ETAG and Roofs

The design of an inverted warm flat roof – where the thermal insulation is above the waterproofing layer – allows for the possibility of rainwater passing through the insulation layer. Moisture can have an effect on both the insulation itself, and on the thermal performance of the roof as a whole. Both must be taken into account as part of inverted roof U-value calculations.

ETAG 031, the document that sets out how inverted roof systems should be assessed, describes procedures for adjusting the thermal conductivity of insulation, and establishing the effect of rainwater cooling if water is present below the insulation layer.

What is design thermal conductivity?

The thermal conductivity of an insulation product as it comes out of the factory is its declared thermal conductivity. Where the lambda is adjusted to account for specific conditions – such as exposure to moisture in an inverted roof – it is known as the design thermal conductivity.

Account is taken of moisture absorbed by diffusion, and additional absorption due to freeze-thaw cycling. The adjustment is calculated by multiplying the declared thermal conductivity by a moisture conversion factor (Fm).

Extruded polystyrene (XPS) insulation is less affected by moisture than expanded polystyrene (EPS). The result is a conversion factor of no more than 1.1 for XPS insulation. Using the performance of Polyfoam Roofboard Extra to illustrate:

  • Declared thermal conductivity: 0.033 W/mK.
  • Design thermal conductivity: 0.034 W/mk (insulation 100mm or thicker).
  • Design thermal conductivity: 0.035 W/mK (insulation less than 100mm).

How is rainwater cooling effect measured?

An allowance must be made for the effect of rainwater absorbing heat energy from the structure and increasing the rate of heat loss from the roof. Three factors are taken into account to calculate it:

  • p, measured in mm/day: the average rate of rainfall during the heating season, based on location-specific data. To achieve an accurate U-value calculation, p should reflect the location of the building, or the closest location with data available.
  • f, a ‘drainage factor’: expressing the percentage of the rainfall (p) that reaches the waterproofing membrane.
  • x, measured in W.day/m².K.mm: the factor for increased heat loss, which is a standard value of 0.040 W.day/m².K.mm.

f and x are typically combined by multiplying them together, so it’s common to see fx values quoted.

Annex C of ETAG 031 describes the test method for establishing a value for f. The test must be on the thinnest insulation board and the thinnest, most permeable ballast. If a product is intended for zero falls roofs then the tested build-up must also be without falls.

A document published by the BBA – Information Bulletin No.4 – quotes the following values for f which may be used without testing (and which are conservative as a result):

  • 0.5 for “roof gardens, green roofs and parking decks with cast concrete finish”.
  • 0.75 for “insulation with rebated joints and an open covering”.
  • 1.0 for “insulation with butt edged joints and an open covering”.

Section of ETAG 031 supports all of this, quoting respective fx values of 0.020, 0.030 and 0.040 for the three categories listed above, which may be used without the need for testing.

ISO 6946 – the international standard that describes the calculation of U-values by the combined method – says that “different types of roof build-up” can enjoy the benefit of a lower fx value where “the effect of the measures are documented in independent reports”.

Usually, this means the use of a water control layer (WCL) – an impermeable (to liquid, but permeable to water vapour) membrane, resistant to rot and UV decay. Following the test described in ETAG 031, a WCL may be so effective that a zero value for f is recorded.

What is the benefit of a water control layer in practice?

BBA Bulletin No.4 – is clear that a water control layer cannot be assumed to be waterproof. A minimum f factor of 2.5% is recommended, resulting in a value for fx of 0.001 (0.025 x 0.040).

This fx value is used in U-value calculations featuring Polyfoam XPS inverted roof solutions that include our Slimline Zero membrane – all independently verified in our BBA certificate. The value is so low that is has almost no impact on the end result.

Both of these moisture correction calculations are widely accepted within the construction industry, but their application is subject to some ongoing debate. The effect of water on insulation, and the application of the moisture correction factor, has been raised by the publication of the 2018 version of BS 6229. You can read more about that debate in this blog post about inverted roof guidance in the revised BS 6229​.

