Comparing XPS vs PIR Insulation For Your Flooring Insulation Project

When comparing XPS vs PIR insulation for a building project it is important to understand how the properties of XPS insulation differ from those of PIR insulation. Although both are lightweight rigid insulation boards, their methods of manufacture and composition result in differences in thermal performance, compressive strength and moisture resistance. Selecting the most suitable insulation can help streamline installation, support compliance with building regulations, and contribute to long-term durability and energy efficiency.

In this article, Celotex looks at the differences between XPS and PIR insulation and discover which insulation type is better suited for different insulation projects.

How are XPS and PIR manufactured?

Polyisocyanurate, or ‘PIR’ insulation as it is more commonly known, is made by reacting two liquid chemicals – an MDI and a polyol, with a blowing agent. The blowing agent is trapped inside the cells of the foam that are formed by the reaction between the two liquid chemicals. The foam rises and is cured in a continuous process between two facers that form the top and bottom of the PIR insulation board. The gastight properties of the board are needed to help prevent the diffusion of the trapped gas.

XPS insulation, or extruded polystyrene insulation, is made by mixing polystyrene with a blowing agent. The resulting fluid is forced under pressure through a die in a continuous extrusion process and it expands into a foam where a smooth skin forms on the top and bottom of the XPS insulation board. 

The blowing agents used in XPS and PIR have lower thermal conductivity than air, reducing heat transfer through the board. PIR is more efficient than XPS at trapping blowing agents within its closed cell structure and has a better insulation performance than XPS. However, XPS insulation does not need facers to retain the blowing agent and maintain its thermal properties.

XPS has a more uniform closed-cell structure than PIR, giving it higher compressive strength and greater resistance to moisture absorption. The key differences between XPS and PIR insulation that determine their suitability for a project are their thermal performance, compressive strength and resistance to moisture. 

Thermal performance

The typical thermal conductivities, or lambda values of each insulation are:

• XPS insulation: from 0.029 to 0.036 W/mk
• PIR insulation: from 0.021 to 0.028 W/mk

The lower the thermal conductivity, the better the insulator. Therefore, PIR has an improved insulation property to XPS. In practical terms on a project, this means that less thickness of PIR will be required to achieve the required thermal performance of a building element. Or, if you have limited space in an application, fitting PIR insulation, rather than XPS will maximise the thermal performance of the element.

Compressive strength

The typical compressive strengths of each insulation are:

• XPS insulation: from 300 to 700 kPa
• PIR insulation: from 120 to 150 kPa

XPS has a higher compressive strength than PIR, making it well-suited for applications where insulation must withstand greater loads. This does not rule out PIR, as this can still be used under concrete screed for flooring where the loading allows, however, there will be situations where high static or moving loads will be better served by using XPS insulation. In these situations, guidance should be taken from the project structural engineer as to expected loadings and preferred floor build-up and insulation type.  

Water-resistance properties

XPS insulation is better at resisting the ingress of moisture than PIR insulation.

XPS has a hard skin on its surface with a strong closed cell structure throughout the board, whereas PIR has facers on its surface that help to protect the cells of the PIR foam.

Although PIR can get wet for short periods, it should be dried out before use. The exposed edges of the PIR boards can absorb water, as can any areas where the facer has become detached or damaged. Therefore, when used in applications where moisture is present a damp proof membrane (DPM) needs to be in place to protect the PIR boards.

By contrast XPS insulation does not require such protection from moisture and can be used in floor applications without the need for a DPM or in inverted roofs.

What applications are best suited to XPS vs PIR insulation?

Ground floors are in contact, either directly or indirectly with the ground and can be subject to high loading, so this makes XPS insulation well suited to this application. 

The insulation can be positioned below a ground bearing slab in strip or raft foundation, together with a ground bearing concrete floor. The strength and moisture-resistant properties of XPS insulation provide durability in applications where ground moisture exposure is a concern.

Equally, in cases where insulation is placed above a ground-bearing slab, suspended concrete floor, or concrete beam-and-block floor, XPS provides structural strength and moisture resistance to support long-term performance in these conditions.

Whilst PIR cannot match the compressive strength or water-resistant properties of XPS insulation, its ability to offer a thin solution for solid floors where loadings are not as high makes it common for many domestic projects, including floors. Where underfloor heating (UFH) is installed within the screed, Celotex PIR insulation is commonly used above the slab to help improve heat transfer and response time. The clips for the UFH pipes are pushed through the vapour control layer into the PIR boards and then the screed is poured on top.

When retrofitting suspended timber floors, in a similar way to the rafters in the room-in-roof example, the project goal is to maximise the thermal performance of the space between the floor joists. Installing PIR boards between the joists can help reduce the U-value of the existing floor, enhancing its thermal efficiency due to PIR’s low lambda value.

Where can I find out if XPS or a PIR solution is best for my project?

David Milner, technical team leader at Celotex, describes the benefits of being able to offer both XPS and PIR insulation: “Our deep understanding of how our manufacturing processes shape different performance parameters within XPS and PIR insulation allows us to be able to confidently recommend the most suitable insulation solution for individual projects.

“XPS and PIR insulation are widely used in the construction industry, but no single insulation type is suitable for every project. Our customers value recommendations that are based on technical and practical requirements rather than commercial preference.”

Celotex has a wide range of both XPS and PIR insulation solutions for floors, as well as other areas such as walls and roofs. If you would like to learn what U-values they can achieve, the manufacturer has an online U-value calculator ready to use or you can contact Celotex’s technical team. They will be happy to run through some calculations and can offer advice on the best insulation choice for your project and even send you some samples.

www.celotex.co.uk