Insulation terminology: what do different words and phrases mean?

Specifiers and installers who regularly research construction products will be familiar with certain commonly-used words and phrases. Sometimes, these words and phrases are applied generically across a large range of products. Other times, they may be specific to a certain type of product – like thermal insulation – or used in a specific way depending on the product in question.

The meaning of words and phrases used to describe the performance of construction products has been brought into sharp relief by the Code for Construction Product Information, or CCPI. The Code’s aim is to improve clarity and accuracy in the marketing of product information, and that raises questions about how careful a manufacturer is in describing their products – whether they are undergoing a Code assessment or not.

Inspired by the CCPI publication Words and Phrases to Avoid Using, this post describes the meaning of selected terminology that we use regularly in connection with our extruded polystyrene (XPS) insulation. Where one exists, we provide a source for the terminology. For words and phrases that don’t have a formal source, we explain our justification. In this way, we hope to further demonstrate our commitment to communicating transparently about our products, until such a time as we formally apply for CCPI Assessment.

Fundamental characteristics of XPS

When it comes to the essential performance characteristics of our insulation products, certain terminology is simple and straightforward. All of the following are tested as described in specific standards, as required by, and declared in accordance with, the harmonised standard for extruded polystyrene, BS EN 13164. The declared performance is published in the product’s Declaration of Performance (DoP).

• Thermal performance.
• Compressive strength.
• Moisture resistance (see further explanation below).
• Fire performance (see further explanation below).

In terms of moisture, ‘moisture resistant’ is distinct from ‘moisture tolerant’. Some other rigid foam insulation products are resistant to moisture vapour, or they absorb low quantities of liquid water (but still a higher quantity than XPS). They are described as ‘moisture tolerant’ but their thermal performance is negatively impacted by the moisture absorbed, or by foil facings coming away from the foam core due to the moisture exposure.

This is best highlighted by typical solid ground floor constructions. All manufacturers of foil-faced rigid foam insulation expressly state that their products should not be laid in direct contact with the ground. Instead, they should be above the damp proof membrane (DPM) to ensure their performance is unaffected by moisture in the ground. Thanks to its water absorption performance, XPS has no such issues and can be laid below the DPM without loss of thermal performance.

Where fire performance is concerned, further classifications may apply where a product is used within a specific build-up and tested as part of a complete element, rather than as a standalone material. In either case, you should never see us use misleading phrases such as ‘fire retardant’ or ‘fireproof.’

Is XPS insulation ‘easy to handle’ and ‘easy to install’?

A word you are likely to see us use often in relation to our products is ‘lightweight.’ The CCPI documentation says use of this phrase depends on what the product is being compared to.

A lot of manufacturers of rigid foam insulation boards, including Polyfoam XPS, commonly use the term – not least because the insulation is usually lightweight in comparison with the other materials used in the same build-up.

For instance, an XPS board used in a ground floor construction is lighter than the hardcore, concrete or screed that also forms part of the construction.

As a result of being lightweight compared to other associated materials, it’s also common to see rigid foam insulation manufacturers say their product is ‘easy to handle’ and/or ‘easy to install.’ The CCPI documentation says ‘easy to install’ must not be used as it is reliant on user experience.

However, our BBA certificates say the products should be used by experienced installers. As a result, we still tend to refer to our products being easy to install. When we do, it is on the basis of experienced installers using our rigid foam insulation board in an application where it is does not need to be mechanically fixed (i.e. ground floors and inverted flat roofs).

What sets XPS apart from other rigid foam insulation?

In the final part of this post, we are talking specifically about the following three terms.

• Durable.
• Robust.
• High performance.

Like the characteristics discussed earlier, durability is tested and declared as required by BS EN 13164. The declaration is published in the relevant DoP. In addition, our BBA certificates refer to the durability of the material.

For example, our BBA certificate for flooring says: “The products are rot proof, dimensionally stable and, when installed with the overlays specified in this Certificate, will remain effective as an insulating material for the life of the building in which they are incorporated.”

While there is no issue with ‘durable’ the CCPI documentation does have reservations about the term ‘robust.’ It classes ‘robust’ as a general adjective that isn’t appropriate. However, XPS is not friable (easily crumbled), and we know from anecdotal evidence that the material is well able to stand up to the rigours of use on site, in a way that other insulation materials do not always achieve.

It is in this context that we use the word ‘robust’ to describe our XPS products.

All of which discussion leads us to the phrase ‘high performance,’ which the CCPI documentation says is never acceptable.

“With a view to future assessment under the CCPI, we have sought to reduce our use of the term ‘high performance’ in product literature and marketing,” said Rob Firman, Technical and Specification Manager at Polyfoam XPS. “However, as a phrase that has been used commonly across the construction industry, it is virtually impossible to identify all historic uses of it.”

In any situation where you see us use the phrase ‘high performance,’ it is designed to reflect that XPS is a material manufactured to provide a desirable combination of characteristics (in particular: thermal performance, compressive strength, moisture resistance, durability and robustness).

From experience, we know this desirable combination of characteristics is capable of contributing positively to a wide array of building types, including ones certified to voluntary standards that surpass the minimum requirements of national building regulations.

To learn more about how the characteristics of XPS can benefit your current project, contact us to discuss your specification or installation.

April 2026 note: this blog post is based on content written for the first edition of The Polyfoam Orange Book of XPS Insulation, our comprehensive guide to extruded polystyrene (XPS) published in March 2026. Written for designers, specifiers and contractors, you can learn more about the book, including how to request a copy, here.