Fire Performance of Construction Membranes

Good morning everyone, my name is Keira Proctor, and welcome to our first webinar of 2022. Our series of webinars has been running now since 2020, and if you’ve missed any you can go back and review them all right here on our YouTube channel.

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Today we will be discussing the fire performance of construction membranes.

We will first discuss the basics of fire and combustion. Then move on to what fire classifications mean and how they are tested, along with the requirements for construction products in various applications. Following this, we will discuss each of our membranes, and how they relate to these tests before finishing up with our usual Q&A session with our team of experts.

Fire Science Basics

To start off, let’s take a brief look at the science behind fire, and how the process of combustion works in practice.

Fire is defined as a chemical reaction between oxygen and a fuel, which produces heat and light.

The correct mixture of fuel, oxygen and heat is very important to create a sustainable reaction, so when considering fire protection, we visualise this as the “fire triangle”. If any of these three sides are removed then the reaction cannot sustain itself and the fire will go out.

In the past fire brigades would pull down buildings to limit the spread of fires by removing the fuel source, a technique which is still used in fighting wildfires. In the built environment however, most firefighting techniques today rely on the removal of heat or oxygen from the fire, either by using fire supressing foams or blankets to remove the oxygen, or using water to cool the burning material, removing the heat.

Removing the heat source to extinguish the fire works because most materials do not burn directly, but rather heat causes them to decompose into flammable vapours via a process called pyrolysis. The ease with which these vapours are produced, and how readily they burn ultimately dictate the reaction to fire of the specific material.

Once ignited, the heat produced by this burning vapour drives further decomposition in adjacent material, which may then ignite, causing further heat and leading to a chain reaction. The speed of this chain reaction is determined by the materials “combustibility ratio”, or the relationship between the heat it will release when burning, to the heat it takes to ignite it.

During this growth stage of the fire, it will continue to grow until it becomes limited by the availability of oxygen or fuel. In an enclosed space, materials may also become hot enough to spontaneously ignite from the radiant heat alone, and this process is known as a flashover.

What’s important to bear in mind at this stage is that smoke produced by the fire is comprised of unburnt gasses and vapours produced by pyrolysis. The smoke, as opposed to flame, is simply the result of the balance of heat, fuel and oxygen in the mixture being unsuitable to support combustion.

This distinction is important because if a fire in a building depletes the available oxygen in a space, there may still be a significant amount of this unburnt flammable vapour present, as well as a large amount of residual heat. If oxygen is reintroduced, for example by opening a door or structural collapse, the fire may rapidly reignite.

In order to ensure buildings are designed to limit the scope for fires to develop and spread, it’s important to quantify and clearly define their reaction to fire, both in terms of ignition and fire development, and this is done using the “Euroclass” fire rating system.

Fire Performance: A Definition

The European standard EN 13501-1, which was first released in 2007 and updated in 2018, assigns materials a reaction to fire classification based on their performance in a number of tests designed to assess the capacity of a material to spread fire. Classifications range from A1, which is given to materials that offer no contribution to fire, through to F, for materials with no tested performance.

Different tests are required based on the classification a material is intended to achieve. No tests are needed for class F, as materials this classification are simply declared as having “no performance determined” in their product documentation.

Class E requires a test covering the ignitability of the material, to the standard EN ISO 11925-2, the “single flame source test”.

In this test, small samples of the product are vertically oriented, and a single small flame from a propane gas burner is used to determine how readily the material ignites. The degree of ignitability is determined form the length of time the burner is applied to the sample, and the resultant flame height produced.

Class E materials are defined as contributing to a fire, and only being able to resist ignition by a small flame for a short period. Under the test conditions, a Class E material must produce a flame not exceeding 150mm within 20 seconds when exposed to the burner for a 15 second period. A typical example of this sort of material is polythene.

Products which fail to meet these criteria automatically fall into Class F.