As blue roof constructions become more common, the role of temporary rainwater retention and whether it increases rainwater cooling is also being discussed by industry. We cover that in more detail in this blog post about inverted blue flat roofs.

Published in August 2019.


Under floor insulation for loadbearing

After discussing the different types of insulation material commonly specified in ground floor constructions in this previous blog, we explained why it’s important not to assume that all insulation performs the same way – with regard to thermal performance and loadbearing capacity. Here, we offer guidance on how the compressive strength of ground floor insulation compares to that of other layers in a floor.

Types of Floor Load

Commonly-specified and widely-available building insulation types are not intended for ‘loadbearing’ applications – meaning they should not be expected to take the structural loads of the building, where the weight of the roof transfers through the walls, to the foundations and into the ground.

Generally speaking, floor insulation is part of what a structural engineer might call the ‘secondary structure’, which takes the loads imposed by the ‘building use’.

Building use includes people, fixtures and fittings, furniture and potentially vehicle movements too, if the building has a car park deck or is used for warehousing. All of these loads transfer through the floor insulation, in addition to the dead weight of any screed or slab, so the insulation product has to be sufficiently strong as well as performing thermally.

Some ground floor constructions, for example, those formed by a raft slab, can be part of the primary structure, but insulation must be specified carefully to ensure it is only exposed to loads it is capable of bearing.

Loads can broadly be grouped into two types. An active load, sometimes referred to as a ‘live’ or ‘imposed’ load, is anything imposing load which can move around, changing where and how the load acts. A dead load, or ‘self-weight’, is a constant, static load. Either load type can also be categorised as one of the following;

Distributed load: a load acting evenly over a surface area, such as the weight of a screed or floor slab.

Point load (or concentrated load): a single load applied to a localised area. Most insulation types should not be subjected to point loads; for example, it’s possible to leave a footprint impression on an insulation board that is otherwise capable of withstanding several tonnes of distributed load.

A point load is the most extreme loading condition and the most onerous consideration. Because insulation is always covered by other materials, it is the distribution of load through the floor construction that tends to dictate insulation specification.

Compressive Strength

Insulation is typically the weakest layer in a floor build-up, and therefore the layer subjected to most scrutiny. Even the strongest insulation pales into comparison with concrete screed and slab layers.

An insulation material’s ability to resist the loads applied to it is assessed and declared by measuring its compression behaviour. The standards covering the manufacture of insulation materials specify that declarations should be based upon a sample being compressed by up to 10 percent of its thickness.

Insulation would never be used in a situation where it could compress by 10 percent; the consequences of that movement could not be tolerated. The test method simply offers a reference value to which safety factors can be applied to obtain ‘in use’ values.

Compressive strength measurements for any insulation product boards are declared in kilopascals (kPa). The pascal is the SI (International System of Units) for pressure, defined as one newton per square metre.

1 Pa = 1 N/m2

1 kPa = 1 kN/m2

For easy conversion, 10 kPa or 10 kN/m2 is about equivalent to one metric tonne per square metre.

By contrast, the strength of concrete screeds and slabs is measured in megapascals (MPa), an order of magnitude greater than kPa (i.e. 1000 kPa = 1 MPa).

For structural engineers and flooring industry professionals who are used to thinking in MPa, it can be tricky to contextualise measurements expressed in kPa. There is a risk of confusion, but kPa is what the standards for insulation require. This is why ensuring the correct insulation specification is so important.

Insulation Specification

The British and European Standard BS EN 1991-1-1 gives maximum expected distributed and concentrated loads for all types of buildings and their uses.

When carrying out structural calculations and checking the suitability of proposed build-ups, there should be no need for structural engineers to go beyond the values in BS EN 1991-1-1. However, they are free to do so if they deem it necessary.

As we have already touched on, insulation manufacturers apply safety factors to their compressive strength declarations. Doing so gives recommended maximum loadings for the product in use, to help structural engineers with their assessments.

The risk is that structural engineers apply safety factors of their own at the same time – for example, by assuming loads in excess of those given by the standard. The compound effect of these multiple safety factors leads to the insulation being completely over-specified.

