How is a rooflight’s thermal performance declared?
These days, 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 air between the panes.
The measure of a rooflight’s thermal performance is its U-value, or the amount of heat energy that transfers through it. U-values should be for the whole unit, including glazing and frame, and the lower the U-value the better (because less heat is being transferred from inside to outside).
Sometimes ‘centre pane’ U-values are quoted – in other words, the thermal performance of the glass only. They have application in conservation projects, for example, where traditional frame designs offer no meaningful thermal performance but there is a need to demonstrate that the glazing area is helping to meet regulations.
The appearance of rooflights has a direct influence on thermal performance. Units with thinner frames and more glazing offer better U-values, and in that sense choosing one large roof window over two smaller ones is better in terms of heat loss.
Taking a wider view of the building design, however, positioning a number of smaller units in different parts of the roof can deliver more natural light and better illuminate the space. If the goal is to maximise the number of natural daylight hours and limit the use of artificial lighting, then the compromise of more framework might be acceptable.
Frames are typically manufactured from timber, plastics, aluminium and steel, or are composites of metal and plastic or wood. The choice of materials may be driven by thermal performance requirements or a desire to use more sustainable materials; composite products usually offer the benefits of both materials, such as combining the appearance of timber with the low maintenance of metal.
A relatively recent innovation is to fill the sealed space between glass panes with an inert gas like argon, which has a lower thermal conductivity than ‘normal’ air. Some manufacturers use krypton and xenon, both of which offer further improvements in thermal efficiency but are more expensive.
The U-value of a typical double glazed unit is likely to be around 1.1 W/m2K; a triple glazed unit could be expected to offer a U-value of around 0.70 W/m2K, and a quadruple glazed product around 0.60 W/m2K.
Low U-values are achieved in part through the application of a microscopic coating of tin, silver or zinc to some of the glass surfaces within the build up of a rooflight unit. The coating reduces the emissivity (e) of the surface, meaning it emits less thermal radiation from inside the building than if the coating hadn’t been applied.
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. As the names suggest, hard coat is more durable; soft coat remains delicate, is only applied to the sides of panes facing into the sealed airspace, and has a lower emissivity than hard coat.
The declared U-value for a rooflight takes all of these features into account. The specification of rooflights – including pane thicknesses, ratio of glazing to frame, type of coating and gas filling the voids between panes – varies from manufacturer to manufacturer.
So when you find a rooflight that meets the needs of your project, it helps to stick with that choice – because, unlike other building materials, it is difficult to simply swap to a like-for-like alternative that costs less or is more readily available.