Pro Installer December 2019 - Issue 81 - Page 32

32 | DECEMBER 2019 Products Read online at U-WHAT?! Paul Trace from Stella Rooflight offers some guidance on the thermal performance (U-values) of rooflights and what to look out for in the specification process. One of the most im- portant elements of modern building materials is thermal performance, which is measured in terms of heat loss. In the construction industry this is commonly expressed as a U-value or even some- times an R-value. U-value calculations will invariably be required early on in the building strategy stage as it provides an indication as to how much heat loss a building is likely to suffer upon completion. The products used in the build are normally required to be tested and a figure for each component given. One example of where this is an essential requirement is rooflights. Thermal transmittance (U-value) is measured in units of W/m²K which stands for Watts/meter square Kelvin. The lower the U-value the more efficient the construction is at keeping heat flow through the structure to a minimum. It is worth noting that it’s not just the building materials and products that have an impact on the thermal performance of a building, as both workmanship and installation standards can strongly affect the thermal transmittance. If insulation is fitted poorly, with gaps and cold bridges, then the thermal transmittance can be considerably higher than desired, no matter how good the individual products are. Thermal transmittance takes heat loss due to con- duction, convection and ra- diation into account. The amount of heat conducted through a material of a given volume, in a unit of time i.e. the rate of con- duction is why the units are measured as W/K. There are guidelines in the UK, set out in Build- ing Regulations Approved Document Part L (Conser- vation of Fuel and Power), that give the maximum U-value that materials and structures are allowed to have in a range of build- ings, including domes- tic properties. It sets a national standard to ensure that homes must be built to a certain performance level of energy efficiency for both the reduction of carbon emissions and the reduction of residents heat- ing bills. A U-value is one of the most difficult thermal measure- ments to calculate and so it is important that any figures are produced using reliable software from a bona fide source. When it comes to rooflight sup- pliers providing U-value figures for their products, we all want the lowest possible number to prove that our rooflights give the best thermal performance, which ultimately reduces heat loss for our custom- ers. In the rush to be the best, it is not inconceivable that figures get a little mas- saged so it is always best to ask for a copy of the test performance report to ensure that a) the figures are genuine and b) that the figures were produced in the correct way. Despite them being used in their millions across the country, you might be surprised to find out that there is no specific test for a roof- light. Instead rooflights are tested to BS EN ISO 10077-1:2017, which is a thermal performance test for windows, doors and shutters. So does this actually matter? Well, actually yes it does because most roof- lights cannot be used in the same way as windows and this will result in a change to the U-value. For example, the pitch of the roof will change the ther- mal performance of your rooflight. However, the testing of rooflight perfor- mance is based on either a vertical (above 60 degrees) or horizontal (0 degree) pitch. Both of these posi- tions are usually outside the maximum and mini- mum pitch that rooflight manufacturers recommend for their rooflight prod- ucts. There is quite a bit of difference in the U-val- ues given to rooflights at both ends of the scale. For example, our own Stella rooflight can achieve a U-value of 1.5 W/m²K in the horizontal position but this improves to 1.1 W/ m²K when used above 60 degrees. Now when you ask most companies what the U-value is for their rooflight which figure do you think you will be given? This is why it is important to either ask for both figures or to request data to support the figures quoted. The design of a roof- light is also critical to the thermal performance. Most modern rooflights are produced from thermally broken aluminium, which is then clad internally with wood to provide very good U-values. Whilst making the frames more thermal- ly efficient, this approach also makes the rooflights chunky, which often results in them sticking several inches above the roofline and providing poor frame to glass ratios. It is not uncommon for some roof- light frames to make up over 40% of its overall size,