Of course, compressive strength is only one characteristic – it should not be the sole deciding factor in choosing a product. In our previous blog, we talked about how other features and characteristics affect the specification and use of different insulation types.

Extruded polystyrene insulation (XPS), such as the Polyfoam XPS Floorboard range, is manufactured with several different compressive strengths. Lower grades are ideal as a flat, dimensionally stable insulation layer for an ordinary domestic ground floor.

And with its tolerance for wet environments, and robustness when installed in contact with the ground, higher grades are ideal for applying to the outside of basement structures or for insulating swimming pool basins.

A knowledgeable manufacturer will always be willing to give advice about the best use of their products and assist the design team, even if the final sign-off is the responsibility of the project’s structural engineer.

At Polyfoam XPS, we manufacture and supply extruded polystyrene insulation products for ground floor constructions, backed up by comprehensive technical support and U-value calculations. If you have any questions you can contact us here.

This article appeared in the Contract Flooring Journal April edition 

Published in May 2019.


City Balcony View

At the end of June 2019, the UK Government issued an advice note regarding balconies on residential buildings and fire safety. The advice note followed the amendments made to the Building Regulations at the end of 2018, which we wrote about in this blog post.

A fire at a block of flats in Barking at the beginning of June is widely seen as being the driver for the issuing of new advice. The balconies on the development were clad in timber.

The latest advice note from the Ministry of Housing, Communities and Local Government (MHCLG) is available here.

What does the new advice note say about balconies?

Paragraph 1.4 of the advice note says:

“The view of the Expert Panel is that the removal and replacement of any combustible material used in balcony construction is the clearest way to prevent external fire spread from balconies and therefore to meet the intention of building regulation requirements…”

There is no reference to balconies as ‘specified attachments’ (a term used in the Building Regulation revisions at the end of 2018), or how a flat roof that forms part of the thermal envelope and acts as a balcony should be considered.

Using XPS insulation in inverted warm roofs acting as balconies

As with any sensitive topic like fire safety, consultation with the appropriate authorities and professionals should always take place when considering any design and specification. On this issue, that could be a fire engineer, the local fire service, or the Building Control Body carrying out inspection and approval work.

Paragraph 2.1 of the new advice note says:

“Balcony fires can spread to the adjacent balconies or into the building. If combustible materials have been used in the balcony or external wall system, it is possible that fire may spread rapidly across the façade. The risk is increased if combustible materials are used extensively (i.e. in floors and facades of balconies and in certain geometries).”

It does not mention flat roofs, but the use of the word “extensively” in connection to floors may lead people to interpret that a thermal insulation layer comprising combustible material is not acceptable.

However, we would still maintain that this only applies in the case of a balcony being outside the thermal envelope of the building. The ‘floor’ of a balcony could comprise timber boarding or another combustible finish. As we wrote in February 2019:

“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.”

Paragraph 2.4 of the new advice note would appear to support this view, as it says:

“Building owners need to ensure that any balconies do not compromise resident safety by providing a means of external fire spread.”

Preventing external fire spread in flat roofs

An inverted warm flat roof, featuring extruded polystyrene or any other type of combustible insulation, would achieve this aim. It is not part of the external wall construction, and the specified build up would achieve the external fire spread requirements for a flat roof.

However, the advice note does not provide this level of clarification, and the possibility remains for confusion over flat roof constructions that accord with the regulations, but which have been designated as balconies as part of the scheme design.

We will continue to work with our industry colleagues to try and achieve the level of clarity needed to avoid this confusion. In the meantime, 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 us to discuss the design and specification.

Published June 2019.


How does compartmentation in a building impact inverted roof design?

A fire usually starts inside a building, so internal fire spread is as critical a part of fire safety regulations as external fire spread. It’s not enough to think only about the performance of a roof’s external covering, it’s also important to consider its resistance from the underside.

Internal fire spread is split into two areas: linings and structure. The internal ceiling or underside of the roof deck needs to meet the requirements for linings, so has little impact on the roof insulation specification above the deck.

How does compartmentation help the fire resistance of the structure?