To achieve class B, C, or D, a material needs to be tested to the same EN ISO 11925-2, as class E materials do, but must achieve a higher standard. In this case the flame height should not exceed 150mm within 60 seconds, following a 30 second exposure to flame.

Additionally, materials in these classes also must be assessed using the single burning item test, or SBI, conducted according to EN 13823. This test assesses the heat release rate, smoke production, flame spread and amount of burning droplets or particles produced by the material.

The method of this test uses a right-angled corner of two wings of the material to be tested. A flame is then provided in the corner, representing a simulation of a single burning item in a room corner near the material. This source releases 30kW of heat for a 20 minute period.

During the test the combustion gases produced are collected and measured and this data is used to determine the Total Heat Released during the test, as well as the total smoke production. Alongside this, the test also determines the rate of growth of both heat and smoke, assessing how quickly the fire is developing.

Finally, a visual assessment is made of the lateral spread of flames, and the production of flaming droplets.

These results are used to place the material into classes between B and D depending on the heat and growth rates. Smoke production is measured on a scale of 1 to 3, while the quantity of droplets is rated from 0 to 2. These values are then used with the lettered classification to denote the full fire classification, for example, B-s1,d0.

Most materials with a tested fire performance fall into one of these categories. They are defined as products that have some measure of flammability, ranging from Euroclass D being a material that contributes to fire; through class C, which have a limited contribution, up to class B: a very limited contribution.

Some useful context to these classes is given in part 2 of the Scottish Building Standards Technical handbook. This classifies materials attaining an E or F as very high risk, a D as high risk, C as medium risk and B-s3,d2 or better as low risk.

The final class, Euroclass A, which Approved Document B lists as having limited combustibility, is split into two sub-categories. A1 materials, which are non-combustible, and A2 materials, which make some minor contribution to a fire. Notably, some countries, including Scotland and much of Europe, define any class A material as non-combustible.

To achieve class A2, a material would need to pass a test to either EN ISO 1182, or both EN ISO 1716 and EN 13823. This latter test is the same single burning item test used in determining the performance of materials from classes B, C or D, and to achieve an A2 rating, it is paired with EN ISO 1716, which determines the gross heat of combustion, or calorific value of a material.

This is also known as a bomb calorific test, and provides information on the amount of energy released when burning. In this test a sample of the material is ground into a powder and the maximum heat produce when it is burned up completely is measured.

The other option to achieve an A2 rating is EN ISO 1182 - a non-combustibility test. It assesses the performance of the material in a special furnace. This furnace exposes the sample to a temperature of 750 degrees centigrade for 30 minutes, and the test measures the sample mass loss, duration of any flames, and the amount the material contributes to the temperature of the furnace.

A material can be classified as A2 if it loses less than 50% of its mass, has flames that last for less than 20 seconds, and introduces a temperature rise of no more than 50°C. A2 materials include ones such as fibre cement boards.

Both the bomb calorific test and the non-combustibility test are required for a material to achieve class A1, and they have stricter targets to meet. For example, in the non-combustibility test, materials must produce no flame, and have an increase in temperature of no more than 30°C. The single burning item test is not required, but many manufacturers include it, as it adds the additional smoke and droplets classification, though, on an A1 membrane, neither of these would be expected.

Fire Resistance and Fire Retardation

Fire resistance and fire retardation are separate ideas, which are often confused in practice. Fire resistance is a time, provided in minutes, for which any given system will resist certain criteria of fire.

Typically, this consists of some combination of resistance to structural collapse (or, loadbearing capacity); resistance to fire penetration (integrity); and resistance to the transfer of heat (insulation). This typically applies to tested systems, for example, full wall build-ups which provide an hour’s fire resistance. Individual components, such as plasterboard may state that they provide a certain fire resistance on their own, which would be included in the overall element’s fire resistance.

Materials which offer an element of fire retardation work to actively suppress a fire. They typically include an intumescent material, which expands to restrict the spread of flame. This expanded material can provide additional levels of thermal insulation protecting the underlying layers from heat, such as in fire protection coatings applied to steelwork.

