Comparing XPS vs. PIR insulation: Understanding the differences for your 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.
In this blog we will look at the differences between XPS and PIR insulation and discover which insulation type is better suited for different insulation projects.
How are the performance differences between PIR and XPS insulation defined by their manufacture?
To understand the performance differences between PIR and XPS it is worth quickly exploring how each is made, as this plays an important role in explaining the variation between the properties of these two closed-cell insulation materials.
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 facers on the board are needed to help prevent the diffusion of the trapped gas.
XPS insulation, or extruded polystyrene insulation to give it its full title, 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.
Armed with this information, we can now see how the manufacturing process and structure of XPS and PIR affect key performance parameters when selecting the most suitable product for an insulation 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.”
What are the key differences between XPS vs. PIR insulation that determine their project suitability?
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 of XPS vs PIR insulation
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 of XPS vs. PIR insulation
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, such as raft foundations, basements, or intensive green roofs. 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 of XPS vs. PIR insulation
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.
Selecting the most suitable insulation can help streamline installation, support compliance with building regulations, and contribute to long-term durability and energy efficiency.
David explains why customers appreciate being given a choice of insulation options: “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.”
What applications are best suited to XPS insulation?
XPS insulation is commonly used in applications requiring high compressive strength and/or exposure to moisture.
XPS insulation as insulation for a ground floors
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.
XPS insulation for use in basements
As XPS has a resistance to water exposure it can be used to insulate a basement structure outside of the basement’s waterproofing layer. The strength of XPS insulation also means that it can resist the pressure placed upon it by the surrounding ground.
XPS Insulation for use in inverted flat roofs
In an inverted roof system, insulation is placed above the waterproofing layer and beneath a ballasted roof finish, which may include paving slabs, tiles, gravel, or stones. Rainwater passes through the ballast and insulation joints before being drained away. Because insulation in this system is exposed to water, it must be strong, durable and highly moisture-resistant – qualities that XPS insulation provides.
What applications are best suited to PIR insulation?
PIR insulation is commonly used in applications where space is limited, as its lower thermal conductivity allows for thinner insulation layers while meeting thermal performance requirements.
PIR insulation is frequently used to optimise available space while achieving the required thermal performance of a building element.
PIR’s high thermal performance, lightweight and rigidity make it a common choice for insulating walls, floors and roofs where space efficiency is a key consideration. The gastight facers can also be taped to form an air vapour control layer.
PIR insulation for use in cavity walls
Maximising internal space while maintaining energy efficiency is a key consideration in cavity wall construction. A wider cavity can impact available floor area and may lead to increased material usage, including wider foundations. PIR insulation, with its low thermal conductivity, provides a space-efficient solution that helps achieve target U-values without significantly increasing cavity width.
Designed for both partial-fill and select full-fill applications, PIR insulation offers excellent thermal performance while supporting compliance with building regulations. Its rigid structure and high insulation value make it a preferred choice for developers looking to optimise wall construction without compromising energy efficiency.
By incorporating PIR insulation into cavity walls, projects can benefit from thinner insulation profiles, improved thermal performance and a streamlined approach to meeting energy targets. This can help to create more efficient buildings without sacrificing valuable space.
PIR insulation for floors
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. 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.
PIR insulation for roofs
When the space inside a pitched roof is to become part of the heated habitable space, in either a new build project or in a room-in-roof conversion for an existing property, the insulation is installed at rafter level.
In new builds, PIR insulation’s low thermal conductivity allows for thinner insulation layers when installed between rafters, combined with either over-rafter or under-rafter insulation methods. This utilises the space inside the roof and helps to keep the roof construction material sizes down. For example, lower-performing insulation materials may require deeper rafters to achieve the same U-value, which could impact material costs and structural design.
Where an existing roof is being refurbished the rafter size is limited to that of the original build. Therefore, using PIR insulation can improve the U-value of the existing construction while preserving internal space in a room-in-roof conversion.
For new build flat roofs, PIR is a popular choice when used in conjunction with waterproofing membranes. The low thermal conductivity of PIR allows for a thinner insulation layer within the roof build-up, which may help optimise material use while achieving the required U-value. Specialist PIR boards can be used in conjunction with single ply membranes, either mechanically fixed or adhered.
Where an existing flat roof requires refurbishment, incorporating PIR insulation can help improve thermal performance and support compliance with building regulations, while limiting the overall roof build-up thickness.
David Milner describes the company’s approach to advising its customers: “We are pleased to offer our customers two widely used insulation types: XPS and PIR. We take pride in providing insulation recommendations based on the specific performance requirements of each project. In this way, our technical team can recommend an insulation solution tailored to project needs, ensuring performance characteristics align with long-term building requirements.”
Where can I find out if XPS or a PIR solution is best for my project?
At Celotex we have a wide range of both XPS and PIR insulation solutions for walls, floors and roofs. If you would like to learn what U-values they can achieve, we have an online U-value calculator ready to use or you can contact our 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.