When calculating the U-value of an inverted flat roof, the thermal conductivity of the insulation layer should be the product’s design lambda value and not its declared value. Because of the insulation’s position above the roof waterproofing layer, it is expected to be exposed to some rainfall and therefore a moisture correction factor should be applied accordingly.

Testing of inverted roof systems carried out in accordance with the technical guidance document ETAG 031 often generates a result that suggests water does not penetrate the system. In accordance with BS 6229:2018 and, by extension, BBA Bulletin No. 4, the inverted roof installation above the waterproofing should not be assumed as waterproof – meaning only design values are appropriate.

What happens if the declared lambda is used for inverted roof U-value calculations?

Using declared thermal conductivity in an inverted roof U-value calculation gives an unrealistic impression of the performance of the roof. If the chosen insulation product has a relatively high water absorption value then when it is exposed to moisture the rate of heat loss will be much higher than allowed for in compliance calculations.

Find out more about the low water absorption of Polyfoam XPS’s extruded polystyrene (XPS) insulation boards, and the different methods by which it is tested. Contact us for reliable and accurate inverted roof U-value calculations featuring the design lambda values of our Polyfoam Roofboard Extra and Roofboard Super products.