WHAT IS THE DIFFERENCE BETWEEN WATER ABSORPTION BY IMMERSION AND DIFFUSION?

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, and a range of testing options exist accordingly.

Testing by diffusion is an additional test for products used in below-ground or inverted roof applications, such as extruded polystyrene (XPS) insulation. XPS 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 in an inverted warm roof construction, it performs consistently and predictably in wet and dry conditions.

Water absorption testing methods are described in European standards, part of a suite of test standards which are common to thermal insulation materials. In a separate blog post we describe what effect water absorption has on the thermal performance of insulation products, and how Polyfoam XPS products perform when tested to both standards.

How is water absorption by immersion tested?

The equipment and procedure for establishing water absorption by immersion is detailed in BS EN 12087. There are two test methods: the first is for partial immersion, and the second for total immersion. Results for both types of testing are determined by the change in mass of the test sample after 28 days of partial contact with water, or complete immersion in water, respectively.

For partial immersion, the bottom face of the test sample must be 10mm below the surface of the water. During total immersion testing, the top face of the sample must be 50mm below the surface of the water.

For both partial and total immersion, there is also an A and B option for dealing with excess water adhered to the surface of the product, but which is not absorbed. Option A involves sitting the tested sample on a mesh rack and allowing it to drain for ten minutes. Option B deducts initial water uptake.

A mathematical formula is then applied, depending on the method followed, to determine long term water absorption. EN 13164, the harmonised European Standard for extruded polystyrene (XPS) products, specifies that method 2A (total immersion with excess water allowed to drain) should be used. Harmonised standards for other insulation products will specify the most representative method for the specific material.

How is water absorption by diffusion tested?

The equipment and procedure for establishing water absorption by diffusion is detailed in BS EN 12088. It measures the increase in mass of the insulation sample when subjected to a vapour pressure difference and temperature gradient over a 28 day period.

The sample is placed into a container and sealed so that one side is exposed to water heated to 50 deg.C and the other side is cooled to a temperature of 1 deg.C. The sample is turned every seven days. At the end of the total test period, the sample is removed, any surface water wiped off, and the sample is weighed. A mathematical formula is then applied to determine water absorption.

Do the tests measure long term water absorption?

Both standards – BS EN 12087 and 12088 – are clear in their intention to measure long term water absorption, though some in the roofing industry question how well they replicate that long term exposure. While laboratory testing cannot recreate the conditions that might occur on a site at any given time, it can create a set of ‘extreme’ conditions that test a product to its limits.

BS EN 12087, for example, says its test method, “is intended to simulate the water absorption caused by long term water exposure. The long term water absorption by total immersion is not directly related to the conditions on site, but has been recognised as a relevant condition of test for some products in some applications.”

Where an inverted warm flat roof construction is concerned, operating on the principle that the thermal insulation is exposed to some water, the insulation would never be submerged to the extent that its top face is 50mm below the level of the water.

Even in the extremely unlikely event that such a scenario occurred, it would not be for the 28 day period over which the test is carried out – at that point we would be talking about a wider failing of the building design and/or construction.

Similarly, BS EN 12088 says its method, “is intended to simulate the water absorption of products subjected to high relative humidities, approximating to 100%, on both sides and subjected to a water vapour pressure gradient for a long period of time e.g. inverted roof or unprotected ground insulation. The test is not applicable for all types of thermal insulating products. The product standard should state for which of its products, if any, this test is applicable.”

XPS insulation is tested by both immersion and diffusion because it is one of the few materials offered for inverted roof and unprotected ground applications. It achieves its declared performance levels when tested in these demanding conditions, precisely to give confidence that it will continue to perform that way if, for example, a water flow reducing layer (WFRL) is not installed to the correct standard on site.

For more information on the use of Polyfoam XPS in any of the applications mentioned in this blog post, please contact us.

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