For a flat roof construction – be it a cold roof, a warm roof, or a warm inverted roof – the calculation method for year-round condensation risk set out in BS EN ISO 13788, known as the Glaser method, is almost always sufficient. As long as the roof accords with good practice then a more complex calculation method should not be necessary.

Good or best practice in terms of condensation risk means following the guidance of BS 5250 ‘Control of condensation in buildings’. Ensuring the roof includes a suitable vapour control layer on the warm side of the insulation layer is an appropriate starting point.

What does BS 6229 say about condensation risk in flat roofs?

For a flat roof design to be considered good practice, it should also follow the recommendations of BS 6229 ‘Flat roofs with continuously supported flexible waterproof coverings – code of practice’. We have written about the revised version of BS 6229, published in 2018, in this post.

Section 4.7 of BS 6229 offers the standard’s own summary of how to deal with condensation risk, and says the risk of surface and interstitial condensation should be analysed based on BS 5250 and BS EN ISO 13788.

The key part of this section, and the reason for this blog post, is where it says the risk should be assessed “using an external temperature of -5 deg.C for 60 days during the heating season, to allow for the cooling effect of clear sky radiation.”

In simple terms, and as the name suggests, ‘clear sky radiation’ refers to the radiative cooling that takes place when there is no cloud cover to restrict the emission of longwave radiation from the Earth’s surface.

Are winter parameters for flat roof condensation risk analysis new?

The recommendation to use an external temperature of -5 deg.C for 60 days during the heating season (i.e. winter) is not new. It has featured in versions of BS 6229 since the 1980s, but has only come to prominence with the substantial revision of the standard that took place in 2018.

Calculation software, which normally produces a Glaser method condensation risk analysis alongside a U-value calculation, does not automatically apply the lower temperature. The climate data used by software is location-specific, not application-specific. Since the recommendation to use a lower winter external temperature is specific to flat roofs, it is incumbent upon clients to ask for the data to be amended, or for software users to amend it if they want to follow the recommendation.

In truth, for cold, warm or inverted warm flat roof designs that follow best practice guidance, using a winter temperature of -5 will not make a difference to the outcome of a condensation risk analysis. It will change the output of the analysis slightly, in terms of the temperature and dewpoint charts, but it will not change the outcome of no predicted condensation risk.

Where a difference might be seen is in non-standard roofs that do not follow best practice, such as hybrid (or ‘over and under’) flat roof constructions. These types of build-up are not recommended as they heighten the risk of condensation anyway, so adherence to the winter external temperature recommendation will only serve to reinforce the unsuitability of these roofs.

What is Polyfoam XPS’s approach to winter condensation risk in flat roofs?

Since the revision of BS 6229 brought to prominence this recommendation for a lower winter external temperature, Polyfoam XPS have adopted the measure as standard. We always aim to accord with best practice and are happy to apply this measure to U-value calculations and condensation risk analyses for both conventional warm flat roofs and inverted warm flat roofs.

The standard does not specify which 60 days of the heating season the measure should be applied to, so we elect to amend the external temperature for January and February. Customers are also free to specify other parameters if their project requires. For example, we recently dealt with a technical enquiry where the external temperature was adjusted for three months of winter – but that is a rare occurrence.

For more information about our U-value calculation service, or to discuss your project, contact us.

Published May 2019.


Polyfoam XPS in a green roof

A blue roof is a flat roof construction – either a warm roof or an inverted warm roof – where additional elements are incorporated into the build-up to provide temporary control of the flow of rainwater during heavy rain. The aim is to provide a sustainable urban drainage solution (SUDS) on sites where other solutions are impractical.

Blue roof solutions are not water storage. Their aim is controlling the flow of water, by use of a restrictor, to avoid overwhelming the storm drainage on site and away from the site. Following rainfall, the rainwater should be successfully drained within 24 hours – although only the most extreme weather event would require a full day to drain.

This post focuses on inverted blue roofs, the covering of which can be gravel ballast, paving, or a green roof covering, just like a ‘standard’ inverted warm roof.

What is current flat roof industry guidance on inverted blue roofs?