We examine the role of roof coverings in protecting the roof structure from external fire spread in this blog post. Internal fire spread is tackled by designing the building layout to restrict a fire to discrete compartments of the building, making it more likely that fire and rescue services can tackle the blaze and bring it under control.

In addition, concealed cavities should not allow the uncontrolled and unseen spread of fire through a building.

A compartment wall divides two separate compartments and should provide a certain level of fire resistance to help contain a fire. Where a compartment wall meets the underside of the roof, it imposes certain requirements on the roof covering.

How should I design an inverted flat roof over a compartment wall?

Approved Document B in England says: “If a fire penetrates a roof near a compartment wall there is a risk that it will spread over the roof to the adjoining compartment.”

One solution is to carry the compartment wall up through the roof structure to create a definitive break, but this is often undesirable or impractical. The preferred option is for the roof construction to continue uninterrupted over the compartment wall, but this can be the subject of some confusion because of statements made in some BBA certificates for inverted roof systems.

The Polyfoam XPS BBA certificate, for example, says at paragraph 9.4: “The system should not be laid over compartment walls.”

That leads specifiers and contractors to believe a complete inverted roof construction should not span across two different compartments. Following discussion with the BBA, we know they are only referring to the insulation and water control layer in their use of the word ‘system’.

This is unusual, since these components are always laid on a structural deck, and would not be supported directly by a compartment wall. A compartment wall can therefore meet the underside of an inverted roof, and should be fire stopped accordingly (a detail that is outside the scope of an insulation manufacturer’s advice).

What is an appropriate inverted roof specification over a compartment wall?

Approved Document B currently requires a roof covering with a minimum rating of AA, AB or AC for a distance of at least 1500mm either side of a compartment wall. To align with European classifications, that should be read as a minimum rating of BROOF(t4).

Inverted roof systems completely covered by an inorganic covering automatically achieve this standard. Departing from one of those standard coverings may force alternative solutions, which is down to the design professional to address and make appropriate enquiries about.

This blog post is largely based on the guidance contained in Approved Document B for England. The requirements for other countries of the UK are similar, and even identical in some respects, but care should be taken to ensure that any design complies with the individual regulations that apply. 

For more on the use of Polyfoam XPS products in different types of flat roof construction, visit our flat roof application pages. Section 4.9 of BS6229:2018 also provides useful guidance on the topic of fire safety in flat roofs. If those don’t answer your question, then contact us with any questions and we will get back to you.

Published in June 2019.


How is the fire performance of building elements declared?

As one of the two main areas of building design and construction where extruded polystyrene (XPS) insulation is commonly specified, it’s useful to understand how ground floor constructions are viewed by national building regulations when it comes to fire safety.

A separate blog post deals with the difference between reaction to fire and fire resistance generally, while we have also looked in some depth at the fire performance of XPS insulation in flat roof build-ups. For information about the impact of the December 2018 revisions to the Building Regulations in England, check out this blog post.

Are ground floors classed as an ‘element of structure’ in regulations?

A building’s structural elements are required to maintain stability in a fire, and resist fire spread from one part of a building to another.

The way in which fires start and develop is largely independent of how a floor is constructed or specified. As a result, the lowest floor of a building is not considered to be an ‘element of structure’ as defined in regulations, and therefore isn’t subject to these requirements.

What about internal fire spread and floor finishes?

Similarly, the performance of floor surface finishes and linings, and their ability to resist internal fire spread, are not included in regulations because they make little contribution to the early development of a fire.

Concrete slabs and screeds are, by their nature, non-combustible. For anybody concerned about the use of combustible materials in a building, an insulation layer covered by concrete poses no risk – but even where a combustible insulation material is installed above a concrete slab, it will not make the performance of the floor any worse in terms of fire.

Conclusions about ground floor fire performance

Discussions around the combustibility of building materials tend to focus on walls and floors – the main ‘elements of structure’ to which fire performance regulations relate. Ground and basement floor constructions have not become part of the debate, and there is no compelling reason for them to.

