What is the difference between declared and design thermal conductivity (or lambda)?

‘Declared thermal conductivity’ is the lambda value of an insulation product as it leaves the factory. In the context of inverted roofing, ‘design thermal conductivity’ is the lambda value of the insulation once it has been adjusted for potential moisture absorption.

The difference between the two values depends on the insulation material. For a material that has very low water absorption, its thermal conductivity will be affected less because the material in situ is closer to the factory production specification. Where a material is capable of absorbing a relatively greater volume of moisture, its design lambda will be correspondingly worse.

How are declared and design lambda values established?

For an insulation product manufactured under a harmonised European standard, the thermal conductivity declared must be a ‘lambda 90/90’ value. That is, there is a 90% statistical certainty that 90% of tested samples have a thermal conductivity equal to or lower than the declared value.

The design lambda value used for the insulation product in U-value calculations for inverted roofs is the declared value with a moisture correction factor applied to it. The moisture correction factor varies depending on the material type, as described above.

Extruded polystyrene insulation, such as Polyfoam XPS Roofboard products inverted roofs, has a moisture absorption of just 0.7%. For a board greater than 100mm in thickness, that means the declared lambda is only adjusted from 0.033 W/mK to a design lambda of 0.034 W/mK. For further advice on the correct application of design thermal conductivity in U-value calculations, contact our technical team.


Can declared thermal conductivity be used in inverted roof U-value calculations?

When calculating the U-value of an inverted flat roof, the thermal conductivity of the insulation layer should be the product’s design lambda value and not its declared value. Because of the insulation’s position above the roof waterproofing layer, it is expected to be exposed to some rainfall and therefore a moisture correction factor should be applied accordingly.

Testing of inverted roof systems carried out in accordance with the technical guidance document ETAG 031 often generates a result that suggests water does not penetrate the system. In accordance with BS 6229:2018 and, by extension, BBA Bulletin No. 4, the inverted roof installation above the waterproofing should not be assumed as waterproof – meaning only design values are appropriate.

What happens if the declared lambda is used for inverted roof U-value calculations?

Using declared thermal conductivity in an inverted roof U-value calculation gives an unrealistic impression of the performance of the roof. If the chosen insulation product has a relatively high water absorption value then when it is exposed to moisture the rate of heat loss will be much higher than allowed for in compliance calculations.

Find out more about the low water absorption of Polyfoam XPS’s extruded polystyrene (XPS) insulation boards, and the different methods by which it is tested. Contact us for reliable and accurate inverted roof U-value calculations featuring the design lambda values of our Polyfoam Roofboard Extra and Roofboard Super products.


What is a water flow reducing layer (WFRL)?

A water flow reducing layer (WFRL) is a loose laid membrane used as part of inverted flat roof constructions. It is installed over the roof’s thermal insulation layer, and below the roof covering. A WFRL is a barrier to rainwater, reducing the volume of water that can reach the waterproofed roof deck. It also stops fines from entering the roof system.

The properties of a WFRL are defined in the technical guidance document ETAG 031. It should be a synthetic non-woven membrane that is water resistant, diffusion open, UV stable and rot resistant.

Crucially, a WFRL is not a waterproof layer. The membrane may perform well in independent testing, but some rainfall is always expected to reach the roof deck. That means design lambda values should be used for the insulation layer, and an appropriate rainwater cooling correction applied to inverted roof U-value calculations.

Why is a WFRL important?

Without a WFRL, more water reaches the waterproofing layer and increases the effect of rainwater cooling on the roof U-value calculation. A significantly thicker layer of insulation is required to compensate, increasing the depth of the roof build up and the cost of the inverted roof system.

Specifying WFRLs, and accounting for them accurately in U-value calculations for inverted roofs, is an essential part of ensuring that buildings achieve their intended performance once occupied and in service. Polyfoam XPS have been at the forefront of industry efforts to improve guidance around WFRL installation, leading to the creation of LRWA Guidance Notes 14 and 15.

Roofboard Extra and Roofboard Super are extruded polystyrene insulation products suitable for use as part of inverted roof build-ups, together with Polyfoam Slimline Zero membrane performing the function of WFRL. For any technical queries or U-value calculations relating to inverted roofs, please contact us.


