Five years on from Grenfell, reducing fire risk in outer walls is still an area of widespread debate, with progress hampered by a lack of understanding about what represents best practice.

Fire safety experts are advocating the adoption of a higher level of protection throughout external wall systems in high-rise and high-risk buildings. In many cases, some key elements of the building envelope are not meeting the same levels of safety required by law for cladding. Breather membranes and vapour control layers are two of the most under-scrutinised and misunderstood elements. With that in mind, ITP have developed this fire safety hub to provide all the essential information you need to ensure compliance and maximise fire safety in breather membranes and vapour control layers, including classification, testing, certification and best practice.

If you have any questions or require further guidance on fire safety in façade design, please contact one of our expert Building Team for a free consultation.





Fire safety classifications for building products and building membranes

Fire performance data is an essential factor in building design. The Grenfell disaster threw an unforgiving spotlight on the issue of flammable building materials and all stakeholders, from contractors to insurance and warranty providers, are aware of the fundamental importance of fire testing in the shaping of a project.

More than one testing standard is available to provide assessment. A useful source is the BS 476  Fire Test Series (based Approved Document B) which evaluates materials across a broad range of categories, including Non-Combustibility, Ignitibility, Fire Propagation and Surface Spread of Flame.

The fire safety of materials is tested according to part 7 of BS 476 which includes classes 1-4. This can be upgraded with further testing in Part 6, relating to heat release. In each case, the maximum score is 0.

In 2002, to harmonise the classification of the reaction to fire for building materials, the European Commission introduced the Euro Fire Class System (also known as Euroclass) based on EN ISO 13501-1. The Euroclass system provides a common classification framework for building products based on their reaction-to-fire performance. Reaction properties include ease of ignition, spread of flame, evolution of smoke and toxic gases, and heat release rate of the burning material.

Euroclass has become a standard reference for building regulations related to fire safety in building materials – for example, the Building (Amendment) Regulations 2018 (S.I. 2018/ 1230) and amended Approved Document B and Approved Document 7 Guidance uses the Euroclass classification to determine the legality of materials used in external wall systems.  Approved Document B (Fire Safety) Volume 1 (dwelling houses) and Volume 2 (buildings other than dwelling houses) specifies the minimum standards (BS 476 and EN ISO 13501-1) for all materials used in the construction, with specific installation requirements.

Euro Fire Class System and National Fire Class Systems*

Euroclass EN ISO 13501-1 UKBS 476 Parts 6 & 7 Germany DIN 4102
Class A1 NA A1
Class A2 Class 0 A2
Class B Class 0 B1
Class C Class 1 B1
Class D Class 1 B2
Class E Class 2 B2
Class F Class 3 B3

*Indicative purposes only; test methods and standards vary

The Euroclass system

In the Euroclass system, materials are divided into seven classes on the basis of their reaction-to-fire properties: A1, A2, B, C, D, E and F.

The main properties to assign classification are non-combustibility, ignitability, flame spread, calorific value as well as the development of smoke and burning droplets. Depending on the outcome of the various properties, the product is assigned a fire classification as shown below.

The diagram includes additional classes for smoke development:

  • s1 the structural element may emit a very limited amount of combustion gases
  • s2 the structural element may emit a limited amount of combustion gases
  • s3 no requirement for restricted production of combustion gases

The diagram also includes additional classes for burning droplets

  • d0 burning droplets or particles must not be emitted from the structural element
  • d1 burning droplets or particles may be released in limited quantities
  • d2 no requirement for restriction of burning droplets and particles

The below table shows the target safety level for each Euroclass rating and examples of materials/products that typically fall under each classification.

Euroclass Target safety level Examples of materials/products
A1 No contribution to fire even under fully developed fire conditions Natural stone, concrete, bricks, ceramic, glass, steel, aluminium
A2 Only negligible contribution to fire even under fully developed fir conditions; no spread of fire from the area of the primary fire in the fire development phase Products similar to those of class A1, including small amounts of organic compounds
B In the fire development phase, no spread of fire from the area of the primary fire and very limited contribution to the fire Gypsum boards with different (thin) surface linings
C Under the conditions of a fire in the development phase, very limited spread of fire and limited energy release and ignitability Phenolic foam, gypsum boards with surface linings (thicker than in class B)
D Under conditions of a fire in a fire development phase, limited spread of fire and acceptable energy release and ignitability Wood products with thickness greater than or equal to10mm and density greater than or equal to 400 kg/m2 (depending on end use)
E In the case of a very small fire (match flame) acceptable reaction to fire (ignitability, flame spread) Low density fibreboard, plastic-based insulation products
F No requirements concerning reaction to fire Examples of products not tested (no requirements)