No British or European standard yet exists which deals exclusively with blue roofs. Over the last two or three years, a committee convened by the National Federation of Roofing Contractors (NFRC) drafted and published the NFRC Technical Guidance Note for the Construction and Design of Blue Roofs.

At the end of 2018, a revised version of BS 6229 Flat roofs with continuously supported flexible waterproof coverings – Code of practice was published, featuring recommendations relating to inverted blue roofs and some commonality with the NFRC Technical Guidance Note. However, the updated BS 6229 also appears to draw some of its own conclusions.

There are already concerns about the approach to inverted warm roof thermal performance in BS 6229, as we detail in this post. The current guidance on blue roof construction makes statements that are causing confusion within the flat roofing industry. Because no formal standard exists, there is no formal test method to determine how blue roofs behave.

As a result, there is a risk of different stakeholders seeking to undertake modified versions of existing tests to try and demonstrate the performance of a blue roof construction, with no joined-up approach to achieving a common conclusion.

Why is there confusion about inverted blue roof guidance?

In an inverted blue roof construction, the ‘attenuation zone’, or the part of the roof where a limited amount of water temporarily sits during heavy rainfall, is directly above the water control layer.

The water control layer is recognised as a drainage layer in its own right, but is not a waterproof layer. In a standard inverted roof, it significantly restricts the volume of water that moves through the insulation layer and reaches the waterproofed structural deck. The test that measures this water flow, however, is not considered appropriate to blue roof construction, due to the potential presence of a head of water that is not accounted for.

At face value, both the NFRC Guidance Note and BS 6229 are clear in what approach to take. The fx value used to correct for rainwater cooling should be 0.04 where square-edged insulation boards are used, and 0.03 when insulation boards with a profiled edge are installed.

BS 6229 goes further than the Technical Guidance Note, saying these fx values apply “irrespective of the covering” and “no correction to fx is permitted for inverted blue roofs”.

Insulation manufacturers supply inverted roof solutions with testing and third-party certification that allow a significant correction to fx, based on the standard water flow test. The confusion exists because those who work with blue roof systems on a daily basis feel the standard does not accurately represent the systems they supply.

How does a blue inverted roof actually perform?

In discussing the amount of water that will be present in the attenuation zone of a blue roof, the NFRC Technical Guidance Note says, “In reality, it is exceedingly unlikely that the roof will ever reach full capacity, as it will start to drain as soon as it starts to rain.”

This reaffirms how a blue roof is not a water storage solution. Even if the presence of a slight head of water was enough to cause some increased water flow through the water control layer and insulation layer, the waterproofing on the deck still provides secondary drainage as normal.

However, BS 6229 says, “the thermal insulation in inverted blue roofs is regularly saturated and therefore the roof system might remain permanently damp and not achieve the expected thermal performance.”

Blue roofing professionals believe this misrepresents how their systems perform.

What next for inverted blue roofs?

As with BS 6229’s criticisms around workmanship of water control layers, its approach to blue roof design does not seem to have been founded on a thorough evidence base. The typical design of blue roof attenuation zones is not reflected in BS 6229’s assumption of an ‘open roof covering’.

This is leading to a questioning of the published guidance, which is not helpful when the document is relied upon as an authoritative code of practice.

We are working with other organisations across the flat roofing industry, including members of the Liquid Roofing and Waterproofing Association (LRWA) and the National Federation of Roofing Contractors (NFRC) to collectively voice these concerns to ensure we are working with clear guidance that avoids ambiguity and we look forward to the issues outlined here being addressed at the earliest opportunity.

Published in May 2019 and updated July 2019.


Polyfoam XPS Flooring

Rob Firman, Technical and Specification Manager at Polyfoam XPS highlights common misconceptions about ground floor insulation and offers guidance on specification and installation.

How many types of rigid insulation can you can name off the top of your head? And no, this isn’t an attempt to shame anybody for being unfamiliar with insulation, and there are no prizes for being able to name the most!

The point is that there are many misconceptions about floor insulation within the flooring industry. Materials are as likely to be identified by colour and visual appearance as they are by knowledge of their performance characteristics.