However, it’s common to encounter a lack of confidence, awareness and understanding about how individual materials work together in complete build-ups. We therefore think it’s important to address the performance of ground and basement floors to allay any potential concerns at an early stage.

For more on the use of Polyfoam XPS products in different types of floor construction, visit our floor application pages. If those don’t answer your question, then contact us with any questions and we will get back to you.

Published in June 2019.


What is the fire performance of XPS insulation in flat roofs?

As one of the two main areas of building design and construction where extruded polystyrene (XPS) insulation is commonly specified, it’s useful to understand how flat roof constructions are viewed by national building regulations when it comes to fire safety.

A separate blog post deals with the difference between reaction to fire and fire resistance generally, while we have also looked in some depth at the fire performance of XPS insulation in ground floor and basement build-ups. For information about the impact of the December 2018 revisions to the Building Regulations in England, check out our post on AMENDMENTS TO BUILDING REGULATIONS FOR FIRE SAFETY.

Is a flat roof a loadbearing element of structure?

Fire safety regulations do not generally class roofs as ‘loadbearing elements of structure’. Unlike the walls, a roof does not usually take the weight of other parts of the building – unless it performs ‘the function of a floor’, such as forming part of an escape route or being used for parking vehicles.

In those specific situations, where an inverted roof with XPS is commonly specified because of the extra load imposed, the roof should have a minimum fire resistance as specified by the regulations. For a roof forming part of a means of escape, the typical standard is 30 minutes when measured from the underside (putting the onus on ceiling and deck specification).

For a roof acting as, say, a parking deck, the performance requirement is much more dependent on the variables that affect how a fire might develop in the specific building – things like building height and occupancy, and whether the building is sprinklered or not.

How is resistance to fire spread declared for roofs?

National building regulations are concerned with the potential for fire to spread from one building to another, and across the external walls and roofs of buildings.

A roof’s resistance to external fire exposure, in terms of fire spread across the surface and penetration through the construction, is classified in accordance with EN 13501-5. A roof can be rated one of the following, from best performance to worst:

    • BROOF(t4)
    • CROOF(t4)
    • DROOF(t4)
    • EROOF(t4)
    • FROOF(t4)

EN 13501-1 refers to four separate roof tests, detailed in ENV 1187. The (t4) in the above classifications refers to the use of test 4, which is the only one sufficiently rigorous to demonstrate compliance with UK fire safety requirements. The performance rating of a roof typically dictates how far from a boundary or another building the particular construction may be used.

Like reaction to fire classifications, the external fire exposure of roofs used to be measured under national tests. While the European classifications should be the norm, approximate national class equivalents, when tested to BS 476-3, are as follows:

    • BROOF(t4) – AA, AB or AC
    • CROOF(t4) – BA, BB or BC
    • DROOF(t4) – CA, CB or CC
    • EROOF(t4) – AD, BD or CD
    • FROOF(t4) – DA, DB, DC or DD

In Scotland, the terms low, medium and high vulnerability are also used.

Restricting fire spread across an inverted flat roof

ETAG 031 is a European technical assessment guidance document which defines the criteria against which inverted roof ‘kits’ (thermal insulation plus water control layer) are assessed.

It points readers toward a list of roof coverings that meet the necessary standard for fire spread without needing further testing. Among the options are kits fully covered by one of the following inorganic coverings:

    • Loose laid gravel at least 50mm thick, or with a mass greater than 80 kg/m2 (subject to maximum and minimum aggregate sizes, to resist wind scour).
    • A sand cement screed at least 30mm thick.
    • Cast stone or mineral slabs at least 40mm thick.

These inorganic coverings feature in the majority of specifications, and drive the design of most inverted roof systems. Roof coverings not listed in the annex should be tested to ENV 1187 and classified to EN 13501-5.

Fire performance of (inverted) green roofs

Green roof designs should follow guidance issued by the Green Roof Organisation (GRO), featuring details of fire testing. The advice includes the correct use of fire breaks and non-combustible growing medium, and employing sufficient irrigation and maintenance to guard against a build-up of dry vegetation.

