrared radiation (solar heat) allowed into a building is the g-value. A g-value can be anything from 0 to 1, where 0 represents no solar heat gain and 1 is the maximum possible solar heat gain. It is calculated by dividing the total solar heat gain by the incident solar radiation (the amount of solar radiation received on the surface during a given time).

The lower the g-value, the lower the percentage of solar radiation allowed through the glass. Like U-values, performance figures can be quoted for the glass alone or for a complete glazed unit. Better, lower g-values also result in lower light transmission.

How do different gases change the thermal performance of rooflights?

Most people understand the benefits of having a glazing unit that is more than just single glazed. Double glazing, triple glazing and even quadruple glazing improve the thermal (and acoustic) performance by introducing sealed layers of gas between the panes.

A well-known feature of products is to fill the sealed space between panes with an inert gas like argon, whose thermal conductivity is some 34% lower than still air. Some manufacturers use krypton and xenon, both of which offer further improvements in thermal efficiency, but which are more expensive.

Do low emissivity coatings impact on rooflight U-values?

The function of low emissivity (low-e) coatings is to impact on the loss of solar radiation back out of the building.

A material’s emissivity determines the amount of thermal radiation emitted from its surface. Low-e surfaces emit less thermal radiation, and glazing units benefit from this through the application of a microscopic coating of tin, silver or zinc to certain faces of the glass panes in the unit.

In contrast to the short-wave radiation from the sun that heats the building interior, the heat energy transferring back through the building fabric, from warm inside to cold outside, is long wave radiation. Glass with the low-e coating reflects long wave radiation, effectively keeping more heat energy in the building.

There are two types of coating: hard and soft. Hard coat is applied while the glass is still molten, whereas soft coat is applied later in the process. Hard coat is more durable, as its name suggests. Soft coat remains delicate, is only applied to the sides of panes facing into a sealed airspace, and has a lower emissivity than hard coat.

The difference in emissivity between the two means argon-filled glazing with a hard coat treatment will typically offer a centre pane U-value of 1.4 W/m²K, while a soft coat treatment will see that improved to 1.1 W/m²K. It’s a meaningful distinction, yet some manufacturers will simply claim their glazing to be “low-e”.

Making a hard coat treatment sound like a similar benefit to soft coat is another reason for specifiers to be clear about the features of the products they’re selecting.

About Glazing Vision

Large areas of glazing are architecturally impressive, but more likely to require solar control in an effort to control the effect on internal temperatures. This is particularly relevant as specifiers seek to meet new overheating requirements of building regulations, alongside thermal and ventilation requirements – and especially with the Future Homes Standard due to be implemented in 2025.

Glazing Vision can help projects to exceed the requirements of current building regulations, looking towards the Future Homes Standard. Our expert technical support helps you to meet daylight, ventilation and roof access requirements. We provide all the details required for correct specification, including U-values and CAD drawings.

For further advice and guidance on how Glazing Vision architectural rooflights can help to future-proof  your project, download our ‘Homes of tomorrow today’ white paper or contact us.