Unless it is a system requiring specialist installation equipment, like blown cavity wall insulation, there are no specialist insulation installers in the construction industry. Floor insulation tends to be considered an ‘extra’, carried out by other specialists – like screeding contractors – as part of their work.

There are however, key differences between rigid insulation types and it’s important to know what these are. That’s because if one material is specified, but an alternative material is offered, purchased and installed, that other material may not offer the same performance.

If it has a worse thermal performance, the building may not comply with energy efficiency requirements. If it is not capable of bearing the same loads, there could be a failure of the floor.

So, here’s our guide to the different types of ground floor insulation available and what to consider when specifying and installing these products.


In the history of our built environment, floor insulation is still a relatively recent phenomenon. Experienced designers and contractors may remember when it did not feature in build-ups at all.

Over the last couple of decades, the performance required from buildings has made high levels of thermal insulation a necessity in all building elements. As a result, floor insulation’s primary function is to resist the flow of heat energy from the building’s interior down into the ground.

However, a ground floor construction also bears the load of all the people, furniture, fixtures and fittings in a building – even vehicle movements in some cases. Those loads transfer through the floor insulation, in addition to the dead weight of any screed or slab over it, so it has to be sufficiently strong as well as performing thermally.

The loadbearing nature of ground floor insulation means compressible materials are unsuitable. Rigid insulation materials, sold as boards, or sheets, slabs, or panels, depending on your preferred terminology, not only bear loads with relative ease, but their large-format sheet sizes can be laid quickly and easily over the floor’s surface area.

Insulation comparisons

Rigid insulation materials are mainly lightweight, plastic-based rigid foam insulations.

In recent years vacuum insulated panels (VIPs) have begun to offer an alternative to the more common board stock materials. However, these products are best suited to refurbishment projects where the depth of insulation is constrained. The table shown highlights how VIPs compare to other insulation types.

Insulation typeKnown asTypical thermal conductivity (W/mK)Typical compressive strength (kPa)
Extruded polystyreneXPS0.033 to 0.036, depending on grade200 to 500, depending on grade
Expanded polystyreneEPS0.032 to 0.038, depending on grade70 to 250, depending on grade
Phenolic foamPIF0.018120
Vacuum insulated panelsVIP0.007150

Both phenolic and PIR foams derive some of their long-term thermal performance from facing materials that restrict the loss of the gas in the foam structure. Protecting those facings is important to ensure they perform for the life of the building.

That means keeping water away from the insulation boards, regardless of them being closed cell materials with low rates of water absorption. They should always be installed above the damp proof membrane (DPM), and never laid exposed directly to the ground.

Similarly, although EPS insulation has no facings that are susceptible to damage from alkalis or moisture, its capacity for moisture absorption means it must still be installed above the DPM.

As the table shows, EPS is capable of much greater loadbearing capacity than phenolic and PIR foams – but is also made available in lower compressive strengths. Even with compressive strengths exceeding 120 or 140 kPa, manufacturers of phenolic and PIR products can be extremely cautious about offering their insulation for anything more than light commercial applications.

Comparing EPS and XPS insulation

As different types of polystyrene insulation, price tends to be the common differentiator between EPS and XPS. However, the significantly increased compressive strength offered by higher grades of XPS makes it the only choice for certain applications – particularly those involving vehicular traffic.

Like EPS boards, XPS products are typically unfaced. But they offer another important advantage: lower moisture absorption. They are more tolerant of wet conditions, so can be installed against the ground without affecting performance over time.

It’s not uncommon for phenolic and PIR foam manufacturers to receive questions about the suitability of their products for being laid on the ground, usually because they were installed below the DPM and somebody has queried the correct sequencing. More often than not it is too late to do anything, since there is an understandable lack of appetite to break up a freshly-poured concrete slab.

The floor will be accepted as-is, with no penalty to anyone except the end user (and their energy bills) if the thermal efficiency gradually worsens over time. XPS might need a slightly increased thickness to reach the same intended U-value, but can offer a more robust and flexible solution as it can be positioned above or below the DPM and is resilient if exposed to moisture.

Making the right choice

As we have sought to learn more about other areas of the floor industry such as slabs, screeds, and floor coverings, and how our products work with them, we’ve discovered that awareness and understanding of insulation is not always what it should be.

