Understanding the Compressive Strength of Insulants

Compressive strength is a crucial characteristic when evaluating insulants, particularly for applications where flat roofs are subjected to various active or dead loads. Two tests are commonly taken out to work out the compressive strength properties of a material, these are: short-term testing (Compressive Behaviour BS EN 826:2013) and long-term testing (Compressive Creep BS EN 1606:2013).

It is important to understand what these tests are, as the results can determine if the product is suitable for its application. Failure to install a suitable insulant can lead to deterioration of the insulation system, leading to potential damage to the waterproofing layer, along with mechanical systems and other equipment above.

In the short-term test, the insulant is subjected to a load that steadily increases. The point at which the material reaches its limit of elastic behavior (it fails) or compresses by 10% relative to the original sample thickness is recorded. The result indicates the maximum load that product can withstand before failing, which can then be passed to a structural engineer to perform appropriate loading calculations. In real world applications, the insulant will never be subjected to its tested loads and the manufacturer will advise on the figure they are happy with the product being subjected to, whilst being mindful that different materials can perform differently under stress.

The long-term test differs, in that it is conducted to determine how the product will perform in long term intervals, for example, over a 10, 25 or 50 year period. In this test, the insulant is subjected to a constant load under specified conditions of temperature and humidity and the deformation of the material is measured repeatedly. Typically, three different loads of varying weights relative to the figure identified in the short-term test are applied to three separate samples (or a specific load can be placed on all three samples to determine the long-term impact that this load has on the insulant). The insulation manufacturer will have an expected target of deformation which they would like to not exceed. Again, the results of the test can be passed to a qualified specialist to assess whether the product is suitable for its intended application.

Compressive strength can vary among insulants depending on its material properties. Flat roofing grade PIR products often exceed 150 kPa at 10% compression when tested to BE EN 826:2013. In contrast, flat roofing grade stonewool products typically have a compressive strength of 70kPa+ at 10% compression which is considerably lower. It is important to note that these figures are taken from the short-term test and the material should not be subjected to these loads in real world applications (real world data can be retrieved following the long term test) which is why it is important to open conversations with appropriate specialists to confirm if the material is suitable.

Non-combustible products are being specified more often in trafficked roof areas and at time of writing, limited data is available on point load performance in the marketplace. TaperedPlus have developed a non-combustible solution in the form of ‘ROCKDeck’ which offers enhanced point load performance when accessed under long term testing to EN 1606:2013). For more information about this product, or any queries regarding compressive strength, please contact our technical department at technical@taperedplus.co.uk.