Euroclass requirements for external walls

The current regulations require an outerwall system to have a minimum Euroclass A2-s1,d0 fire safety rating, denoting limited combustibility. However, breather membranes installed behind the cladding and vapour barriers installed on the inside of the facades are subject to less stringent standards. Approved Document B Volume 2 (Aug 2019) states that “there is no requirement for membranes which are part of external wall construction over 18m for any building (and over 11m in Scotland) to be limited combustible.” Membranes are currently exempt from being part of an A1 or A2-s1, d0 vertical wall system; they are required by law to have a minimum Euroclass rating of B,s3-d0. It is expected however that this will change in the future and that class A membranes will become mandatory.

For further guidance about Euroclass ratings for external wall systems, please visit our Best Practice section.

Testing methods used for Euroclass fire safety classification

The Euroclass system has been widely adopted with reference to fire safety regulations applied to building materials. For an explanation of the Euroclass system, please visit our Fire Safety Classification section. The various classes of reaction to fire performance for construction products are based on the following testing methods:

  1. Non-combustibility test EN ISO 1182

Designed to identify the products that will not, or significantly not, contribute to a fire.  Tests insulation products of mineral wool, fibre reinforced cement, silicate boards, sealing compounds etc.

  1. Gross calorific potential test EN ISO 1716

Determines the potential maximum total heat release of a product when burned completely. Tests homogenous and multi-layered building products, for example insulation products of mineral wool with or without facings, fibre reinforced cement, silicate boards, sealing compounds etc.

  1. Single burning item (SBI) test EN 13823

Based on a fire scenario of a single burning item, e.g. a wastebasket, located in a corner between two walls covered with the lining material to be tested. The SBI test is used for all types of construction products excluding floorings. The purpose of the SBI test method is to use a larger sample model compared to the standard test methods in order for the test results to better correspond to a full-scale reference fire development phase.

  1. Ignitability test EN ISO 11925-2

Subjects the specimen to direct impingement of a small flame with classification criteria based on observations whether the flame spread reaches 150 mm within a given time and whether the filter paper below the specimen ignites due to flaming debris. In addition, the occurrence and duration of flaming and glowing are observed. Tests all types of building products.

  1. Radiant panel test EN ISO 9239-1

Determines the burning behaviour using a radiant heat source and is used to test a range of flooring products e.g. wood floor, plastic floor, rubber floor, linoleum floor etc. The radiant heat is applied by means of a gas-fuelled panel, inclined at 30º, and directed at a horizontally-mounted floorcovering specimen. Measures flame spread, smoke production and the heat flux towards the surface that is essential for flame spread.

The below table outlines the testing methods relevant to each class.

Class Test methods
A1 EN 1182 and EN 1716
A2 EN 1182 or EN 1716 and EN 13823
B EN ISO 13823 and EN 11925-2
C EN 13823 and EN 11925-2
D EN 13823 and EN  11925-2
E EN 13823 and EN  11925-2

Testing methods applied to building membranes

Two tests are used in the classification and certification of building membranes: EN 11925-2 Single Flame Source Test and EN 13823 (SBI) Single Burning Item (SBI) Test . To obtain the necessary B,s3-d0 classification, a product must be tested by a certified independent laboratory.

EN 11925-2 – Single Flame Source Test

This test is generally conducted in order to obtain a Euroclass in accordance with EN 13501-1, however it may also be called upon in other applications. The method specifies a test for determining the ignitability of products by direct small- flame impingement under zero impressed irradiance using vertically oriented test specimens.

Although the method is designed to assess ignitability, this is addressed by measuring the spread of a small flame up the vertical surface of a specimen following application of a small (match-sized) flame to either the surface or edge of a specimen for either 15sec or 30sec. The determination of the production of flaming droplets depends on whether the filter paper placed beneath the specimen ignites.

