How can using XPS help to reduce performance gaps?

Extruded polystyrene (XPS) insulation’s combination of performance characteristics can help to deliver energy efficiency, control of moisture and loadbearing capability in ground floors and flat roofs. Specifiers and installers might prioritise the former, but not think about the other requirements until it is too late to specify build-ups accordingly. That’s when shortfalls in performance can occur, resulting in poorer outcomes for clients.

The term ‘performance gap’ is nothing new in construction, unfortunately. It is most commonly used to describe how buildings experience higher rates of heat loss, or greater energy consumption, in-use than was predicted at design stage. However, performance gaps can exist in any area of building fabric specification, and often do so despite the best intentions of specifiers or installers.

That is where XPS comes in, as it can help to achieve closer to the desired end result – without specifiers and installers necessarily having to change their processes. Let’s look at each performance requirement in turn.

Performance requirement 1: energy efficiency

The primary role of the thermal insulation layer in a floor or roof is to help deliver energy efficiency. Whichever insulation material is chosen, it typically contributes the vast majority of the build-up’s thermal resistance – and so has the biggest say in what U-value is ultimately calculated.

As we point out in our CPD session introducing extruded polystyrene, there are insulation materials that offer lower thermal conductivities than XPS. If you are specifying a floor or roof for the thinnest possible build-up, therefore, XPS would not seem to be your first choice on paper.

However, the reason performance gaps exist is because designs and calculations don’t translate to real-world, in-use performance. And a big part of making sure they do translate is choosing materials that retain their performance characteristics once they have been subjected to the rigours of on-site handling and installation.

“Insulation boards can get roughly treated on site,” said Rob Firman, Technical and Specification Manager at Polyfoam XPS. “It leaves foil facings peeling off and board corners and edges damaged. When that happens, the performance of the product is affected, and the performance of the floor or roof as a whole is compromised.”

Unlike other rigid insulation products, XPS is unfaced and not friable (easily crumbled). Anecdotal evidence tells us it is well able to stand up to use on site, in a way that other materials do not always achieve. It’s therefore not unreasonable to say that, coupled with a good standard of installation, XPS can help to deliver a floor or roof that is more likely to achieve its as-designed performance.

Performance requirement 2: control of moisture

Architects and design professionals don’t always retain control of their designs once a project moves to site. However thorough their specification, if a contractor or installer changes the specification in some way, or even makes a simple error, the designer does not have an opportunity to rectify what is about to go wrong.

Members of our technical team have extensive experience of dealing with technical enquiries relating to insulation. There are few situations they haven’t encountered.

One such situation is a general contractor laying floor insulation directly onto the prepared ground. They didn’t lay a damp proof membrane (DPM) first. A membrane was then installed over the insulation, followed by the concrete floor slab.

The insulation material was not XPS, and the manufacturer’s instructions made clear the insulation should not be exposed directly to moisture. The error was only realised after the floor slabs had been poured, and the contractor was then desperate to prove that the performance of the insulation would not be affected – something the manufacturer couldn’t possibly guarantee.

In contrast, XPS insulation can be laid directly on prepared ground, with a single membrane layer over it acting as both vapour control layer (VCL) and DPM. The low moisture absorption of XPS is proven by testing, and ensures that it continues to perform even if exposed to moisture.

As a designer and specifier, should you have to plan for every contingency and assume mistakes will be made? In an ideal world, no. But innocent oversights and deliberate cost-saving exercises do happen. One way to make it less likely your design is affected by them is to specify an insulation material that can tolerate a range of conditions without being adversely affected.

Performance requirement 3: loadbearing capability

A common feature of the technical enquiries we deal with at the Polyfoam XPS helpdesk is the way in which specifiers remain focused on U-values first and foremost. Other considerations, like compressive strength, follow after.

This is understandable. It’s important to know what thickness of insulation layer to accommodate to meet the target U-value. However, it also carries some risk.

For typical domestic projects, the loadings on ground floors are unlikely to be significant enough for the insulation’s compressive strength to become a factor. But in heavier duty use cases – such as commercial developments, and certainly industrial buildings – selecting floor or roof insulation with inadequate compressive strength can have knock-on effects like increasing the depth of the build-up or forcing a reassessment of structural calculations.

Much like we discussed above, designing for the thinnest possible build-up is pointless if the floor or roof cannot deliver the performance needed. XPS can provide declared compressive strength values of 300 kPa or 500 kPa, making it versatile enough to cover a range of application types and loading conditions (including car park decks).

In situations where precise loading conditions are uncertain, specifying XPS from the outset can give the confidence and reassurance that both thermal and loadbearing needs will be met.

What to do next

There are many and varied reasons why performance gaps occur in construction projects. The key is to understand the outcomes required, consider what could prevent those outcomes from being achieved, and specify products and materials – like XPS – that can play a role in mitigating issues between design and construction.

Earlier, we noted that the thermal conductivity of XPS cannot quite match the lowest values offered by other rigid board insulation materials. The good news is that, with improvements in manufacturing processes, the difference is less than it has ever been.

The ‘penalty’ of extra thickness that comes with using XPS is therefore reduced too, while retaining all of the benefits discussed in this post about moisture resistance and compressive strength. To learn more about outcomes-based insulation specification, contact us or download a copy of our comprehensive specification guide, The Polyfoam Orange Book of XPS Insulation.