For example, there can be little difference in how XPS and EPS products are perceived, which is concerning when there is such wide variation in the performance range of EPS products. And because PIR and phenolic foams offer superior thermal performance, it’s not uncommon for it to be assumed that they are superior in compressive strength too.

Sometimes, insulation is simply used as a void-filling material. While there is nothing wrong with that, the desire to use the cheapest material might lead to the conclusion that the cheapest material will suffice for any ground floor construction.

It’s not hard to see why confusion can arise regarding insulation specification and installation. However, a greater awareness and understanding of insulation products can significantly reduce risks on site and ease installation.

It’s important to value the thermal and loadbearing performance of materials over how much they cost to ensure ground floor construction performs as expected. And if a product substitution can’t be avoided, make sure the same material type is used with an equivalent performance.

If in doubt, seek advice from the insulation manufacturer’s technical team who should be able to guide you on specification and installation, including u-value calculations.

This article appeared in the Contract Flooring Journal February edition


Ian Exall

Ian Exall, National Sales Manager of Polyfoam XPS gives his view.

Fundamentally, price remains the driving factor on construction projects, but priorities have shifted. The focus is no longer on specifying or purchasing the cheapest products. The priority for most of our customers is value for money. They want affordable products, but which can still perform to high standards and are seeking assurances through third party guarantees and certifications such as BBA before purchasing. Cost certainty is another key consideration, which is being fuelled by Brexit and fears of the unknown. We are getting lots of requests for long-term pricing for projects, especially for larger schemes which could take two years or more to complete. Many contractors are worried about the risk of product and labour inflation, so want the assurance that prices will remain stable despite potential changes to market and/or economic conditions. Whilst this is understandable, it simply moves the risk along the supply chain where margins may be insufficient to withstand potential inflationary pressures. At the same time, we have noticed a big crackdown on specification switching and so has our distribution network. This is a trend which follows the recommendations in the Hackitt Review and the push for more competency across the construction industry. Long-term, this will have a positive impact on our sector as switching specified insulation during construction can lead to inaccurate U-values and potential non-compliance with Building Regulations. However, following the Grenfell tragedy, minimising risk is another priority for construction professionals and this is affecting all aspects of the industry, including specification. As a result, we are seeing some insulation specifications being held even if there is a better product available both in terms of cost and performance. This suggests that roofing contractors, main contractors and other specifiers are reluctant to take on the responsibility of any liability that may come with such a change. Uncertainty won’t go away in 2019 and inevitably, that means our customers will continue to put price in the priority list but it’s no longer just a numbers game. The whole industry is moving towards a more forensic approach that shines the spotlight on quality, performance and risk, and this will continue to affect purchasing decisions. For manufacturers of construction products, this underlines the ever growing need to provide assurances, advice and support which help contractors and specifiers make more informed decisions. This commentary appeared in the January 2019 edition of Total Contractor


Amendments to Building Regulations for fire safety

December 2018 saw the publication of amendments to the Building Regulations in England, as part of the response to the Grenfell Tower disaster. The amendments came into effect in January 2019.

Which areas of the Building Regulations have been changed?

Part B of the Building Regulations, supported by the guidance published in the two volumes of Approved Document B, is the natural starting point for looking at what has been altered.

Two amendment documents have been published and are available via the government website. The first covers both volumes of Approved Document B and consists largely of some new introductory text for the two volumes, re-numbering of paragraphs affected by the amendments, and some changes to the appendices.

The second document covers volume 2 of Approved Document B only (buildings other than dwellinghouses), and comprises a revised wording of requirement B4 (external fire spread), together with replacement text for the whole of section 12 (construction of external walls). There are also some changes to other appendices. The revision to requirement B4 and section 12 of volume 2 goes so far as to also include the text of the amended regulation 7 of the Building Regulations.

Regulation 7 deals with materials and workmanship, previously consisted of one paragraph only, and is supported by its own Approved Document. The decision to reproduce the text in the amendment to Approved Document B volume 2 arguably reflects the extent to which many construction professionals are unaware of the full extent of the Building Regulations and how they should be approached holistically.

