A PRACTICAL GUIDE TO THE COMPRESSIVE STRENGTH OF INSULATION

Compressive strength performance is one of the main characteristics declared for thermal insulation materials used in floor and roof applications. Insulation is part of a building’s ‘secondary structure’, and a material’s compressive strength must therefore be capable of bearing the loads imposed on it by the building’s use.

Nevertheless, the main function of thermal insulation is to provide thermal performance. As a result, products are often specified for their thermal efficiency first, and their compressive strength second. This can lead to situations where an insulation material with good thermal performance but average compression behaviour is specified.

Only later does it become clear that a product with a higher compressive strength should have been specified – leading to possible problems with installation or complying with national building regulations.

Why should the compressive strength of insulation be given more consideration?

Take a typical ground floor construction. Compared to concrete slab and screed layers, which are capable of bearing greater loads, thermal insulation can be considered the ‘weakest’ layer. It is – or should be – subject to the most scrutiny, although this scrutiny often only happens later in a project when a structural engineer is engaged.

One possible reason for insulation materials being poorly specified from a compression behaviour point of view could be confusion over how their performance is declared.

We’ve previously written about the test standards that apply to compressive strength declarations. In this post we’re going to look a bit more closely at what those declarations actually equate to. A better understanding of this could help you to make more informed decisions at the design and specification stage, and avoid changes being made on site that could result in a shortfall of thermal or compressive strength performance.

Compressive strength and compressive stress

Based on the applicable test methods, behaviour under compression should be talked about as compressive strength or compressive stress.

● When a load is applied to the material and the material fails (is at yield) before it has been compressed by 10% then that load is the compressive strength of the material.

● If the material can be compressed beyond 10% relative deformation before being at yield (i.e., its compressive strength is not reached) then the load at which 10% relative deformation occurs is the material’s compressive stress at 10% strain.

Across the insulation sector, behaviour under compression is generally compressive strength, as most common insulation materials fail before they are compressed by 10%. Some product literature may say that a given figure is “at yield”, referring to compressive strength.

However, there can be confusing uses of terminology in product literature and certification, especially around the 10% compression threshold. We would always recommend engaging with a manufacturer directly to confirm the exact behaviour.

Alternative compressive strength declarations

Confusion can also arise due to the way declarations are made for other applications. For example, due to the historic use of expanded polystyrene (EPS) insulation in civil engineering projects, where tighter tolerances are required, declarations are made based on 1% compression, as well as 10% compression.

Extruded polystyrene (XPS) is not used in the same applications. At Polyfoam XPS, we don’t make declarations for 1% compression and it is inappropriate to try and equate the two different declarations, as the products are intended for different uses. For typical floor and flat roof insulation applications, a 1% declaration is not relevant.

Why might a structural engineer from compressive strength declarations?

This post is about what insulation manufacturers declare when it comes to compressive strength, and why. And in another post, we clarify the units of measurement used for compressive strength.

Ultimately, however, it is a structural engineer who must determine whether a material’s compressive strength is suitable for the intended application. Insulation manufacturers simply do not have structural engineering qualifications or experience.

It must also be kept in mind that the compressive strength results declared by testing do not equate to the loads that can actually be applied on site. If they did, products would fail in use.

To help keep the use of products within safe limits, factors of safety are employed. Applying a safety factor accommodates uncertainty in the design process, emergency situations, and higher-than-expected loads. However, using a safety factor should not be taken to mean that a construction is ‘safe’ – there are too many other factors in play to draw that conclusion.

Safety factors can be applied at various stages of design and specification. In fact, there is a risk that several can be mixed together, leading to a product being deemed unsuitable even when its use would be acceptable.

For example, following the lead of our BBA third-party certification, Polyfoam XPS uses a safety factor of 2.5 to determine recommended maximum loadings for our products. A structural engineer has actually expressed surprise at how high this safety factor is – after all, the insulation is manufactured in a controlled environment to tight tolerances.

A structural engineer is likely to employ lower safety factors, often from 1.3 to 1.7, depending on their confidence in test data, risk involved, and the likelihood of issues during installation, among other considerations. However, they might also use safety factors in other areas of their calculations.

We have seen, for example, situations where an engineer has taken recommended loadings for a given building use in BS EN 1991-1-1 and applied a safety factor to give a design loading. When a relatively conservative safety factor is also applied to the insulation material’s performance, it’s easy to see how over-specification could start to occur.

Conclusion

Loadings and compression behaviour are a complex topic. For any given project, insulation manufacturers should be just one voice in a conversation that should also feature clients, designers, specifiers, and structural engineers.

Only through this level of discussion is it possible to be clear about what is required and how best to achieve it, taking into account appropriate safety factors at the right stages.

To find out more about Polyfoam XPS, the compressive strength of our range of insulation products, and how we can support your residential, commercial and industrial projects, contact us.