Calculating the U.Value

Tapered Insulation Thermal Performance
– Calculated NOT Estimated.

A tapered insulation scheme is made up of manufactured components with a fabricated fall. Unlike a more straightforward elemental U. Value calculation where constant thickness insulation is used on a structural deck with falls, tapered insulation needs a more detailed calculation.

Determining the performance of the tapered elements must be calculated in accordance with BSENISO 6946:2017 Annex E. Using the principles of thermodynamics, this guidance outlines the formulae that need to be adhered to in order to determine the ‘effective’ performing thickness of tapered elements. It is NOT simply the average thickness of insulation.

Consideration should be given to the thermal performance at the thinnest point of the scheme to minimise the risk of interstitial condensation and to ensure a balanced level of thermal performance. ‘Guidance’ within BS6229:2018 recommends that at no point of a roof area should there be a heat loss exceeding 0.35 W/m²K. This is certainly good advice and where possible should be followed. The reality of some projects is that either due to other complications or simply a long distance of fall, some areas of the scheme may need to exceed this heat loss. These areas should be kept to a small percentage of the roof area and the risk of interstitial condensation assessed.