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All You Need to Know About: water absorption of inverted roof insulation

by Jennie Ward

Designers and installers of flat roofing solutions are often told what they ‘should’ know about products and standards. In this ongoing series, Rob Firman of Polyfoam XPS looks at different aspects of flat roofing design and construction, and helps to demystify them. This month he looks at the water absorption of insulation for inverted flat roofs.

Rob Firman of Polyfoam XPS.

In last month’s column I talked about the ways in which the thermal performance of an inverted roof has to be adjusted to account for the possibility of water entering the roof system. One of those adjustments involves changing the thermal performance of the insulation material to account for how its behaviour changes if it absorbs water.

As a result, only insulation materials that can tolerate being exposed to water and that have low water absorption should be used in inverted roofs. Even within that category of suitable materials, different insulation materials have different rates of water absorption and it is useful to understand how water absorption impacts on thermal conductivity.

Which water absorption tests are carried out on insulation materials?
Two different water absorption tests are referenced in the harmonised European Standards for the manufacture of thermal insulation materials. Results can be declared for long-term water absorption either by immersion, or by diffusion.

Equipment and procedures for the two tests are described in BS EN ISO 16535:2019 Thermal insulating products for building applications – Determination of long-term water absorption by immersion (which replaced BS EN 12087), and BS EN ISO 16536:2019 Thermal insulating products for building applications – determination of long-term water absorption by diffusion (which replaced BS EN 12088).

The difference between testing thermal insulation products by immersion and diffusion is to assess the level of water absorption when the material is sitting/submerged in water, or exposed to high humidities, respectively. Immersion testing applies to more insulation types.

Testing by diffusion is an additional test for products designed for environments where water can be present, like inverted roofing. It subjects the insulation sample to a vapour pressure difference and temperature gradient over a 28 day period, and measures the subsequent increase in mass.

Does testing measure the long-term water absorption of insulation?
Both immersion and diffusion testing are intended to measure the long-term water absorption of insulation materials. While laboratory testing can have its limitations, and does not always replicate on site conditions, the tests are designed to create a set of ‘extreme’ conditions.

Testing water absorption by immersion, for example, involves submerging the insulation so that its top face is 50mm below the level of the water – conditions that would never be encountered on an inverted roof in-service.

Similarly, testing by diffusion subjects the insulation sample to high relative humidities of approximately 100% on both sides, and a water vapour pressure gradient, for a long period of time.

The low rates of water absorption achieved show that insulation materials such as XPS will perform as declared in the challenging environment of an inverted roof build-up. Even if the roof is constructed in such a way as more rainwater penetrates the system, the water absorption of the insulation will not change. While the impact of rainwater cooling on the overall U-value may be more pronounced, the design thermal conductivity of the insulation material can continue to be used as supplied by the manufacturer.

How are water absorption test results used to calculate design thermal conductivities?
Design thermal conductivities for inverted roof insulation are calculated by applying a moisture conversion factor. The effect of moisture depends on location, with northern European countries being treated differently to Mediterranean locations.

In northern Europe, including the UK, the effect of freeze thaw must also be taken into account. The results of freeze thaw testing are used in the formula, along with the water absorption by diffusion test results, to give the overall moisture conversion factor.

Find out more about inverted roof U-value calculations: www.polyfoamxps.co.uk.

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