How does rainfall affect inverted roof U-value calculations?

There are two ways in which rainfall alters the way that U-values should be calculated for inverted flat roof build-ups. An inverted roof should drain at the level of the water flow reducing layer (WFRL) membrane but, because it is not a waterproof layer, the insulation will be exposed to some rainwater that drains at the waterproofing level instead.

The first way in which the U-value calculation alters is to account for any water absorption by the insulation. The thermal conductivity value for the material should be altered from the declared lambda value to the design lambda value. Because the design of an inverted roof means some water is expected to pass through the insulation layer, it is not appropriate to assume that the insulation will be completely unaffected by moisture.

What is the rainwater cooling correction?

The second way is through the use of a rainwater cooling correction, applied as part of the combined method calculation described in BS EN ISO 6946:2017. The correction is calculated using three values: p, which is the average rate of precipitation during the heating season; f, a drainage factor expressing the fraction of p that reaches the waterproofing; and x, a standard value that is the factor for increased heat loss caused by rainwater flowing over the waterproofing layer.

When carrying out a U-value calculation, knowing the project’s location allows an accurate value of p to be established, ensuring the accuracy of the calculation. The value for f is based on the testing of the inverted roof system including WFRL, and the guidance contained in BS 6229:2018 and BBA Bulletin No. 4.

Polyfoam XPS ensure all U-value calculations carried out for inverted roof constructions featuring Roofboard Extra or Roofboard Super, together with the Slimline Zero WFRL membrane, meet all recognised standards and guidance. Contact us for more information about accurate inverted roof U-value calculations for your project.


How much rainwater penetrates an inverted roof?

The volume of rainwater that reaches the waterproofed deck of an inverted roof depends on two things. First is the building’s location, and the average rate of rainfall that location experiences during the heating season. The second is the result achieved from testing the thermal insulation and water flow reducing layer (WFRL) in combination to see how effective a barrier to rainfall the system is.

These two values are used as part of the rainwater cooling correction applied to inverted roof U-value calculations. The average rate of rainfall is denoted by the value p and based on location-specific data from the Met Office.

A building in the south-east of England, for example, experiences significantly less average rainfall than one in the Outer Hebrides. Inputting accurate rainfall data for the location improves the accuracy of an inverted roof U-value calculation.

How is an inverted roof tested for rainwater penetration?

The technical guidance document ETAG 031 describes how thermal insulation and a WFRL should be tested in combination to establish what fraction of the rainwater volume denoted by p will reach the waterproofed roof deck. This fraction is denoted by the value f.

Test results are usually extremely good, but a WFRL should never be assumed to be waterproof. The guidance of BBA Bulletin No.4 – which is referenced in the code of practice BS 6229:2018 – says that a minimum fraction (and therefore f value) of 0.025 should be used to represent 2.5% of the rainwater reaching the waterproofing layer.

As of the first quarter of 2020, an insulation committee set up by the Liquid Roofing and Waterproofing Association (LRWA) is working with the BBA on new test procedures that will enhance the accuracy of the value of f used in rainwater cooling correction calculations.

Polyfoam XPS is heavily involved with this work and we continue to update our blog and our calculation procedures in accordance with industry findings. For any queries regarding accurate U-value calculations for inverted roofs on your project, contact us and we will be happy to discuss current best practice.


What is the water absorption of inverted roof insulation?

The water absorption of inverted roof insulation varies depending what insulation material is used. Extruded polystyrene (XPS) is the most common type of insulation used on inverted roofs, while expanded polystyrene (EPS) is also popular. Technical guidance related to inverted roofs usually mentions both, though XPS has the longer history of use in the application.

The water absorption of insulation materials is established by testing, and varies depending on the foam structure of the product. Both absorption by immersion and absorption by diffusion are usually tested, and you can read more about the differences between XPS and EPS here.

Why is water absorption relevant to inverted roofing?

The thermal conductivity of an insulation material gets worse as the quantity of water it absorbs increases. Because an inverted roof is designed in such a way that the insulation will be exposed to some water, it is important to ensure that the thermal conductivity of the insulation is adjusted accordingly.