Other examples of intumescent materials in use include fire barriers which expand to close off cavities when they exposed to excessive heat. Flames can spread through voids and cavities in building if these spaces are not designed to restrict fire spread.

Because heat rises by convection, fires are particularly good at spreading upwards. This rising airflow can also draw additional oxygen to fuel the growth of the fire. By allowing the fire to develop upwards, and drawing in additional oxygen to support combustion, vertical voids can rapidly spread fire.

Fire barriers are used to subdivide such voids horizontally and vertically, preventing this convection driven fire spread. But in subdividing these voids without considering the effect of ventilation and moisture movement, problems with drainage and vapour egress can arise.

So in addition to ensuring materials and systems are appropriately rated for the intended application, it’s vital to ensure the systems are properly designed and detail to work together.

Requirements

Following the tragedy at Grenfell tower in 2017, fire considerations in construction were made much more prominent. Approved Document B was updated in December 2018, providing significant changes to what would be usable on the external walls of relevant building. It has since been updated a few more times to provide clarity.

Relevant buildings are defined as those with an inhabitable storey which is 18m above ground level and which contain one or more dwellings; an institution; or a room for residential purposes, excluding hostels, hotels and boarding houses.

Buildings close to a boundary are also subject to certain requirements. In particular, buildings of any height and function which are less than one metre from a boundary need to have at least the relevant wall constructed to B-s3,d2.

Places of assembly or education need to protect 10m above any accessible space or roof, with materials of C-s3,d2 or better. Any section of wall above 18m on any building needs to be built using materials of or exceeding class B-s3,d2.

Scottish Building Regulations define a high rise as being over 11m, rather than 18, but imposes generally similar requirements on the materials that can be used.

In these “relevant buildings”, the amended Approved Document B specified that all materials used in the construction of external walls should achieve at least class A2-s1,d0, with a number of exceptions. These exceptions were provided for materials that are difficult or impossible to manufacture to meet class A2 or better.

These include:

· Cavity trays in masonry cavity walls · Roofs · Door frames and doors · Electrical installations · Insulation and water proofing materials below ground level · Intumescent and fire stopping materials, where required. · Membranes · Seals, gaskets, fixings, sealants and backer rods · Thermal break materials where required. · Window frames and glazing Some of these are included by necessity – intumescent materials react to fire by definition, and so cannot be non-reactive by the very nature of the material. This type of material includes intumescent paints which protect steelwork, some cavity barriers – particularly in rainscreen cladding systems, and our own Fireshield breather membrane.

Other systems are included in the list of exemptions as they are made of materials that do contribute to the spread of flame, but which there a no alternatives for. Typically, these materials are ones that occupy only a small area of the building envelope, so while they contribute to the development of the fire, this contribution is extremely limited, and is outweighed by the benefits provided.

Subsequently, section 10.15 of the same document draws attention to the requirement for membranes as part of an external wall construction to meet at least class B-s3,d0. As we discussed earlier, this means that they will have only a limited contribution to fire, and although they are allowed to produce any amount of smoke, they should not produce any flaming droplets.

Membranes are typically located at interfaces where they would not be overly exposed to sources of fire, such as between or behind layers of plasterboard, or between sheathing boards and insulation, all of which would be class A1 or A2. Therefore, there would be limited opportunities for a membrane to propagate the spread of flame under normal conditions.

Membranes in External Wall Construction

There are two main types of membrane used in external wall constructions: vapour control layers and breather membranes.

A vapour control layer, as the name suggests, is designed to restrict the movement of water vapour in the construction. It is located on the warm side of the insulation, typically installed at the internal lining. Commonly, these are made of polythene or foils.

Warmer and more humid air creates a vapour pressure, which moves from warmer to colder spaces. In the UK and other “heating climates” this usually means from inside to out. As warm, moist air follows this path through a construction element, it cools.

