The water absorption of inverted roof insulation varies depending what insulation material is used. Extruded polystyrene (XPS) is the most common type of insulation used on inverted roofs, while expanded polystyrene (EPS) is also popular. Technical guidance related to inverted roofs usually mentions both, though XPS has the longer history of use in the application.

The water absorption of insulation materials is established by testing, and varies depending on the foam structure of the product. Both absorption by immersion and absorption by diffusion are usually tested, and you can read more about the differences between XPS and EPS here.

Why is water absorption relevant to inverted roofing?

The thermal conductivity of an insulation material gets worse as the quantity of water it absorbs increases. Because an inverted roof is designed in such a way that the insulation will be exposed to some water, it is important to ensure that the thermal conductivity of the insulation is adjusted accordingly.

The declared lambda value of the insulation is multiplied by a moisture correction factor to arrive at a design lambda value. The design lambda can then be used in U-value calculations for inverted roofs. Open space in the structure of EPS foam allows more water into the material than the closed cell structure of XPS does. The moisture correction factor for EPS is therefore greater than for XPS.

Test results of Polyfoam XPS products such as Roofboard Extra and Roofboard Super are available in our product data sheets, BBA certificate and Declarations of Performance. Access all of them through the downloads section of our website. For advice on the correct use of design lambda in U-value calculations for your inverted roof project, contact us.

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