Fire performance of conventional warm roof constructions

An inverted roof imposes performance requirements on the roof covering (ballast, paving etc.). By contrast, a conventional warm roof imposes requirements on the waterproofing. Occasionally, extruded polystyrene insulation boards are specified in warm roof constructions. The external fire exposure of a warm roof is tested to the standards described above, but the onus is usually on the waterproofing manufacturer to comment on fire performance.

Whatever the type of roof construction, we continue to encounter a lack of confidence, awareness and understanding about how individual materials work together in complete build-ups. While this post is not intended to be a comprehensive guide to flat roof fire performance, we hope it is useful in summarising the key criteria and terms, and allowing specifiers to identify the best solution for the proposed building design. 

For more on the use of Polyfoam XPS products in different types of flat roof construction, visit our flat roof application pages. Section 4.9 of BS6229:2018 also provides useful guidance on the topic of fire safety in flat roofs. If those don’t answer your question, then contact us with any questions and we will get back to you.

Published June 2019.


What is the fire performance of XPS insulation in ground floor constructions?

When considering the fire performance of building elements, generally it is necessary to distinguish between the fire performance of individual components (such as an insulation board) and the performance of a construction element (such as a roof) that combines multiple components.

It can be misleading, for example, to quote a poor individual test result for a component that can be safely used in a tested construction.

Over the last few years, as the construction industry has found itself under scrutiny and started to hold itself to greater account, risk aversion towards fire performance and product specification has understandably increased. In some cases, however, that is leading to specifications being written without necessarily understanding what is being asked for, or why.

What are the different types of fire performance?

National building regulations in the UK deal with fire safety in the following documents:

    • England: Approved Document B, volumes 1 and 2.
    • Wales: Approved Document B, volumes 1 and 2.
    • Scotland: Section 2 of the technical handbooks.
    • Northern Ireland: Technical Booklet E.

The aim of the regulations in each country is broadly the same but the specifics of how those aims are achieved can vary. Solutions differ depending on building type, use, occupancy, layout, height and construction, as well as the distance from surrounding buildings.

Essentially, regulations deal with external and internal fire spread. (They also cover access for fire and rescue services, means of escape, and the fire performance of internal linings – although none of these are directly relevant to the products offered by Polyfoam XPS.)

When it comes to construction products and applying the regulations, we are interested in the reaction to fire performance of individual components. For an element build-up or system approach, we are more likely to be interested in the fire resistance.

What is reaction to fire?

Products are classified according to EN 13501-1, which defines reaction to fire. It is a measure of how a product behaves when exposed to a fire, and how it contributes to the fire as it decomposes as a result of that exposure.

From best to worst performing, the Euroclass system is: A1, A2, B, C, D, E and F.

For classifications from A2 to E, a designation for the production of smoke (s1, s2 or s3) and/or flaming droplets/particles (d0, d1 or d2) are added. 

European classifications should be the standard method of declaring performance, but it remains common to see national designations used, based on testing to BS 476-6 and -7. The following equivalent national class terminology is generally accepted, though is not directly comparable:

    • A1 – non-combustible
    • A2 – limited combustibility
    • B – Class 0
    • C – Class 1
    • D – Class 3

What is fire resistance?

Fire resistance has three aspects, all measured by the number of minutes that elapse during standard tests for each:

    • Resistance to collapse (R), which applies to loadbearing elements only.
    • Resistance to fire penetration (E), which is also referred to as integrity.
    • Resistance to the transfer of excessive heat (I), otherwise known as insulation.

A construction element may need to meet all three aspects, in which case the performance would be written REI 30 (or 60, 90 or 120, depending on the period of resistance achieved/required). EI relates to an element that is not loadbearing, and sometimes E alone is required – again, both followed by the number of minutes.

Classifications are determined from test data, in accordance with EN 13501-2 (construction products and building elements excluding ventilation services), -3 (fire resisting ducts and fire dampers) or -4 (components of smoke control systems).

How should fire performance declarations be applied?

Buildings – particularly large buildings, non-domestic buildings, and buildings split into multiple uses and occupancies – can be incredibly complex. The more complex the building, the more specific the requirements in terms of fire safety and performance.