Pass standard:

Flame spread (Fs) <150mm within 60sec (when exposure time is 30 seconds)

EN ISO 11925-2 Principles and Methodology (Source: Efectis)

The EN ISO 11925-2 Ignitability of building products subjected to direct impingement of flame – Part 2: Single-flame source test results are required for a classification E – B (combined with test results from EN 13823), as well as for classification Efl – Bfl (combined with test results from EN ISO 9239-1).

Test principle

The test determines the ignitability of a vertically oriented test specimen when exposed to a small flame, either at the edge or the surface of the specimen. The burning behaviour of the specimen is observed for flame spread, and the occurrence of burning particles and droplets.

Test report

The test report contains information about the test specimens and test results. The following information is given in the test report:

  • the exposure time used in the test (15 or 30 s)
  • the exposure mode (edge – surface)
  • whether the flame tip reaches and/or passes the 150 mm mark above the flame application point, and the time at which this occurred
  • whether burning droplets were observed, which ignited the filter paper in a tray underneath the test specimen

The test results described above can be used for assessing a classification according to EN 13501-1. Normally, this is done based on the average of 6 – 12 single tests. The criteria depend on the classification which is aimed for. For classification D – B, 30 s exposure is required, and the flame height should not exceed 150 mm within 60 s after the start of the test. For the (other) classifications (E and Efl ), 15 s exposure is required, and the flame height should not exceed 150 mm within 20 s after the start of the test.

For the flooring classifications, and for end-use applications where edge exposure is not possible, only surface exposure is applied. In all other cases, edge exposure is applied in addition to the surface exposure.

Test specimens

The specimens shall be representative of the product in its end-use application, as far as possible. This applies specifically to the use of standard substrates, as described in EN 13238.

Essentially flat specimens shall be used: Dimensions: 250 mm x 90 mm.

Thickness: maximum 40 mm (including substrate when used).

Eight specimens must be cut in the direction of production and four specimens must be cut at 90° to the direction of production.

Test method (source: Exova Warringtonfire)

The specimen is mounted vertically into the test frame and two pieces of filter paper are placed below the specimen. The defined test flame may be applied in a number of manners, dependent upon the requirements of the specification document:

  • 15 seconds surface application
  • 15 seconds edge application
  • 30 seconds surface application
  • 30 seconds edge application

The test duration is as follows

  • 15 second flame application time, total test duration = 20 seconds
  • 30 second flame application time, total test duration = 60 seconds

In all cases, observations are made as to whether the flame tip reaches a point 150mm above the original flame application point within the test duration and whether the filter paper was ignited by any flaming debris / droplets, etc.

Generally six specimens are tested with each flame application type required.

EN 13823   – (SBI) Single Burning Item (SBI) Test

The specimen is mounted on a trolley that is positioned in a frame beneath an exhaust system. The reaction of the specimen to the burner is monitored instrumentally and visually. Heat and smoke release rates are measured instrumentally and physical characteristics are assessed by observation.

Pass standards:

  • Fire Growth Rate Index (FIGRA) must remain below 120 W/s
  • Total Heat Release (THR) in the first 600sec of exposure must be <7.5MJ
  • Lateral Flame Spread (LFS) must not reach the end

EN 13823 overview (source: Exova Warringtonfire)

This test provides data on a products heat release, surface spread of flame properties and smoke production when exposed to a thermal attack by a single burning item (SBI) – a sand box burner supplied with propane.The burner is representative of a waste paper basket or small chair fire, and has a heat release rate of 30kW. Heat release is a measure of the contribution that a burning material makes to a fire in progress, for example, a high rate of heat release will produce a high rate of temperature increase in surrounding un-burnt material and accelerate fire spread.

Test method

The specimen is mounted on a trolley that is positioned beneath an exhaust system. A base line of average heat release is taken from the ‘auxiliary’ burner over a period of three minutes. Once this baseline has been obtained an identical ‘primary’ burner is ignited, which impinges on the specimen, and is run for a total of 21minutes. Heat release and smoke production rates are measured instrumentally throughout the test. This is obtained by air flow drawing the effluent gasses through the duct. The smoke density is measured by obscuration of a white light beam and the heat release is measured by gas analysis (oxygen depletion method). At the end of the test the heat release from the baseline is deducted from the heat release from the primary burner and the specimen, this then gives the heat release contribution from the specimens.

