WHAT UNITS OF MEASUREMENT ARE USED TO EXPRESS COMPRESSIVE STRENGTH

Key to understanding compressive strength, as with any characteristic of a construction product, is being aware of the units used to express the performance. In fact, it could be argued that it’s especially important to understand the units for compression behaviour in order to avoid confusion when dealing with loadings.

Loads, the forces they exert, and the compressive strength of insulation materials, can all be expressed in different ways. It is a useful skill to be able to switch between these different units and understand why certain figures are being expressed a certain way.

This blog post about compressive strength units can be read in conjunction with our practical guide to understanding compressive strength, our blog post on compressive strength testing, and our blog post about the types of load imposed on insulation.

Why is the compressive strength of insulation declared in kPa?

Compressive strength measurements for insulation boards are typically declared in kilopascals (kPa). The pascal is the SI unit for pressure, defined as one newton per square metre.

1 Pa = 1 N/m2

1 kPa = 1 kN/m2

A newton is the SI unit of force, so a measurement in pascals equates to the pressure exerted by that force, perpendicularly, over a square metre. For the purposes of working out loads on insulation materials, 1 kN can be taken as about 100kg of load.

For easy conversion, 10 kPa is about equivalent to one metric tonne per square metre. In terms of compressive strength declarations, then, using Standard grade Polyfoam as an example:

200 kPa = 200 kN/m2 = 20,000 kg/m2, or approximately 20 metric tonnes per m2

As we discuss in our guide to compressive strength, it’s important to remember that 20 metric tonnes is simply a test result rather than a real-world load that the insulation board is capable of being exposed to. Appropriate safety factors need to be taken into account, as well as the effect of other material layers within a floor or flat roof build-up.

Is the compressive strength of insulation ever measured in megapascals?

The Polyfoam technical helpdesk occasionally receives enquiries where a specification states measurements in N/mm2.

In the same way that kN/m2 equate to kPa, N/mm2 equate to megapascals (MPa). Concrete strength is measured in MPa, which is an order of magnitude greater than kPa (i.e., 1000 kPa = 1 MPa).

For people used to working with insulation and/or measurements in kPa, coming across measurements in MPa is confusing. The opposite is true too – structural engineers and people in the concrete industry used to thinking in MPa can find it difficult to contextualise measurements expressed in kPa.

Specifications for insulation seen expressed in N/mm2 format include the following.

0.12 N/mm2 = 120 kPa

0.50 N/mm2 = 500 kPa

How can I easily convert between different compressive strength measurements?

For ease of reference, we have compiled a table summarising the different conversions discussed above. Hopefully this helps with understanding what different kPa ratings mean when comparing different types of insulation.

While the primary function of thermal insulation is to restrict the flow of heat energy, it must also be capable of bearing the loads that will be imposed on it in any given application. Getting the right thermal performance and compressive strength performance is critical to successful specification.

To find out more about extruded polystyrene and how it offers the best combination of thermal and loadbearing performance compared to other common insulation materials, contact us.