Today we're going to talk about the fire performance of construction membranes and how this can be accounted for an integrated into a fire-focused design process.

Since the tragic events at Grenfell in 2017 a good proportion of the construction industry has quite rightly been focused on how we can improve the fire resistance of our buildings, and how we can prevent situations where the "as built" fire performance tragically fails to be realised in practice.

The 2018 report by Dame Judith Hackitt, "Building a Safer Future" was highly critical in its assessment of both the industry and the regulations. Its references to "negotiating down" the standards given in the approved documents, and to the opaque and confusing nature of that same guidance will be all too familiar to everyone in the industry.

Following its publication, the report received criticism for not suggesting an outright ban on combustible materials in high rise residential, but this is perhaps unfair given the focus of the document.

What it highlights as being of primary importance is clarity of information and guidance, and the implementation of clear and transparent procedures to ensure that information is both correct and is distributed to where it is needed with the design team.

So we're going to base today's presentation around four main aims:

  • Gaining an understanding of the fire design process across disciplines
  • Familiarity with the fundamental regulations and guidance
  • What product information and testing is critical to ensure compliance
  • How to ensure as built performance matches the design criteria.

We'll consider how this can be achieved from our perspective as manufacturers, and from that of specifiers and installers, and look at how we can ensure we use the correct tools to transfer the correct information throughout the design and build process.


As manufacturers of membranes which can be used in fire safety critical applications, we have a responsibility to ensure the systems and material we supply are fit for purpose, but we must also provide specifiers with sufficient information to integrate those systems into the wider design without causing problems.

So let's begin by considering some of the fire test that are used, and how they relate to the specifics of particular designs.


Historically, BS476 was the most commonly used form of fire testing the UK, and it's comprised of various different sections, or parts. The most commonly referenced of these parts were parts 4, 6, 7 and 11, which variously deal with the combustibility and fire spread characteristics or materials.

BS476 test however, are bench-scale tests, so are conducted on very small samples, and a tightly focused on a particular aspect of performance in isolation.

For example, BS476 Pt 4 uses a small furnace heated to 750 degrees into which is placed a 40x40x50mm sample of the material under test.

For many years this was the only test used to determine combustibility of material in the UK, although as you may have noticed, there's no flames involved, nor does the test setup resemble a realistic sub-assembly of a building.

The situation is similar with the BS476 Part 7 test, which uses a combination of radiant heat and pilot flames to measure the rate at which flame propagates across a surface. As with the other parts of BS476, this is a perfect valid and repeatable test method, but not one which has any particular resemblance to the application of a material in a building context.

It's also possible, as was the case with some types of foam cored aluminium panels, that a product or material can achieve a good result in the surface spread of flame testing while remaining essentially combustible.

The building regulations which referenced BS476 did not always account for these factors, which was a contributing factor leading to the use of combustible ACM cladding on Grenfell tower and other projects, most of which are now undergoing extensive remediation works.

EN ISO 13501

The EN ISO 13501 "Euroclass" fire classification system is a newer and more extensive test regime, introduced to harmonise fire testing across the EU.

The classification systems relies on data from several test methods, some of which are similar to those by BS476, such as EN ISO 1182 which tests combustibility, and EN ISO 1716 which determines the calorific value of energy release associated with combustion.

This energy release is a particularly important consideration for membranes, which while often combustible, will have virtually no effect on overall fire development due to their lack of calorific value.

The crucial step forward from the older test methodologies is in both the EN13823 "Single Burning Item" test and in the simplified classification system, which expresses many performance criteria in a single easily comparable value.

Its also worth noting at this point that as this is a Europe wide test, it can be found quoted in a variety of ways. In the UK it maybe be referred to as "BS EN ISO 13501" while Germany for example may use a "DIN EN ISO" prefix to reference the exact same test.

The single burning item test test is an intermediate scale test where a standardised test assembly is exposed to direct ignition from a flame source.

The amount of heat and smoke produced during the test is then measured and used to determine the following characteristics

Total Heat Release>(THR) & Total Smoke Production>(TSP) - simply the total amounts of heat energy and smoke produced by combustion during the 20 min test period.