Applying general principles to buildings where specific provisions are required is likely to compromise fire safety, and nothing in this post should be a substitute for following the guidance in regulations, or seeking advice from a fire engineer or other specialist. Section 4.9 of BS6229:2018 provides useful guidance on the topic of fire safety in flat roofs, however some projects require consultation directly with the local fire service.

In this post we have tried to summarise the measurement and declaration of fire performance of buildings and materials, to aid discussions on the topic. Improving the confidence of designers, specifiers, contractors and insurers who make decisions is critical to the success of construction products.

We have written separately about the specifics of using XPS insulation in flat roofs and ground floors, and we can be contacted for more information about the use of our extruded polystyrene products.

Published June 2019.


Installing a water control layer on an inverted warm flat roof

A water flow reducing layer (WFRL), is a loose laid membrane installed over inverted warm roof insulation. The performance of a WFRL is independently tested, and the result incorporated into the Agrément certificates of inverted roof solution providers.

It is not a waterproof layer, but acts as a barrier to severely reduce the volume of rainwater entering the roof’s insulation layer and reaching the waterproofed roof deck. The primary benefit of a WFRL is to reduce the rainwater cooling effect in thermal transmittance calculations, allowing target U-values to be met using thinner insulation solutions.

What does BS 6229 say about water flow reducing layers?

At the end of 2018, a heavily-revised version of BS 6229, the code of practice for ‘flat roofs with continuously supported flexible waterproof coverings’, was published. The standard’s updated guidance referred to “imperfections” occurring in the WFRL when installed on site.

A separate post goes into more detail about our concerns with the way this guidance has been written.

Putting to one side issues over the stance adopted by the standard towards inverted roof construction and water control layers, one thing the construction industry can unite on is the general need to address issues of construction quality. Otherwise, the performance gap – the discrepancy between building performance as-designed and building performance as-built – will never be closed.

Polyfoam XPS is not aware of widespread issues with the way WFRLs are installed. Our colleagues in the inverted roof sector have expressed similar surprise at the revised standard. Nevertheless, in an effort to improve the availability and accessibility of information, a blog post on WFRL installation seemed like a worthwhile idea.

How to install a water flow reducing layer

The structural roof deck should be fully waterproofed in accordance with the manufacturer’s design guidance and details. Insulation suitable for inverted roof applications – such as the Polyfoam XPS Roofboard range – should then be loose laid in a brick bond pattern, again in accordance with instructions and details provided.

Water control layer installation is as follows:

    • Lay the WFRL – e.g. Polyfoam Slimline Zero membrane – over the roof insulation, at right angles to the slope of the roof.
    • Make sure all side and end overlaps are a minimum of 300mm, and that end overlaps are in the direction of the downward slope.
    • Turn up the membrane at upstands and penetrations so it finishes above the surface of the ballast.

As both the insulation and the WFRL are loose laid, it is recommended to install only as much of the system as can be ballasted at the time. BS 6229:2018 contends that post-construction damage is one cause of poor WFRL performance, so another reason for laying ballast on an advancing front is to protect the membrane from site traffic as materials are moved across the roof.

Getting roof falls and drainage right with a water flow reducing layer

Correct drainage and roof falls are critical to the success of a water flow reducing layer installation. Back falls should never be allowed to occur on a roof, and are a potential reason for worse-than-expected performance of a WFRL. If a back fall causes rainwater flow across the roof in the opposite direction to the intended fall, it may flow under an end lap.

Because a WFRL is an effective (albeit not waterproof) barrier to rainwater, the roof should be designed with dual-level drainage. Detail rainwater outlets so that drainage takes place at both the WRFL level and the waterproofed deck level. As part of that detailing, ensure the WFRL is turned down at outlets.

The Polyfoam XPS BBA certificate for roofing contains further guidance on our solution for inverted warm flat roofs. Up-to-date guidance on the best practice for the installation of water flow reducing layers is also available, in Guidance Note No 14, via the LRWA website or you can contact us with any questions.

Published in June 2019 / Updated March 2020


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.