The warmer air is, the more moisture vapour it can hold, so if this air cools to a temperature lower than what is called the “dewpoint temperature”, the airborne moisture vapour will condense, depositing liquid water. If this happens inside of the wall, it is called interstitial condensation, and could cause damage to materials such as the frame. A vapour control layer limits the passage of moisture laden air into cold spaces, reducing this risk.

There are arguments that a vapour control layer should not be counted as part of the external wall, as it is inside of the structural element. This would mean that it would not need to meet the fire performance requirements.

However, as it is positioned at the internal lining, fire in a service cavity or spreading through a service penetration could be spread on the vapour control layer. It is also vulnerable to fire during construction, before the internal lining is installed. Therefore, it is correct to treat the vapour control layer as any other part of the external wall, and apply the appropriate fire protections.

Breather membranes are the other membrane used across the main section of external walls. These are designed to keep out liquid water, which could be driven in by precipitation. Without a breather membrane, this could lead to damage to structural members, through corrosion or rotting, or could saturate insulation.

This is especially important during construction, before the external cladding is applied, which is also when this element of the construction is most at risk from fire. If the breather membrane here is easily ignited, then it could lead to flames spreading across the façade, and depending on the stage of construction, may allow ingress into the building itself.

Construction sites make use of a range of equipment which could start a fire, so having a membrane that does not contribute to the spread of flame is often essential.

A Proctor Group Products

Procheck FR200

Procheck FR200 is our class B-s1,d0 vapour control layer. It is a black lacquered aluminium foil bonded to low density polyethylene, and strengthened with a glass reinforcement mesh.

We originally developed it for external use in cooling climates, but it quickly gained purchase as a vapour control layer in the UK, particularly in high rise constructions. It can be used in both roof and wall constructions to provide an air and vapour tight seal within buildings. The black side is a fire rated lacquer, giving the membrane it’s B-s1,d0 performance.

A vapour resistance of 220MNs/g makes it ideal for use on offices and residential buildings, particularly with insulation between and outside of the frame.

Procheck A2

Procheck A2 is a membrane which offers a very limited contribution to fire, per the definition in Approved Document B. Section 2 of the Scottish Technical Handbook lists it (and all other class A2 products) as non-combustible. As the name suggests, it achieves class A2-s1,d0, far exceeding the B-s3,d0 requirement for relevant buildings.

The membrane comprises a glass fibre backing on an aluminium foil, which is lacquered with a clear coating. This composition provides it with a robust vapour resistance – an equivalent air layer thickness of over 1500m, which equates to over 7500MNs/g. This makes it ideal for dense residential environments, and an excellent choice for almost any wall construction. It is most commonly used inside of the internal lining, but you could also use it on the sheathing board in warm frame constructions.

As the lacquer on this foil is clear, it has the additional benefit of having a low emissivity surface. The reflective face of the Procheck A2 is able to be installed facing into a cavity, such as a service void. The cavity would then contribute to the thermal performance of the wall. With an emissivity of 0.14, a cavity of at least 20mm would have a thermal resistance of 0.51m²K/W – equivalent to around 20mm of mineral wool.

Tested as a system, our Procheck A2 maintains its fire rating of A2-s1,d0 with up to 10% coverage of our Procheck FR Tape, meaning that air and vapour tightness can be achieved without compromising on the fire performance.

Façadeshield

Moving away from our vapour control layers, we also have a range of breather membranes which exceed the minimum requirement for preventing the spread of flame in applicable buildings. The first of these that we will discuss is our Façadeshield.

This is a black spunbond polyester membrane with a UV resistant coating, which makes it perfect for use behind open jointed cladding. It is UV stable, designed to have up to 30% of its surface exposed behind cladding, providing a striking shadow gap. These gaps can be up to 30mm wide, providing a number of aesthetic options for a façade. Combined with our Façade Tape, it also provides an additional measure of airtightness to a building.