Lateral Flame Spread - the extent of flame movement relative to the edges of the test specimen

Fire Growth Rate Index>(FIGRA) - the rate of increase of heat production during the test

Smoke Growth Rate>(SMOGRA) - the rate of increase of smoke production during the test

Flaming Droplets/Particles - a visual assessment of any droplets with continue to flame for longer than 10 second after reaching the ground.

These are all brought together into the euroclass rating which works like this:

Class A1 materials are fully non-reactive when exposed to fire, while those in Class A2 have an extremely limited reaction in terms of smoke and droplet production. Class B products are combustible but with a "very limited" contribution to the overall fire development, with C-E products making increasingly large contributions to the fire development.

Alongside this the classification adds the s, and d suffixes denoting smoke emission and droplet productions. s1 means smoke emission is weak or absent, through to s3, meaning high intensity smoke production. Likewise d-zero is no droplets, while d2 mean high levels of dripping.

This simple and universal system makes comparisons of fire characteristics simple in regulatory terms, but it's not without problems as the classes can accommodate a broad range of performance.

This test here conducted by the University of Edinburgh is a comparative test based on the single burning item test, and show our Fireshield membrane next to competing material.

Both these materials have an identical Class B, s1, d-zero rating on paper, so could be considered equivalent. In practice though, the difference in performance is obvious.

How this disparity can be tested, quantified and accounted for in regulations is something we are currently discussing with the BBA among others. The Fireshield is a BBA certified vapour permeable membrane for use in timber frame wall and facade applications where fire performance is of paramount importance.


The most recent test method used in the UK is the BS8414 test, revised just this year. This is the largest scale test commonly used, and aims to give an accurate representation of the performance of a fully integrated wall assembly.

In essence this test is a super-sized single burning item test, with the 1.5m high test panel increase to 8m, and the peak heat release from the test fire take from 30kW to 4.5MW.

In this test, products are testing integrated into complete constructions, with additional components making up the entire wall assembly present.

The fire source, called a "crib", is inset at the bottom of the test structure to simulate a fire breaking out of a window and spreading up a facade.

After ignition the systems is observed and thermocouples are used to measure temperature at various points on the sample. The test is run for a period of 30 mins, unless the flames extend above the test sample or the is a risk to personnel or equipment, In either of these cases the test it terminated and the system cannot be classified.

After the test is completed, the system is assessed according to the following criteria.

External Fire Spread - based on external thermocouple temperatures, which should not exceed their initial reading by more than 600 degrees for 30 seconds, within a period of 15 mins form the start time.

Internal Fire Spread - Which uses the same criteria as external, but based on the temperatures of thermocouples located within the structure.

Mechanical Performance - A written assessment of the dame to the test sample detailing any collapse, delamination of components, and general damage.

Because of this test incorporates a full wall build up in a realistic scenario, BS8414 is very much the gold standard of fire testing in the UK, however these tests are expensive to undertake and limited in general applicability as they relate to a very specific construction. There are also limited number of test locations and test slots available.

A BS8414 test report does however contain a lot of useful insight that can be applied across constructions however, so can still be of some use to inform expectations of performance based on EN13501 classifications if an exactly matching test is not available. This should however be done with caution and careful consideration of the overall buildups and material under test.

In this image here you can see the post test condition of our wraptite membrane, which in this case was behind a layer of incombustible mineral fibre and has suffered minimal damage. The Wraptite has a B-s1,d0 classification which means that it's contribution to the development of the fire is negligible both in terms of smoke production and overall energy release, but in this case the overall construction has substantially protected the membrane from ignition, highlighting the importance of integrated system testing.


Alongside these various tests, third party product certification and accreditation can also be vital in determining the suitability of a product or system for a given application.

Documentation like BBA certification can include details on fire ratings, and any restriction that might apply, such as ratings only being applicable to specific substrates, which maybe necessary to consider.

The main thing to consider, whether the information comes from test reports, or from certification, or from manufacturers themselves, is to ensure that the information is applicable to the situation under consideration, and is the best available for that product.


When considering the fire performance of structures, specifiers have to balance combustibility against a range of performance factors. While it's easy to simply say we'll make the entire structure incombustible, this may have impacts on the hygrothermal performance of the building. or on it's air leakage.