Further paragraphs have been added to regulation 7 to make law the requirement to use materials of limited combustibility for the “external wall, or specified attachment, of a relevant building”. Paragraph 4 of regulation defines a ‘relevant building’.

As a manufacturer of extruded polystyrene insulation, Polyfoam XPS has received enquiries relating to the Building Regulations amendments, and the rest of this post explains our understanding of how the use of Polyfoam XPS products should not be affected by the published amendments.

The effect on XPS insulation – roof upstands

Where a flat roof meets an external wall, an upstand is required to help prevent the ingress of water to the building. To limit the cold bridging effect of the junction, the upstand should also be insulated, and Polyfoam XPS offers Upstand Board specifically for this application.

Some people have interpreted the upstand as forming part of the external wall. Where an upstand is on a ‘relevant building’ as defined by the revised regulation 7, their concern is that XPS insulation – which does not achieve a Euroclass A1 or A2 fire rating – is no longer appropriate.

However, paragraph 3 of the revised regulation 7 lists exceptions where the new requirement for materials of limited combustibility does not apply, including “any part of a roof if that part is connected to an external wall”.

The roof upstand constitutes part of the flat roof and it is therefore our view that a Building Control Body should continue to accept the use of a product like Polyfoam Upstand Board on projects that fall under the description of a ‘relevant building’ in paragraph 2 of regulation 7.

The effect on XPS insulation – balconies

The revised regulation includes not only external walls, but also ‘specified attachments’. A number of items are considered to be ‘specified attachments’, including “a balcony attached to an external wall”. This has led to concerns that where a flat roof designed as a balcony abuts an external wall, the materials in the flat roof should meet the requirement for limited combustibility.

Inverted warm roof constructions featuring Polyfoam XPS products are frequently designed as balconies. However, in these situations, if the balcony is insulated then that means it is over a heated space. It therefore becomes a thermal element in its own right – i.e. a flat roof – and is subject to all other necessary Building Regulations requirements – including fire safety.

Our view is that a balcony considered to be a ‘specified attachment’ is something essentially bolted on to the external wall, outside of the thermal envelope and considered on its own merit in terms of fire performance.

Should you or your Building Control Body have any questions or concerns about the use of Polyfoam XPS products on projects where this sensitive area of regulation applies, contact our technical helpline on 01429 855120, or email

Published February 2019.


Technical Support for BS6229

A revised version of BS 6229, the code of practice for ‘flat roofs with continuously supported flexible waterproofing coverings’, came into effect on 30th November 2018. As part of the wide-ranging update, the standard’s guidance about inverted roof construction was amended.

For more information about what BS 6229 covers generally, we have written a separate blog post.

BS 6229 guidance about the thermal performance of inverted flat roofs

In addition to a general description of inverted roof construction, paragraph is the section of most interest in terms of inverted roof construction. It describes how the thermal performance of an inverted roof should be established to include for the cooling effect of rainwater that penetrates the insulation layer and reaches the waterproofing.

This procedure should be familiar to anybody who regularly works with inverted roof insulation manufacturers or system suppliers. Thermal transmittance (U-value) is calculated following the method in BS EN ISO 6946, using the design thermal conductivity of the insulation and rainwater cooling correction, as specified in the accompanying guidance of ETAG 031-1, BRE Report BR 443, and BBA Information Bulletin No. 4.

Furthermore, the drainage factor, f, can be obtained from third-party testing and certification if available – such as Polyfoam XPS’s BBA certificate for flat roofing.

What has changed in BS 6229’s guidance on inverted roofs?

The 2018 revision features a supplementary note to paragraph It says that imperfections occur in the water flow reducing layer installed over the inverted roof insulation, due to poor workmanship, poor detailing, or post-construction damage, increasing the volume of water likely to reach the waterproofing layer.

In turn, that increases the water cooling effect on the insulation and worsens the in-service thermal performance of the roof, beyond what was anticipated at design stage. To compensate, the supplementary note suggests increasing the thickness of the insulation layer by 10% until such a time as further evidence and performance testing is available.

What are the issues with this revised inverted roof guidance?

Polyfoam XPS has a number of issues with this supplementary note in BS 6229: 2018.

