BUILDING ENVELOPE
For rooflights, skylights and roof windows (of which the definition is crucially important), the relevant consideration is the thermal transmittance. This is measured as a U-value in units of W/m²K (which stands for Watts per metre squared Kelvin). The lower the U-value the more efficient the construction is at keeping heat flow through the structure to a minimum. The new regulations deem the worst acceptable U-values to be 2.2 W/(m2 for rooflights and 1.6 W(m2
K) K) for roof
windows. To correctly assess whether an element meets the new limiting U-value figure, the U-value must be calculated for the element in the appropriate plane – either horizontal or vertical. Now this makes a big difference, as testing the same product in either a horizontal or vertical position will make a significant difference to the resulting U-value figure – with the vertical position providing a much lower (better) U-value figure. This is where the definition of the terms and roof window prove crucial in determining how they should be tested and what the relevant U-value should be for each. According to the Approved Document, the following definitions apply: • Rooflight – A glazed unit installed out of plane with the surface of the roof on a
LIKE-FOR-LIKE
kerb or upstand. Also sometimes referred to as a skylight.
• Roof window – A window installed in the same orientation as, and in plane with, the surrounding roof.
It is important to understand that U-value calculations for roof windows
As with most things in life, you tend to get what you pay for, and this is particularly true when comparing rooflights in the market. Prices often reflect the quality of manufacturing and materials used to produce roof windows. While prices may vary considerably, there is a good reason for this and the difference in quality between the bottom and high end of the rooflight market can be night and day. The rooflight may be one of the last things that you budget for, but it is also one of the most important finishing touches to your home. Opting for the cheapest product may come back to haunt you in the long run.
Paul T race is dir ector of Stella Rooflight
and rooflights must be undertaken by an approved UKAS accredited product certification agency, who will calculate accurate thermal performance figures based on the individual make up of components in the product’s construction. It’s always worth checking with your supplier to see proof of performance as U-values are often misquoted, and can refer to the centre glass pane rather than the rooflight as a whole.
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www.sbhonline.co.uk
may/june 2022
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