Some weatherproofing systems, while incombustible, do not allow sufficient moisture transfer to comply with the recommendation of BS5250, the code of practice for condensation control, so in upgrading fire performance we may create problems in other areas.

The challenge therefore is to balance the fire related aspect of the design with other factors like ensuring acceptable energy performance and healthy indoor environments, without compromising on the critical safety considerations.

So how can this be done?

The key to achieving the delicate compromise is good information based on the criteria we discussed earlier. Ensuring primarily that materials used in the design are incombustible where appropriate and practical, but that where combustible products must be used, any fire risk is minimised and clearly understood in the overall context.

Without good test data for products this is very difficult, and even if the data is available, successfully integrating designs will require expertise form all parts of the design process.

It's no longer sufficient to simply specify a material as par to of, for example, a facade buildup and trust that detailing around feature like windows, fire breaks and element junctions will be adequately resolved.

Input from all parties, form product manufacturers to site operatives is the key to ensuring a robust design is developed that not only meets the on paper requirements of the building regulations but actually provides meaningful performance.

To return to Dame Hacketts quote about "negotiating down" standards, it's important to comply not just with the letter of the regulation, but also with the spirit, something not always taken sufficiently seriously in the past.

Building Regulations

A good understanding of the materials and constructions must sit alongside a good understanding of the building regulations themselves.

There are three main approaches that can be taken, one based on the building regulations and approved documents, a risk management approach using the principles given in BS9999>("double nine, double nine") and BS9991>(double nine, nine one), and a fully engineered approach from first principles.

The latter two are considerably more project specific methods, so today we'll mainly consider a regulation based approach, which is by far the most widely applicable, and the closest thing available to a standardised method.


In the case of fire, that means Part B in England and Wales, Section 2 in Scotland, Part E in Northern Ireland and Technical Guidance Document B in the Republic of Ireland. As always there are some differences which we'll highlight, but we'll try and keep as broadly applicable as we can.

The building research establishments BR135 guidance document is also important when considering fire design, as is regulation 7 covering materials and workmanship.

Regulation 7 and Approved Document B>(ADB) were historically somewhat ambiguous and therefore the reading and interpretation of them could result in flawed or even potentially dangerous outcomes in design and specification, this is of particular importance in high rise applications.

In late 2018 they were both updated to improve, strengthen and clarify the message, language, terminology and examples, in order to simplify the information and remove any ambiguity regarding what is required. The most recent version of ADB is the 2019 edition.

One of the main changes was the shift in classification of materials within ADB and the removal of references to BS 476-6&7 which classified products for Spread of Flame. As we saw earlier, the issue with this classification was that it only took into account the reaction of the surface of the material being tested and did not consider the combustibility of the whole product.

Document B also includes the concept of "relevant buildings" being more or less any building intended for residential use which is over 18m in height, and these require most materials used to achieve at least Class A2.

in Scotland similar changes took place in 2005, with the building regulation shifting away from BS476 classifications to requiring cladding materials be either non-combustible or tested as part of a complete system. This applied to new build and refurbishment works.

As we discussed previously however, commonly used types of membrane such a vapour permeable or breather membranes, are not able to meet the necessary criteria for combustibility without severely compromising their ability to perform their essential function, and this is recognised in the building regulations, with ADB stating:

'Membranes used as part of the external wall construction should achieve a minimum classification of European Class B-s3,d0'

A similar exemption is provided in Scottish Technical Standard 2, but the cutoff point for a construction to be considered as "high rise" is lower in Scotland, at 11m as opposed to 18.


The BR135 guidance document uses the following model to illustrate the development of a fire, we covered this in our recent modular and facade webinar but it's worth repeating as it illustrates well the effect and purpose of the reduced combustibility requirements.

A rapid fire spread occurs when an initial fire develops and flashes over, and is then spread to all areas simultaneously by the outer cladding layers, in turn starting fires across all the areas of the building.

Where fire spread is restricted, the initial fire develops and flashes over, but can only ignite a single secondary fire directly adjacent.

The fire will only develop further if this secondary fire also develops. This is a much slower process, and allows far more time to contain each area of fire, and to evacuate occupant.

