‘Declared thermal conductivity’ is the lambda value of an insulation product as it leaves the factory. In the context of inverted roofing, ‘design thermal conductivity’ is the lambda value of the insulation once it has been adjusted for potential moisture absorption.

The difference between the two values depends on the insulation material. For a material that has very low water absorption, its thermal conductivity will be affected less because the material in situ is closer to the factory production specification. Where a material is capable of absorbing a relatively greater volume of moisture, its design lambda will be correspondingly worse.

How are declared and design lambda values established?

For an insulation product manufactured under a harmonised European standard, the thermal conductivity declared must be a ‘lambda 90/90’ value. That is, there is a 90% statistical certainty that 90% of tested samples have a thermal conductivity equal to or lower than the declared value.

The design lambda value used for the insulation product in U-value calculations for inverted roofs is the declared value with a moisture correction factor applied to it. The moisture correction factor varies depending on the material type, as described above.

Extruded polystyrene insulation, such as Polyfoam XPS Roofboard products inverted roofs, has a moisture absorption of just 0.7%. For a board greater than 100mm in thickness, that means the declared lambda is only adjusted from 0.033 W/mK to a design lambda of 0.034 W/mK. For further advice on the correct application of design thermal conductivity in U-value calculations, contact our technical team.