The Standard’s foreword is clear that “notes give references and additional information that are important but do not form part of the recommendations”, but readers may not be aware of that. The inclusion of this note could lead people to wrongly assume their designs and installations do not accord with current best practice, even though the correct calculation procedures have been followed.

Standards are typically written assuming a good level of workmanship. Workmanship is a requirement of national building regulations, and it is impossible to make provisions for every scenario of something being installed poorly. It is unusual to see a comment included in a standard which assumes poor workmanship, for which evidence has not been gathered.

As a manufacturer supplying inverted roof insulation and water control layers, Polyfoam XPS is not aware of widespread workmanship issues. If the committee responsible for BS 6229 has evidence to the contrary then industry – including manufacturers like Polyfoam XPS – should be given the opportunity to respond and address those issues.

Instead, the suggestion of a 10% increase in insulation thickness feels arbitrary. If changes to insulation thicknesses are necessary then they should have been included in the recommendations upon the completion of evidence gathering and further testing – not as a potentially scare-mongering supplementary note in advance of any such exercise.

Mechanisms exist within the calculation method to achieve a similar level of caution, such as using an increased value for the drainage factor, f. It is not clear why the supplementary note, if it had to be written at all, did not suggest this, rather than an arbitrary percentage increase in insulation thickness.


If clients and customers are concerned about the quality of workmanship then we advise them to speak to their contractor. However, if they wish to explore a compensatory factor in their U-value calculations then we are happy to work with them to discuss a measurable level of caution.

Otherwise, the recommendations of BS 6229: 2018 are clear. The current method for calculating the thermal performance of inverted roofs is valid and does not require any change in current practice. Polyfoam XPS will therefore continue to abide by all existing calculation standards and guidance.

The LRWA have published further guidance on the topic and it can be found on the LRWA website. To discuss inverted roof U-value calculations in more detail, contact our technical helpline on 01429 855120, or email

Published February 2019 / Updated March 2020

WHAT IS BS 6229:2018?

What is BS 6229:2018

As a British Standard only (rather than an adopted European or International standard), BS 6229 concentrates on flat roof construction in the UK. It describes best practice in the design, construction and maintenance of cold, warm and inverted flat roofs with a fully supported, flexible waterproofing layer.

Like any British Standard code of practice, the contents of BS 6229 are guidance and recommendations only. The flat roofing industry collaborates to produce this standard, and the guidance therefore carries weight. Any claim of compliance, however, should be made with care – and any deviation from the recommendations should be justified if design and construction work is suggested to be compliant.

What is the current version of BS 6229?

The previous version of BS 6229 was published in 2003. The 2018 version acknowledges that many changes have since occurred in how buildings are constructed, and how waterproofing materials perform. BS 6229:2018 therefore represents a comprehensive and wholesale review of the guidance.

To avoid the duplication of information, BS 6229 no longer gives detailed information on condensation risk, workmanship or acoustics. Instead, it points readers in the direction of other standards specifically addressing those topics: BS 5250, BS 8000-4, and BS 8233 respectively.

What roof constructions does BS 6229 cover?

The standard gives advice and recommendations for flat roofs, which it defines as having a pitch of up to 10 degrees and a deck constructed from timber, metal or concrete. The “fully supported, flexible” waterproofing types referred to by section 5.4 of the standard are:

  • Reinforced bitumen membranes.
  • Mastic asphalt.
  • Plastic and rubber sheets.
  • Hot and cold liquid applied roofing.

Four types of roof system are recognised: warm flat roofs, inverted warm flat roofs, cold flat roofs, and uninsulated flat roofs. Reference is also made to hybrid flat roof constructions and the heightened risk of condensation for which they can be responsible. While BS 6229 does not say outright that hybrid roofs are not recommended, it does make clear that designers “should select the type of flat roof most suitable for the intended building”.

What other areas of roof design feature in BS 6229?

Drainage and minimum falls is a critical aspect of flat roof design. To aid good detailing, not only does BS 6229 state the falls that should be achieved, it also includes typical level access designs for the different roof systems.