In this model, how any external membranes react to fire is important - if the membrane is mechanically fixed and taped, there is the potential for fire to spread on both surfaces of the material as oxygen can feed into the fire on either side.

With a fully adhered membrane like Wraptite, only the outer surface can contribute to the development of the fire, as there are no gaps between the membrane and the substrate. This inhibits the supply of oxygen to the fire and slows down the spread of fire across the membrane.

This again highlights why a more joined up approach to design brings benefits when the overall performance of the structure is considered.


The structural timber association also has design codes and guidance for fire protection in timber frame, and of particular note in this guidance is the minimum separating distances between properties, and the effect a fire resistant construction can have in reducing these distances.

This guidance is based on determining the heat flux from a burning structure and how this is transmitted to adjoining plots, and is based on in part on large scale testing conducted at the BRE.

The STA guidance places timber frame structures into three categories, A, B and C, with differing guidance for each type.

Type A comprised a standard timber frame and sheathing, with no particular fire resisting measures applied.

Type B is an "enhanced frame" built and tested to meet the STAs FR Build standard with sheathing materials of limited combustibility.

Type C is a "fire robust" panel frame which further reduces the combustibility of both the sheathing and internal linings.

Type B and C also introduce FR build or non-combustibility requirements for internal walls and floors to ensure the overall structure is fully protected.

It's also important to remember that this FR Build spec sits separately form the testing we discussed above, and it's something more application to timber frame manufacturers and suppliers than to specifiers as structural systems must be tested in accordance with the STA technical paper procedures as complete assemblies.

That said, it's still worth being aware of this, as the availability of STA FR Build systems may inform specifiers choices of supplier if fire is a critical consideration. Timber frame or panel fabricators should contact the STA directly for further advice on how to comply with this, but our Fireshield vapour permeable membrane is fully tested and suitable for inclusion in such systems.

Once the frame specification is confirmed, this table can then be used to cross reference the classification of the building against the building size. This gives a minimum separating distance based on the emission of heat energy required to cause ignition of adjacent structures.


Another good example of the benefits of this approach is in the detailing of junctions and openings, where without careful consideration of the specification, and the involvement of multiple parties in the design problems can arise.

Here we can see a window frame detail with a thermal break specified at the perimeter.

The purpose of this thermal break is to limit the effects of cold bridging. This doesn't just mean reducing the overall heat loss, but also serves to increase the surface temperature and minimise the risk of condensation in this area.

In theory this is pretty straightforward, add some insulation, vapour barrier and problem solved. But when we introduce a fire requirement, we then have to ensure this can be achieved without compromising either the combustibility, or hygrothermal performance.

On top of that we have to ensure this can be delivered in a way that is easy to install effectively.

In this case, our Spacetherm SLENTEX-A2 insulation was used at the the perimeters, to maximise the insulation performance without compromising on the reaction to fire.

The Class A2, s1-d0 aerogel insulation can be supplied laminated to a variety of facing materials and can also be custom cut to meet project requirements, leading to a faster and more accurate installation procedure.

The detail can then be completed using the Procheck-A2 or Procheck FR200 vapour control layers internally, and Fireshield externally giving a fully protected and highly effective thermal break minimalism heat loss and condensation risk without increasing the fire risks.


Another project where the Spacetherm SLENTEX-A2 found use, was the refurbishment of the 27-storey Balfron tower in Tower Hamlets, London. This iconic 1960 building designed by Erno Goldfinger houses 146 apartments and was recently upgraded to bring it up to modern standards in terms of fire, acoustic and thermal performance.

Development lead architects Egret West identified a particular issue around the concrete stairwell walls, particularly space critical areas and vulnerable to cold bridging. There were also similar issues at window and door junctions to address.

In the stairwell areas, the original specification called for 145mm of insulation to maintain condensation free surface temperature of 16 degrees, however the use of the Spacetherm system reduced this to under 60mm.

Both of these examples illustrate the benefits that involving product manufacturers in the design process can bring, and this can be extended to facade systems both in new build and re-cladding applications.


In this project, student accommodation in Newcastle, the Class B,s1-d0 rated Wraptite membrane was able to deliver low air leakage rates, simply and quickly by minimising the sealing of services that an internal air barrier would require.

