There are two ways in which rainfall alters the way that U-values should be calculated for inverted flat roof build-ups. An inverted roof should drain at the level of the water flow reducing layer (WFRL) membrane but, because it is not a waterproof layer, the insulation will be exposed to some rainwater that drains at the waterproofing level instead.

The first way in which the U-value calculation alters is to account for any water absorption by the insulation. The thermal conductivity value for the material should be altered from the declared lambda value to the design lambda value. Because the design of an inverted roof means some water is expected to pass through the insulation layer, it is not appropriate to assume that the insulation will be completely unaffected by moisture.

What is the rainwater cooling correction?

The second way is through the use of a rainwater cooling correction, applied as part of the combined method calculation described in BS EN ISO 6946:2017. The correction is calculated using three values: p, which is the average rate of precipitation during the heating season; f, a drainage factor expressing the fraction of p that reaches the waterproofing; and x, a standard value that is the factor for increased heat loss caused by rainwater flowing over the waterproofing layer.

When carrying out a U-value calculation, knowing the project’s location allows an accurate value of p to be established, ensuring the accuracy of the calculation. The value for f is based on the testing of the inverted roof system including WFRL, and the guidance contained in BS 6229:2018 and BBA Bulletin No. 4.

Polyfoam XPS ensure all U-value calculations carried out for inverted roof constructions featuring Roofboard Extra or Roofboard Super, together with the Slimline Zero WFRL membrane, meet all recognised standards and guidance. Contact us for more information about accurate inverted roof U-value calculations for your project.

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