The declared lambda value of the insulation is multiplied by a moisture correction factor to arrive at a design lambda value. The design lambda can then be used in U-value calculations for inverted roofs. Open space in the structure of EPS foam allows more water into the material than the closed cell structure of XPS does. The moisture correction factor for EPS is therefore greater than for XPS.

Test results of Polyfoam XPS products such as Roofboard Extra and Roofboard Super are available in our product data sheets, BBA certificate and Declarations of Performance. Access all of them through the downloads section of our website. For advice on the correct use of design lambda in U-value calculations for your inverted roof project, contact us.


Is a water flow reducing layer (WFRL) waterproof?

A water flow reducing layer (WFRL) is not a waterproof layer, and technical guidance for flat roofing is consistent in reinforcing this. It is therefore not appropriate to design an inverted flat roof on the assumption that no rainwater will be able to reach the waterproofing layer.

ETAG 031 is a guidance document that describes the technical characteristics of an inverted roof system, comprising thermal insulation and WFRL. It says that a membrane used as a WFRL should be water resistant, rather than waterproof, and diffusion open to allow the passage of moisture vapour from inside to outside.

What is the advice of the flat roofing code of practice?

BS 6229:2018 carries weight as the relevant code of practice for inverted roof design and construction. It references BBA Bulletin No.4, which advises that a WFRL should not be assumed to be waterproof.

The publication of the revised BS 6229 in late 2018, and the guidance on inverted roof build-ups contained within it, have led to the inverted roofing sector working together to review and enhance the ETAG 031 test method, in conjunction with the BBA.

At the time of writing in March 2020, that work is ongoing. Stay up to date with its progress via the Polyfoam XPS blog, and communications from the Liquid Roofing and Waterproofing Association (LRWA). For accurate inverted roof U-value calculations based on up-to-date guidance, including using design lambdas and appropriate correction factors, contact Polyfoam XPS for technical support.


What is BBA Bulletin No. 4?

BBA Information Bulletin No.4 is a technical guidance document issued by the British Board of Agrément. Its content describes two areas of inverted roof design and specification – the drainage of inverted roofs, and how to accurately calculate a rainwater cooling correction factor for U-value calculations.

The document is referenced in BS 6229:2018 Flat roofs with continuously supported flexible waterproof coverings. Code of practice. The recommendations and guidance of a British Standard code of practice carry weight, meaning the information bulletin does too. In the case of BS 6229, it was developed by the flat roofing sector, for the flat roofing sector.

What does the BBA Bulletin say?

Achieving the correct falls on flat roofs to ensure they drain correctly is a longstanding issue and one that applies to all roof types, not just inverted roofs. BBA Bulletin No.4 was one of the first pieces of official guidance on zero falls, a topic that features strongly in BS 6229:2018. Further guidance on the topic is available in LRWA Guidance Notes 14 and 15, available here.

BBA Bulletin No.4 also sets out industry-standard guidance on the correct application of rainwater cooling correction factors in U-value calculations for inverted roofs. It describes the values that should be used, including rainfall volume and drainage factor, and emphasises that a water flow reducing layer (WFRL) membrane is not a waterproof layer.

Polyfoam XPS keep abreast of all industry guidance and ensure that our U-value calculations are always carried out in accordance with best practice. Contact us with any technical support enquiries or to discuss the requirements of your project, and find out how our products Roofboard Extra, Roofboard Super and Slimline Zero membrane can help you.


Zero falls roof

‘Zero falls’ is a term generally used to describe a flat roof designed to be as close to level as possible. That is extremely difficult to achieve in reality, for which reason zero falls has long been a topic of debate within the flat roofing sector.

Designing for zero falls may be desirable on a roof terrace or roof garden, where the flat roof acts as amenity space for people to use. It’s also an approach commonly used with blue roofs. Whatever the fall of a flat roof, however, it must still achieve the expected standard of rainwater drainage.

What is the definition of a zero falls flat roof?

The issue of zero falls has tended to be tied particularly to inverted warm roofs. The 2003 version of BS 6229 included a note to table 7.2 saying that inverted roofs could be designed to the manufacturer’s documented advice and BBA certification – which often said that systems could be used at zero falls.