It was also able to deliver the reaction to fire characteristics required to be used on a residential structure over 18m in height.

These low air leakage rates can be used to offset performance differences associated with non-combustible insulation materials, giving designers increased flexibility in the design choices they make.

This projects in London used this approach to replace originally specified PIR insulation with mineral fibre, reducing fire risks vvwhile minimising the impact of thermal performance.

Increasingly common are re-cladding projects such as this high rise residential block in London, where Fireshield has been used to enhance the performance of the upgraded facade.

In all these cases, by being involved in the design process we were able as manufacturers to assist specifiers in meeting their objectives by providing calculations and test data to support the use of materials that may not have been initially considered if looked at in isolation.

This integrated design approach can be carried forward to site also, where as manufacturers we have to utilise our expertise not just in regard to specification, but also to the practicalities of installation.

This knowledge can be applied firstly to ensure specifications as-drawn are physically possible on site then to ensure the correct processes are followed to bring the project to fruition


By applying this knowledge and experience to site installation, we can help ensure that installers of fire rated systems are aware of the critical impact their processes have on the overall performance of the system.

This doesn't just mean that systems have to be fitted correctly, but means that switches in specification must be minimised, if they take place at all.

Regular viewers of our webinars will remember we discussed Designing for Manufacturing and assembly a couple of week ago, and these principles for a good basis for considering fire performance also.

A well designed system will facilitate straightforward installation, and this design will form part of a process where material specifications are agreed and fixed, meaning products can be sourced in sufficient time that specification changes will be unnecessary.

This optimisation of deliveries and installation processes will be particularly important in light of any quarantine restrictions post-lockdown as work practices on site will have to be carefully planned and controlled.


In this detail here we can see a common situation in a facade application, a floor junction.

In these areas there are a number of requirements to account for in any installation, and ensuring this is done correctly as these requirements intersect in various ways.

The joints here must be cable of accommodating structural expansion and contraction, they must not introduce a cold bridge, the must adequately resit air leakage and finally must limit the potential for fire to spread between floors up the facade of the building.

Each of these requirements require careful detailing at the design stage, and even more careful installation to achieve on site, but with a good choice of materials it's possible to mitigate these risks by simplifying the process.

The Wraptite membrane used over the sheathing and slab edges is the key to this simplification.

Used of foam compression strips at expansion joints, the flexibility and self adhesion of the membrane allows structural movements to be accommodate without risking over-stressing the air tight layer.

Being external this airtight layer reduces the needs for proprietary tapes and gaskets, which may be difficult to source and may also introduce complications as regards fire performance if suitably fire rated components cannot be easily sourced.

Coupled the faster self adhesive installation process, this greatly simplifies and speeds up the on site procedure.

With airtightness aspect dealt with, the outer layers of the construction can then be applied, with a permeable and incombustible insulation layer preventing fire spread both itself, and by limiting the potential for the membrane to be exposed to flame.

This combined with the fully adhered backing of the Wraptite, reduces the potential for both heat and oxygen to reach to the membrane from either side, removing two key aspect of the "fire triangle" required for combustion.

Fire breaks and cavity barriers can then be added without to much concern as to how they affect air leakage.

What this buildup achieves is breaking the complex interaction of requirements in this area into a straightforward series of linear processes. As manufacturers we also undertake to support site operatives through training both on and offsite to further assist in streamlining these processes and increasing skill sets.

We're obviously not doing as much of this on site training lately, but we're very much looking forward to getting back into our toolbox talks on site, and working with installers across the industry, so please feel free to get in touch if that's something you would be interested in.


We firmly believe as a company that CPD is not just relevant to specifiers, but something that can be applied across the whole design and build chain.

Increasing and transferring knowledge of both materials and processes is the key to achieving a good balanced design strategy across all types of building and is particularly important where safety is a critical consideration.

So hopefully today we've illustrated that type of information and testing that designers should look for, how that information can be used to inform their design choices, and how we as material suppliers can be a responsible and collaborative partner in those choices both for specifiers and contractors.

This Webinar Includes
  • Fire Testing Procedures
  • Fire Regulation Requirements
  • Correct Specification of Construction Membranes
  • Installation Considerations