Test results

A number of different parameters are calculated from the data obtained:

Fire Growth Rate Index (Figra)

Figra = max RHR(av) x 103


Total Heat Release over the first 10 minutes of the test (THR600s)

Smoke Growth Rate (Smogra) Smogra = max RSP(av) x 104


Total Smoke Production over the first 10minutes of the test (TSP600s).


RHR = Rate of heat release

RSP = Rate of Smoke Product

t = time

In addition to the above, visual observations are taken for lateral flame spread (LFS) reaching the end of the long wall of the specimen at a height of between 500mm and 1000mm, and for flaming droplets (as well as any other behaviour occurring during the test).

When is this test conducted?

The most common use of the standard is for classifying the performance of construction products excluding floorings to EN 13501-1. It is also called upon in some other specifications such as in the London Underground Limited (LUL) standard 1-08A classification report against EN 13501-1 and CE marking as required by applicable product standards can be provided by our certification department.

Specimen requirements

Five specimens 1500mm x 1000mm and five specimens 1500mm x 495mm are required. Specimens should incorporate representative joints, and should mounted or fixed as in practice. Products should also be tested over a representative substrate (if applicable).

The above testing methodology should be demonstrated with the correct testing certification – for further details, please visit our Certification section.

The first video below shows  full-scale fire testing of facade breather membranes in Euroclass A2, Euroclass B and Euroclass E. The test was initiated by the Technical Director of FPA and Director RISCAuthority, Dr Jim Glockling. The Euroclass A2 membrane is Serge Ferrari’s Stamisol Safe One system. The test demonstrates how quickly toxic fumes arise with Class B and E membranes. ITP are the exclusive UK suppliers of Stamisol Safe One – for further details about Safe One visit https://itpltd.com/safe-one-class-a-breather-membrane/.

The other two videos show the EN 11925-2 Single Flame Source Test and EN 13823 (SBI) Single Burning Item (SBI) Test.

Fire safety certification for building materials

To ensure that the correct  testing methodology has been applied to assess the fire safety performance of a building system/material, manufactures should provide evidence in the form of recognised certification.

Identifying fully tested and certified membranes

Membranes currently require a CE marking to signify that they have been assessed to meet high safety, health, and environmental protection requirements. CE marking will be replaced in January 2023 by UKCA marking to signify that the requirements have been met.

This change has been prompted by Brexit. The Brexit transition period ended on 31 December 2020, bringing into force a new framework under UK Construction Product Regulations. UK notified bodies and laboratories became UK approved bodies and laboratories, with the same scope and ID number. UK approvals can no longer be used for CE marking unless they have been transferred.

UK Construction Product Regulations are essentially the same as the EU legislation, with standards currently the same. However, a new product on the market that is not CE marked, and is System 1+/1/2+/3 requires UK Conformity Approval through a UK body or laboratory.

CE and UKCA: equivalences and differences*

European Union (EU)

(applies to European Economic Area (EEA) plus Turkey and Switzerland)

United Kingdom

(applies to Great Britain – England, Scotland, Wales)

Regulation (EU) No 305/2011 on construction products

EU Member State laws on enforcement

Construction Products (Amendment etc.) (EU Exit) Regulations 2019

The Construction Products Regulations 2013

EU declaration of performance (DoP) UK declaration of performance (DoP)
Scope / Definitions Scope / Definitions
CE marking (the CE mark) UK marking (the UKCA mark)
Responsibilities: Manufacturer, Authorised Representative, Importer, Distributor Responsibilities: Manufacturer, Authorised Representative, Importer, Distributor
European Union (EU) United Kingdom
Harmonised Standards Designated Standards
European Assessment Document (EAD) /

European Technical Assessment (ETA) /

Technical Assessment Bodies (TABs)

UK Assessment Document (UKAD) /

UK Technical Assessment (UKTA) /

Technical Assessment Bodies (TABs)

Notified Bodies / Notified Laboratories Approved Bodies / Approved Laboratories
Basic Requirements Basic Requirements
Product Areas Product Areas
Systems of Assessment and Verification of Constancy of Performance Systems of Assessment and Verification of Constancy of Performance
Factory Production Control Factory Production Control

*Source: BTTG & Shirley

Variations in testing methods and reporting

Membranes for roofing should be certified to meet the requirements and test methods and  specified in BS EN 13859-1 and membranes for walls should be certified to meet the requirements and test methods and  specified in BSEN 13859-2.