Despite seemingly widespread opposition to zero falls, 2012 saw the BBA publish Information Bulletin No.4. The document addresses drainage, and corrections to U-value calculations, for inverted warm roofs. It defines zero falls (or “zero pitch”) roofs as having a slope between 0 and 0.7 degrees (which is a fall of about 1 in 80).

In the years since, there has been a general acceptance of the concept of zero falls, coupled with a rise in awareness of the importance of designing to minimum falls and ensuring water still drains freely from the roof.

BBA Information Bulletin No.4 says that zero falls doesn’t necessarily mean flat, and that adequate drainage is still a requirement.

When BS 6229 was revised and reissued in 2018, it sought to clarify guidance around zero falls roofs. The definitions section makes clear that a zero falls roof has a slope between 0 and 1:80. However, in order to ensure that such a roof drains correctly, the standard is clear in recommending that it be designed to a minimum fall of 1:80, to allow for settlement of the roof structure and to avoid back falls.

Why is it important to achieve a minimum fall on a zero falls roof?

Designing a roof for one fall in order that it achieves another when constructed is not unique to zero falls roofs. Standards and guidance on drainage for flat roofs give minimum falls that designers should achieve in all cases.

However, this blog post is not about general roof drainage, so we are only focusing on zero falls for the moment.

Roofs can fail to achieve their intended fall for a variety of reasons. Designers may not include sufficient allowance for achieving falls. Contractors may omit a layer, such as a screed laid to falls, that would achieve the designer’s vision. Structural engineers may not account for sufficient load, which causes the roof deck to deflect more than expected.

Any of these things can happen accidentally, or through a lack of awareness of best practice, or because of ‘value engineering’. Whatever the reason, the onus then falls on the insulation manufacturer to be able to offer a system that works at zero falls.

A roof design that doesn’t drain adequately – whether through poor drainage or through roof deck deflection due to structural loads – increases the risk of ponding, which increases the weight of water, which imposes further loading and further deflection, and so on. At too shallow a pitch, that deflection causes back falls, or negative falls, which causes water to flow away from designed drainage outlets.

Inverted roof systems are designed and tested to work with roof falls and appropriate drainage. Back falls introduce a risk of water flowing under the laps of the loose laid water control layer, increasing the proportion of rainfall reaching the waterproofing layer and worsening the thermal performance of the roof.

Achieving a successful zero falls flat roof

For any project where a zero falls roof is desirable, the guidance is clear: design for a minimum fall of 1:80, in order to allow the roof to settle towards zero without causing back falls.

As a supplier of inverted roof insulation and water control layers, Polyfoam XPS will always strive to offer the best advice to help specifiers and installers get the most out of our system. However, even the best products and advice will struggle to overcome a poorly designed or constructed roof that does not follow relevant codes of practice.

Communication throughout the project team is vital from an early stage to make sure the finished roof achieves all of its intended performance criteria, including thermal performance, waterproofing and drainage. Contact us for advice on achieving a successful inverted warm roof, whatever the fall.


Coronavirus Update

After careful consideration, and in light of the rapidly changing market conditions, we at Polyfoam XPS have taken the decision to temporarily reduce the level of our operations.  

From Friday 3rd April we will be reducing our operation and staff levels within our warehouse, offices and sales team. We endeavour to continue to do our best to support customers working on key projects that meet the increased needs of the national healthcare efforts, whilst following Government guidelines. 

Please consider that we are operating with a skeleton staff and will not be working to the normal levels of service. We appreciate your understanding during this difficult time.

For urgent sales or technical enquiries please contact:

Customer Services      sales@polyfoamxps.co.uk       01429 855100    Sales enquiries            ian.exall@polyfoamxps.co.uk  07500  975534 Technical enquiries     technical@polyfoamxps.co.uk  01429 855120

This decision has not been taken lightly but our priority is to ensure the health and wellbeing of our colleagues, customers and suppliers. We will continue to monitor the situation carefully and will keep you updated with any further changes.

Thank you for your ongoing support and we wish you, your families and colleagues the very best.

Stuart Bell
Managing Director
1st April 2020