By Chris Roughneen, Technical Sales Manager at Ravago Building Solutions UK.
In the world of construction, compressive strength and compressive creep are often regarded as interchangeable values, with the former routinely being used when assessing the suitability of insulation materials when it comes to their performance under loading.
It’s a tangible, widely used value that seemingly provides immediate reassurance about a product’s ability to bear high loads. However, relying solely on compressive strength as a measure of long-term performance under loads is a fundamental oversight. The real test of an insulation material’s capability lies in its resistance to compressive creep – the ability to maintain dimensional stability under a sustained load over decades. Without this critical assessment, architects and specifiers risk selecting materials that at a glance may appear to be appropriate but fail in real-world applications.
Compressive strength is a snapshot, a short-term indicator measured under controlled conditions that defines the maximum load a material can withstand before it reaches 10 per cent thickness reduction. It serves a purpose, particularly in classifying insulation products within a manufacturer’s portfolio, but it does not reflect how a material will behave under continuous pressure over time. This is where compressive creep testing is essential. It evaluates a material’s long-term deformation under a constant load, identifying the loading necessary to initially deform the insulation by a maximum of two per cent.
Deformation beyond this can lead to significant consequences. Uneven flooring, water ponding on a sagging flat roof, or shifting loads in buried insulation layers may all result from a failure to consider the long-term compressive creep performance of an insulation product. And yet, this key performance factor is often overlooked in product assessments, with an unhealthy reliance on compressive strength values to differentiate instead.
“Without tested and published compressive creep performance, there is no certainty that the insulation will maintain its dimensional stability over the decades-long life of a building”
Don’t make assumptions…
A key reason for this oversight is that not all manufacturers declare the compressive creep performance of their products. Without this data, architects and specifiers are left making assumptions about long-term performance.Unlike compressive strength which can be determined relatively quickly, compressive creep testing is a rigorous, long-term process. It requires extended periods of load-bearing evaluation – from 122 to 608 days, depending on the period of years being simulated – making it both time-intensive and costly. As a result, many manufacturers opt not to disclose this data.
“Without tested and published compressive creep performance, there is no certainty that the insulation will maintain its dimensional stability over the decades-long life of a building”
While avoiding the cost of testing and not declaring values may enable some manufacturers to offer products at an attractive price point, the long-term implications of choosing an unverified material could be significant. Without tested and published compressive creep performance, there is no certainty that the insulation will maintain its dimensional stability over the decades-long life of a building.
Cost savings at the specification stage should not come at the expense of performance failures that lead to costly remedial work or structural issues in the future. Architects and specifiers should carefully consider whether they are willing to take a risk on materials that do not have a declared compressive creep value considering that liability for failure could suddenly arise decades in the future.
Extruded polystyrene (XPS) insulation, such as that produced by Ravago Building Solutions, is known for its superior compressive creep performance compared to other insulation types like expanded polystyrene (EPS) and polyisocyanurate (PIR).
This enhanced performance is due to XPS’s higher cell density, which provides greater structural integrity under sustained loads. For instance, Ravago’s Ravatherm XPS X 300 SB insulation board demonstrates a compressive strength of 300 kPa at 10 per cent deformation and maintains less than 2 per cent deformation under a sustained load of 130 kPa over 50 years.
Ravago Building Solutions has the confidence to declare its compressive creep values due to its accredited manufacturing and quality control processes. Its laboratory in Rheinmünster, Germany is accredited by FIW München – a leading insulation and building product certification body in Germany – and this commitment to transparency ensures that architects and specifiers have access to a broader spectrum of data when making material selections.
“Architectural integrity depends not just on how materials perform in the moment, but on how they withstand the passage of time and compressive strength is a poor measure of this”
By providing detailed Declarations of Performance (DoPs), Ravago not only declares compressive creep, but also gives customers access to all performance-related results, using NPD (No Performance Determined) only when the value is immaterial.
Architectural integrity depends not just on how materials perform in the moment, but on how they withstand the passage of time and compressive strength is a poor measure of this.
Specifiers must demand full transparency on compressive creep, ensuring that the insulation materials they choose will support their designs for generations, not just at the point of installation. By doing so, the industry can move beyond misleading metrics and embrace specifications that deliver true longevity and resilience.