Comprehensive guidance on flat roof thermal performance is offered, including how point thermal bridges should be treated when they prevent a continuous insulation layer being installed. A separate section is given over to the correction factors applied to inverted roofing (which we discuss in more detail in this blog post). Green and blue inverted roofs feature in their own short sections, including advice not to take into account any thermal performance of growing mediums for green roofs.

A short section introducing the topic of condensation risk analysis includes a recommendation to use an external temperature of -5 deg.C for 60 days during the heating season to allow for the cooling effect of clear sky radiation. While this has been a feature of the standard since the 1980s, awareness of it appears to be limited and is arguably not common practice in the industry.

Guidance on design issues concludes with sections on surface protection – which also addresses ballast for inverted roofs – and rooftop installations such as plant.

Roofing materials, workmanship and inspection

The remainder of BS 6229 lists commonly used materials for the roof structure, waterproofing, thermal insulation and vapour control layers, and gives standards and other applicable guidance to which they should be manufactured or prepared.

A short section about workmanship highlights some of the ways in which roof coverings can fail, and briefly describes how roofing materials should be cared for on site. Finally, a section on roof inspections and maintenance describes how best to ensure the roof system achieves its service life, and the role that a building information manual can play.

To discuss BS 6229 and roof system design in more detail, and how Polyfoam XPS insulation can be part of the roof system for your project, contact our technical helpline on 01429 855120, or email

Published February 2019.


Insulation matters: the importance of correct specification and installation

Poorly specified and/or installed insulation can have a detrimental impact on ground floors. Rob Firman, Technical and Specification Manager at Polyfoam XPS explains the risks and offers advice on what products to choose.

If you’re a flooring contractor, how far would you go to demonstrate that you have correctly constructed a floor?

Would you open it up to allow further inspection, knowing it would have to be made good again? What about opening up the floor, removing a sample of insulation, and trying to arrange with the manufacturer to have it tested to show the method of installation was not detrimental to its thermal performance?

At this point, you may be wondering why you would need to go to these lengths if the floor had been installed correctly – and the answer is that you wouldn’t.

However, these are the steps one contractor tried to take after they had constructed ground floors in a multi-unit residential development and omitted a crucial membrane layer below the rigid insulation boards.

A concern was raised that the floors did not match the specification, and that the insulation layer could be adversely affected by moisture.

Desperate to avoid redoing so much flooring work at their own expense, the contractor was willing to go to significant lengths to prove the performance of the insulation as installed. Unfortunately, with no standard method to check any measurement of thermal performance against, it would have been impossible for any test results to be recognised or accepted as accurate.

The risks of incorrect floors 

This is just one example of the many errors that can occur in the construction of ground floors. Genuine mistakes can, and do, happen, but there can be other reasons behind an incorrect floor which can be difficult to pinpoint. 

It could be the result of poor specification in the first instance. It could be due to misreading the specification, or not reading it at all and ‘doing things the way we’ve always done them’. In some cases, the specification may have been read correctly but an unsuitable product substitution offered or made for one of the floor’s layers. Or the errors could simply stem from the contractor not being up to speed with current floor construction practices.

But whatever causes an error in a floor construction, poorly specified and/or installed insulation is a significant concern, and should not be allowed to remain just because taking up the floor is considered inconvenient.

At worst, this could result in a failure of the floor. At best, it could mean the floor is sound but its intended U-value is not met – a risk that some in the industry seem willing to accept, even though it could rightfully be deemed not to comply with building regulations.

Choosing the right insulation 

For a flooring contractor, distinguishing clearly between different types of rigid insulation for ground floors may seem unnecessary. Some may assume the difference is minimal, but in fact, these products can be very different.

There is also a misconception that a more thermally-efficient insulation offers better performance in other respects. It is assumed that the product with the best thermal performance is typically the most expensive, so must also be the strongest. But that is not the case.

Different rigid insulation boards offer different combinations of characteristics. Some are more thermally efficient but unsuitable for bearing high structural loadings. Insulation materials with a higher compressive strength may need to be slightly thicker to achieve the required U-value but are more robust and tolerant of harsher environments.

