Extruded polystyrene (XPS) insulation is unique among common thermal insulation materials for its low moisture absorption and tolerance of wet environments. Whether installed directly in contact with the ground, or above the waterproofing layer in an inverted warm roof construction, it performs consistently and predictably in wet and dry conditions.

Two different water absorption tests are referenced in EN 13164, the harmonised European Standard for the manufacture of XPS insulation. Results can be declared for long term water absorption either by immersion, or by diffusion.

What are the water absorption tests for XPS insulation?

The two water absorption tests referred to in EN 13164 are described in EN 12087 ‘Determination of long term water absorption by immersion’ and EN 12088 ‘Determination of long term water absorption by diffusion’.

For total immersion, method 2A of EN 12087 should be followed, and a result, Wlt – written as WL(T) in a Declaration of Performance – declared. There are three levels at which performance can be declared, as defined by EN 13164, and no test result can be higher than any of the thresholds given in the table:

LevelVolume %
WL(T)3Less than or equal to 3
WL(T)1.5Less than or equal to 1.5
WL(T)0.7Less than or equal to 0.7

For diffusion, the method in EN 12088 should be followed, and a result, Wdv – written as WD(V) in a Declaration of Performance – declared. There are five levels at which performance can be declared, as defined by EN 13164, and no test result can be higher than any of the thresholds given in the table:

LevelVolume %
WD(V)1Less than or equal to 1
WD(V)2Less than or equal to 2
WD(V)3Less than or equal to 3
WD(V)4Less than or equal to 4
WD(V)5Less than or equal to 5

After testing for either total immersion or diffusion, freeze thaw resistance can be determined through testing, which is another important characteristic of XPS insulation.

Which water absorption test does Polyfoam XPS use?

Polyfoam XPS tests the water absorption of its extruded polystyrene foam by both total immersion and diffusion. Both the Standard and Extra grades of Polyfoam achieve the lowest classifications – WL(T)0.7 and WD(V)1 – for both types of testing.

Super grade achieves the lowest classification for total immersion testing.

What is the relevance of water absorption to thermal insulation performance?

If thermal insulation is installed in a situation where it can get wet and absorb water, it’s declared thermal conductivity worsens and reduces the thermal efficiency of the building element.

This is why other types of rigid foam insulation, such as polyisocyanurate (PIR) and phenolic foam boards, must be separated from the ground by damp proof membranes in floors, for example. As the insulation type with by far the lowest water absorption, XPS can be used in its intended applications with little or no penalty to performance.

Exposure to moisture can mean applying a moisture correction factor, a procedure that is routinely carried out with insulation products for inverted roof applications. Applying a moisture correction factor gives rise to the use of design thermal conductivities in inverted roof U-value calculations, rather than declared thermal conductivities.

Does inverted flat roof insulation become saturated?

At the time of writing, the inverted roof industry is engaged in a lot of discussion around the revised version of BS 6229 published at the end of 2018.

We won’t go into detail about what has prompted the discussions – you can read about that here – but the revisions to the standard essentially suggest that more rain water penetrates the roof construction and passes through the insulation layer than is typically allowed for. It suggests this is the case for both standard inverted roof build-ups, and inverted blue roofs.

This has led to the use of phrases such as, “the inverted roof insulation is regularly saturated” within the updated standard. The water absorption tests we have discussed above clearly show that XPS insulation does not become “saturated”. XPS insulation boards are tested and declared to work in an inverted roof system as it is designed.

There seems to be confusion among those who drafted the standard between the insulation boards behaving as they are intended, and the performance of an inverted roof as a whole. If more rainwater reaches the waterproofing layer then the effect of rainwater cooling will be more pronounced – but the water absorption of the insulation will not change, and the effect of that is already accounted for by design thermal conductivities.

As discussions continue over these issues, we’ll keep our blog up to date with our latest advice and guidance. Connect with us on LinkedIn or follow us on Twitter to receive the latest updates. For any questions over the use of Polyfoam XPS products in the applications we’ve mentioned in this post, or the water absorption results discussed, get in touch with us here.