Back to basics: five common questions about extruded polystyrene (XPS) insulation
Year-on-year increases in the specification of our products can trick us into thinking that everybody knows, and wants to find out more, about extruded polystyrene (XPS) insulation in the smallest detail. Last month (May 2026) we exhibited at Futurebuild, and the questions posed by visitors to our stand were a useful reminder that, for many people, XPS remains unfamiliar compared to other insulants.
Niche topics will always be important when it comes to XPS. We will always explore subjects like specifying insulation with installers in mind, circular economy considerations for insulation, and how updates to British Standards affect water flow reducing layers (WFRLs).
But in order to properly support specifiers and installers, we need to address fundamental questions before we can get to subjects like the correct thickness of insulation for an inverted roof, or what certain insulation terminology means. This blog post is therefore the third in an occasional ‘back to basics’ series, dealing with the five most common questions we received at Futurebuild and serving as an introduction to our XPS insulation solutions.
Previous posts in the series are:
- Back to basics part 1: thermal insulation in buildings
- Back to basics part 2: flat roof design
What is XPS insulation?
XPS insulation is a rigid foam insulation board, which is lightweight when compared to the materials used around it.
(Learn about how XPS insulation is made.)
It is a plastic-based material, and therefore relies on petrochemicals for its manufacture. However, the harmonised European standard setting out how XPS insulation boards are manufactured and tested (BS EN 13164) states the energy savings made in buildings as a result of using the products outweighs the energy required to manufacture them.
Other lightweight rigid foam insulants used in similar applications to XPS include polyisocyanurate (PIR) and extruded polystyrene (EPS).
What makes XPS different from other insulations?
Crucial to the success of any construction project is using the right products in the right applications. XPS is a versatile material that is a good all-round solution for many applications – but that doesn’t mean it is the best solution for every application.
A range of lightweight rigid foam insulants are available, providing unique combinations of features. All of them offer very good thermal performance, but there is still variation between them. (We wrote about this, for example, when looking at different ground floor insulation types.)
Where the key specification decisions lie is in understanding other characteristics beyond thermal performance. XPS excels is in the following areas.
- Moisture resistance – meaning thermal performance isn’t negatively impacted by water absorption.
- Compressive strength – meaning XPS can be used in a wide range of building types with a greater array of loading conditions.
- Robustness – XPS has consistently proven its ability to hold up to the rigours of building site conditions. Also, XPS’s thermal performance doesn’t rely on foil facings, which are easily damaged on site.
Where can XPS be used?
The most common uses of XPS insulation in buildings are as follows.
- Flat roofs (particularly inverted flat roofs), including roof terraces, green/biodiverse finishes, and blue roof systems.
- Ground floors, above and below the DPM.
- Basement walls and floors.
- External walls below DPC level, both in the cavity and to the external face of the wall (‘plinth insulation’).
- Specific areas of detailing in low-energy and Passivhaus construction, especially where moisture might be an issue.
- Swimming pool basins.
- Laminating and fabrication.
Other uses of XPS in buildings include as an infill for composite doors, and as a backing for cladding panels. Underfloor heating boards made from XPS feature routed channels for hot water pipes, and allow the installation of a floating floor finish directly over.
A perhaps surprising use of XPS is in park homes, where it can be used to meet the requirements of BS 3632.
There are also many non-building uses, including temperature-controlled vans, cold stores and pontoons. We have even read about it being used underneath a runway at an airfield in Alaska!
Can XPS be recycled?
The short answer is: yes, extruded polystyrene can be recycled. The longer, fuller answer is explored in our blog post about recycling XPS, where we touch on how recycling is a feature of the production process as well as recycling material taken from site.
You can also learn about how we supported the retrieval and recycling of XPS material from two sites, as a result of what we learned while researching this subject.
Does XPS insulation have a BBA certificate?
Third-party certification – such as BBA certificates – for construction products is important, and XPS is no exception. The nature and scope of any certification varies from manufacturer to manufacturer, so it is essential to understand what solutions a manufacturer has certification for and what build-ups that certification covers.
At Polyfoam XPS, we have BBA certificates covering our Floorboard and Roofboard ranges. Find our latest certificates on our technical support page, or contact us to discuss your project and certification needs in more detail.
What to do next
If this blog post has served as a useful introduction to some of the basics of XPS insulation, then you can delve deeper by downloading a copy of The Polyfoam Orange Book of XPS Insulation. Created with designers and specifiers in mind, the book is a comprehensive guide to XPS which explores everything discussed above and more, allowing you to learn at your own pace.