The benefits of extruded polystyrene (XPS) insulation

Extruded polystyrene (XPS) insulation is one of the most common types of rigid board, but uniquely, it can be used in damp or wet environments without affecting its thermal performance. It can be laid directly on prepared ground, with the damp proof membrane laid over it also acting as the separating layer between insulation and concrete – a saving of both time and membrane material.

In addition, its high compressive strength makes it ideal for installing below concrete floor slabs, to the outside of basement structures and – where required by national building regulations – surrounding swimming pool basins.

As a flat, strong and dimensionally stable insulation layer, it can also be used under floating floor coverings like screed or chipboard, above the ground floor slab.

Alternatively, when used below raft slabs in residential and other low-rise buildings, XPS helps to enclose the building structure within the thermal envelope, reducing linear thermal bridging at junctions between construction elements, and achieving a greater level of building performance, comfort and energy efficiency.

Raising standards

Project quality and building regulation compliance depend on using the right material in the right situation. That’s why dispelling misconceptions about floor insulation is so important and why contractors must understand the qualities of the various types.
Installing the ‘wrong’ insulation board may seem like a small risk, but the impact could be bigger than you think. At a time when the construction industry is being scrutinised on how it delivers projects, such a risk should not be an option.

It is vital that contractors understand the floor insulation they’re purchasing and install it correctly. Not only will this prevent the need to open up completed work but raise quality standards and ensure installers don’t have to resort to desperate measures because they didn’t get it right first time.

This article appeared in the Contract Flooring Journal December edition


Stuart Bell - Managing Director Polyfoam XPS

Stuart Bell, Managing Director at Polyfoam XPS reflects on 2018 and talks of what’s in store for 2019.

Throughout 2018 we continued to see sector growth – although the bad weather at the beginning of the year stalled some roofing projects, the momentum soon picked up and we saw positive results overall.  

The insulation market has remained highly competitive and that has driven the development of new products to meet contractors’ changing demands. Within the last six months for example, we have launched six new XPS insulation board thicknesses in response to the need for more time and cost-efficient products.

We have also continued to work closely with contractors and specifiers to offer technical advice, including raising awareness about the risks of specification switching, which remains one of the biggest challenges for our sector.

Unfortunately, it is not uncommon for specified insulation to be switched during construction and after project-design stage compliance has been achieved. This is an issue which is leading to inaccurate U-values and potential non-compliance with Building Regulations. 

We hope that following the Hackitt Review, we will begin to see this trend shift in 2019. The recommendations in the report include more effective specification and an increased focus on how construction products are developed and tested. These measures would not only significantly raise quality standards, but drive the demand for accredited, independently-tested insulation products which are fit for purpose and ensure buildings perform to the required standards.

We also welcome the move by the Construction Products Association (CPA) to establish best practice marketing and technical information through its Marketing Integrity Group. This initiative could play a vital role in preventing ambiguous and misleading product information filtering into the market next year improving customer confidence and ultimately the quality of the built environment.

This commentary appeared in the December 2018 edition of Total Contractor


NBS National BIM Library Endorsement Stamp

Polyfoam XPS, a leading manufacturer of extruded polystyrene (XPS), has renewed its partnership with NBS to host its growing range of insulation products in the NBS National BIM Library.

A total of seven Polyfoam XPS products can be found on the site, including three grades of Polyfoam Floorboard, providing thermal insulation for loadbearing floors, two grades of Polyfoam Roofboard providing thermally-efficient insulation for flat roofs, the Polyfoam Slimline Zero Membrane, and the Polyfoam Upstand Board for the thermal insulation and protection of upstand and parapet walls.

Free to use, the NBS National BIM Library is the only one of its kind to link directly to market-leading specification software allowing designers to drag and drop objects into a model.

RIBA Product Selector Endorsement Stamp

The partnership also means that information about Polyfoam XPS products, including technical downloads and independent accreditations, are featured on

Rob Firman, Technical and Specification Manager of Polyfoam XPS said: “The NBS offers trusted specification platforms, which are used across the whole construction sector. Partnering with the company ensures our products are available to designers as high-quality BIM objects and that they are armed with all the technical information they need to make informed decisions.”

All Polyfoam XPS products are available in the BIM Library in IFC format as well as 3D CAD Autodesk Revit.