It is important to determine whether a product testing has been carried out externally by an independent testing body or internally by the manufacturer. Internal test methods vary between manufacturers and are often unreliable indicators of performance.

Fire safety tests for membranes should include a declaration of performance linked to an audit and the number of the approved laboratory which carried out the test. In supplying evidence of testing and certification, a manufacturer will provide either a classification report or a test report.

A classification report merely provides a summary of the classification resulting from the test, whereas a test report provides detail on the testing process. This can be an important distinction due to the variation in testing methods. For example, some tests have been known to involve non-combustible layers between the membrane and the test flame, whereas more stringent testing has nothing but an air gap between membrane and flame, providing a more reliable and realistic indicator of its flame retardancy.

A Euroclass Class A rating requires a stricter auditing process to achieve certification. With that in mind, opting for a Euroclass A product has another advantage in providing peace of mind about the reliability of the testing method.

Membrane design: classifications, regulations and best practice

What is the minimum Euroclass rating required for breather membranes and vapour control layers?

Laws introduced in response to Grenfell require a building’s outerwall system to have a minimum Euroclass A2-s1,d0 fire safety rating, denoting limited combustibility. However, breather membranes installed behind the cladding and vapour barriers installed on the inside of the façade are subject to less stringent standards. Approved Document B Volume 2 (Aug 2019) states that “there is no requirement for membranes which are part of external wall construction over 18m for any building to be limited combustible.” Membranes are currently exempt from being part of an A1 or A2-s1, d0 vertical wall system. They are required by law to have a minimum Euroclass rating of B,s3-d0. In Scotland, this requirement applies to an external wall construction over 11m.

Why are are membranes and vapour control layers subject to less stringent requirements?

Breather membranes and vapour control layers represent a large component of the building envelope, comparable in surface area to cladding, and therefore logic dictates that these elements should meet the same levels of fire retardancy. So why is the regulation inconsistent?  It’s largely due to the complexities involved in developing A-rated systems at the time when the legislation was drafted. In the past, there have been challenges in manufacturing breather membranes that could meet the non-combustibility or limited combustibility of Euroclass “A” to  BS/EN13510-3 without compromising their breathability to BS5250 standards with water resistance to W1 standards, negating their primary functions in controlling moisture, reducing condensation and protecting against rainwater.

That may have been the case in 2017, but subsequent advances in design have raised the bar for fire safety with life-saving improvements to this vital area of building performance. We have now reached the point where the development of breather membrane technology has overtaken legislation by redefining what is possible.

Should you choose a Euroclass A1, A2 or B solution?

Certain distinctions apply to different ratings within the Euroclass A classification in the context of breather membranes.  It is important to consider the differences between Euroclass A1 and A2 fire ratings in the context of a membrane’s ability to resist water penetration and allow moisture into the building fabric causing interstitial condensation and rotting the building fabric.

The breather membranes have two primary functions: they allow the building to breathe and to resist the inward passage of water from rain and damp air penetrating the insulation layers, all of which is primary to any fire rating.

Euroclass A1 membranes have been developed for non-combustibility, with no smoke and no hot droplets, but they are limited in other functions.  A Euroclass A1 membrane does not offer the water proofness to the outer wall and give the UV protection that an open façade will need. They allow moisture through to the insulation as the outer coating has been reduced in order to achieve the A1-s1,do fire rating.

The membranes used in safety critical installations are made up of two key components: the base layer, which gives the resistance to fire, and the coating layer, which gives the water protection. Some poorer quality A1 membranes have aluminium layers adhered to the fire retardant layer which is very easily delaminated. They include a base layer predominantly made up of a fiberglass construction, which in  its basic form would be classed as A1. However, this has virtually no resistance to water from the outside of the building. The outer layer or coating is breathable, yet water resistant from the outside in. A single layer of the coating would give a small amount of resistance.

Water tightness is the measure of how the membrane resists the passage of water. The ratings are W1, W2 and W3. W1 is the highest rating and recommended for tall and exposed buildings.

The A rated fire resistance and the W rated water penetration are directly related. An A1 fire rated membrane has only a light coating for water resistance and therefore would be classed W2 at best. An A2 rated membrane includes a heavier coating, giving it a W1 rating combined with a limited-combustible fire rating which is virtually zero flammability.