It achieves a fire rating of B-s1,d0, so is ideally suited for open jointed façades, particularly near boundaries.

Wraptite

Wraptite is our fully self-adhered, airtight breather membrane. It is a triple-layer polypropylene membrane with a micro-porous film laminate, which is fully coated with a proprietary acrylic vapour permeable adhesive. Unlike most membranes, Wraptite is fully bonded to the substrate, removing the requirement for mechanical fixings. It can be applied to most substrates and is most commonly used on the sheathing board.

Applied in this location on a framed building, it can easily be used to create a robust airtight envelope. It can adhere across a concrete frame, spanning from sheathing board to the column or slab edge, and then back to the sheathing board. As it is fully self-adhered, it can be used for this purpose where other membranes would need to be terminated, and sealed with EPDM (Ethylene Propylene Diene Monomer).

EPDMs are typically not rated to B-s3,d0 or above, and so are not suitable for use as membranes, though can be used as seals. Spanning across like this allows Wraptite to simplify detailing, which accelerates the speed of on-site installation. Some EPDM can also be reduced around window details, where it is used as a sealant.

The self-adhered nature of the product also has implications on the possible airtightness that can be achieved using Wraptite. As there can be no air movement behind the membrane, and also as it does not rely on mechanical fixings, Wraptite provides a greater level of airtightness than a similarly performing, mechanically fixed membrane. It has achieved results as low as 0.5m³/m²h at 50 Pascals, and is frequently used on passive house and low energy constructions.

Wraptite-UV is also available which combines these benefits with UV stabilisation allowing it’s use behind open jointed cladding systems.

Both Wraptite and Wraptite-UV are fire rated, and on substrates with limited combustibility will achieve Class B-s1,d0 and Class B-s2,d0 ratings respectively. This type of build up therefore complies with the requirements for use in high rise construction.

The self adhered nature of Wraptite and Wraptite UV mean the substrate plays a role in the fire rating, so for substrate which are not Class A1/A2 rated, please contact us for further information.

Wraptite has has also been tested in some full scale tests in association with cladding manufacturers to a standard called BS 8414. These full-size tests simulate the effects of a fire breaking out of a window and a cladding system being exposed to it.

Because these test are conducted on a specific system supplied by others, the results are not universally applicable, however if this is of interest please get in touch for more information.

Fireshield

Compared to other membranes which achieve a fire classification of B-s1,d0, Fireshield reacts differently.

Where other membranes recede away from a fire, Fireshield expands towards it. It has an intumescent coating on its face, which actively works to suppress the spread of flame. It also significantly reduces the formation of smoke and flaming droplets.

This expansion means that it provides additional protection to the substrate, as shown here. These images were from a series of identical tests, on Fireshield and a membrane which, at least on paper, has the same fire rating.

As you can see, the Fireshield offered far more protection to the substrate, while the best that can be said of the other membrane is that it offered no contribution to the spread of flame.

Fireshield’s unique composition comprises a vapour permeable film sandwiched between a layer of a non-woven spunbonded polypropylene and a glass fibre fleece. This fleece is coated with a graphite compound, which gives it its intumescent properties.

ProBreathe

Probreathe A2 and Probreathe A2 Air, unlike polypropylene based membranes, are made of woven glass fibre, which allows them to achieve a better fire rating.

These class A2 membranes produce less than 1/75th of the heat of a membrane rated class B-s1,d0. Probreathe A2 Air is woven glass fibre with a white surface. It achieves a class W2 water holdout rating, so is suitable for use on walls. It is also highly permeable to both water vapour and air.

Probreathe A2 builds on this glass fibre base with a durable and airtight PU coating. This additional layer reduces the air permeability to 0.0064 m3/hr.m2@50Pa, while maintaining an Sd value of 0.095m.

By delivering Euroclass A2 reaction to fire performance without compromising on vapour permeability and airtightness, Probreathe A2 offers a versatile solution for all types of building and construction type.