The importance of moisture tolerance in ground-floor insulation

Rob Firman, technical and specification manager, Polyfoam XPS.

Rob discusses the moisture absorption of insulation, and explains how it is measured.

THE performance of the thermal insulation layer is critical to ground floor construction. Whether positioned below a concrete floor slab or above it with a floating screed over, the insulation has to help the floor provide a safe, useable surface with the correct quality of finish. The load-bearing capability of the insulation is fundamental to achieving this, but another important factor is moisture tolerance.

Why is moisture absorption important?
If certain types of rigid insulation, such as polyisocyanurate (PIR) and phenolic foam boards, are installed in a situation where it can get wet and absorb water, their declared thermal conductivity worsens and reduces the floor’s thermal efficiency as a whole. They must therefore be separated from the ground by a damp-proof membrane (DPM).

Manufacturers of these materials are careful to explain that their products should not be allowed to get wet, but that does not stop them from being installed incorrectly. The effect of incorrect installation is rarely properly accounted, through applying a moisture correction factor to the insulation’s thermal conductivity, but if the floor’s thermal performance is not what was anticipated at design stage then it may fail to comply with building regulations.

As the insulation type with the lowest water absorption, extruded polystyrene (XPS) insulation performs consistently and predictably in wet and dry conditions. This allows it to be installed below the damp-proof membrane in ground floors without detriment to the performance of the material or thermal conductivity.

How is the moisture absorption of insulation tested?
There are two ways of testing the long-term water absorption of thermal insulation products for buildings. One is by immersion, which assesses the level of water absorption when the material is sitting or submerged in water, and diffusion which tests the material when exposed to high humidity.

Immersion testing applies to more insulation types and a range of testing options exist. Testing by diffusion is an additional test for products used in below-ground applications, like XPS insulation.

Immersion testing
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.

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 specify the most representative method for the specific material.

Diffusion testing
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 50deg C and the other side is cooled to a temperature of one degree centigrade, and the sample is turned every seven days. At the end of the total test period, the sample is removed, and any surface water is wiped off before 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. 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’.

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, for example 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 and moisture absorption testing
For total immersion, there are three levels at which performance can be declared, and no test result can be higher than any of the thresholds given. For diffusion, there are five levels at which performance can be declared, and no test result can be higher than any of the thresholds given.

There are grades of XPS insulation available that can achieve the lowest classifications for both types of testing. After testing for total immersion or diffusion, freeze thaw resistance can also be determined, which is another important characteristic of XPS insulation.
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