To avoid compromising the water protection of the building, it is essential to avoid a W2 or W3. A more effective all-round solution, with a perfect mix of fire rating and water protection,  is a Euroclass A2-s1, d0  membrane. It provides W1 waterproofing to EN ISO 20811, enabling building protection over and above the capabilities of an A1 membrane, while exceeding the requirements of Approved Document B part 4. A good A2 rated membrane will also offer additional breathability to remove moisture and 5,000 hours of UV resistance to EN ISO 13859-2.

ITP is the UK’s exclusive supplier of Safe One, a breather membrane which offers an unrivalled combination of Euroclass A2, W1-rated for waterproofing and breathability to BS5250. Our discussions with developers, contractors and façade specialists suggest that opinion across the sector is increasingly in favour of fully A2 -rated external wall systems incorporating A2-rated breather membranes which are waterproof, breathable and offer a high UV level to enhance safety in buildings.

Where possible, we recommend the use of a Euroclass A2-s1, d0 breather membrane due to its all-round capabilities. We also recommend Euroclass A2-s1, d0 vapour control layers. If this is not possible due to factors such as cost, we recommend the use of a Euroclass B-s1,d0 solution. ITP offer both options within our range of breather membranes and vapour control layers.

What are the testing requirements for breather membranes and vapour control layers?

Assessing different areas of performance can be a challenge, so it is vital to use all the technical resources available. Most importantly, products should be supplied with comprehensive and transparent data on combustibility, breathability and water resistance with reference to the relevant standards. The data should be independently certified by authorised testing bodies.  The Grenfell Tower Fire Investigation discovered the use of non-compliant products and a lack of independent testing.  It exposed the need for a more rigorous specification process and, with that in mind, no breather membrane should be considered without certified evidence of fire safety tested to EN13501-1 which forms the basis of Euroclass standards.

Are regulations likely to change?

The Mayor of London, Sadiq Khan, announced a significant intervention in the regulation of fire safety in buildings. A public consultation on draft guidance that puts fire safety at the centre of the planning process for new developments, the London Plan Guidance (LPG) aims to ensure that fire safety is considered at the earliest design stage of a development and addressed in planning applications to ensure the most successful outcomes are achieved for building occupants and users.  LPG will consider a range of developments, including alterations to external walls.

Sadiq Khan said: “The current building safety situation is a scandal, and I am concerned that almost five years after the tragedy at Grenfell Tower, it appears that the Government are still not willing to properly address it. That’s why I’ve been using all the tools at my disposal to raise the standard of fire safety measures in London, through requirements for developers in my London Plan and lobbying developers and building owners to share vital fire safety information with residents.”

Sadiq Khan and others are advocating a fire strategy which drives the widespread adoption of practices serving a clearly defined building safety concept. To achieve that goal, it is vital to provide education and support throughout the construction supply chain to ensure that specifiers make informed decisions and installers have the key competencies needed to play their part.

BSI, in its role as the UK National Standards Body, opened a public consultation in December 2021 for PAS 8671, a new framework for competence of individual Principal Designers and designated individuals working under the Organisation Principal Designer. The consultation sought input from residents and experts in the housing, construction, fire, and safety industries to develop a new standard as a part of the Built Environment Competence Programme, which supports the government’s Building Safety Bill to raise competence requirements for three newly regulated roles: Principal Designer, Principal Contractor and Building Safety Manager. The conclusions will help to define future best practice and, together with the Building Safety Bill,  they are likely to apply formal responsible responsibilities across the supply chain

In April 2022, the government’s Building Safety Bill received Royal Assent and completed all the parliamentary stages in both Houses to become an Act of Parliament – The Building Safety Act.

It will give residents more power to hold builders and developers to account and toughen sanctions against those who compromise safety. A new Building Safety Regulator will be at the heart of the reforms, overseeing the “safety and performance of all buildings” and implementing the new, more stringent regulatory regime for higher-risk buildings. The regulator will also assist and encourage competence among the built environment industry, and registered building inspectors.

Stricter oversight is likely to have an effect throughout the construction supply chain, especially with regard to the critical area of external wall systems in high-rise buildings. Fire safety experts believe that improvements should be made in façade design with adoption of breather membranes and vapour control layers which are Euroclass A2-rated. Where possible, designers, consultants and Installers should adopt “best practice” when it comes to A2 solutions fitting over the current B S3 class.