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Procura

News & opinion

14 AGO 2018

Trial by fire

Following his review of different facade types, Diego Alves looks at the performance of modern systems during fires.

There is no doubt that the repercussions of the Grenfell Tower fire have brought home to landlords and occupants alike the shortcomings of the building industry, particularly the performance of the modern facade with regard to fire.

The Building Regulations, principally Approved Document B, are intended to ensure that a building and its facade are designed to ensure sufficient time for the safe evacuation of occupants if the property is exposed to fire.

Fundamentally, the regulations are there to preserve and safeguard life, not the asset. To this end, the design, components, selection of materials and installation of facade systems must provide assurance for occupants and owners throughout the life of the property.

Fire spread

In general, the Building Regulations require that external walls on all buildings adequately resist fire spread, and statutory guidance in Approved Document B sets out two ways that they can fulfil this requirement.

The first is for each individual component of the wall, such as insulation or filler, to meet the standard for combustibility. The second is to ensure that the combined elements of a wall, when tested as an entire, installed system, adequately resist the spread of fire to the relevant standard.

Until recently, another method for proving compliance has been to carry out desktop assessments in lieu of actual tests of the facade system. However, the government has undertaken a consultation process on the use of such assessments, though the responses are yet to be published at the time of going to press. The consultation is in line with the recommendations in Dame Judith Hackitt’s interim report on Building Regulations and fire safety.

Dame Judith’s advice should be read in conjunction with all sections of Approved Document B that outline test regimes, performance of materials, products and structures, and which establish the principle of assessments.

Approval standard

Currently, approval of cladding systems for tall buildings is carried out via full-scale tests in accordance with the requirements of BS 8414, which was introduced in 2002.

These involve taking a 6m-high, right-angled sample of the cladding and placing it in a wooden crib comprising 395kg of softwood of specific cross-section and length, arranged in 20 layers. This crib is then set alight, and the behaviour of the fire is measured over 30 minutes.

The concern from various parties is that the prescribed test specimen and its construction do not represent the exact conditions into which the system will be installed. The materials used and construction techniques have changed considerably since the test was introduced almost 20 years ago.

These changes include a sizeable increase in plastic content, which contributes significantly to the fire load and even the height of flames. The test sample is invariably quite idealistic as well, devoid of penetrations such as ducts, pipes and even extra windows, let alone the architectural articulation of the cladding.

Plastic vents and ducts can precipitate fire into the void well before the cavity barriers can intumesce, and this is not currently addressed in BS 8414. There are other factors of concern as well, including the oxygen supply that contributes to the chimney effect; this is a by-product of the need to include a ventilation void in rainscreen cladding systems.

All this and more suggest that testing to BS 8414 is too generic an approach, one that is dated and does not address many key industry concerns, and it therefore requires review. It is expected that the proposed BS 9414 will redress these concerns.

Facade types

Apart from the aesthetics, the prime function of a facade is to resist air and water infiltration, accommodating wind and other forces that act on it while supporting its own dead load. Above all, it must do this safely and without endangering life.

Different systems achieve this in different ways. The materials used in standard curtain walls are generally manufactured from aluminium and glass, both of which are non-combustible and comfortably comply with the primary requirement of Approved Document B to prevent the spread of fire through the external wall. Curtain walls are generally durable, need little maintenance and provide excellent aesthetics.

The rapid rise of rainscreen cladding globally in the past decade or two demonstrates that it is an economic and simple alternative to curtain walls for new properties or over-cladding older buildings, as detailed in my previous article (Building Surveying Journal May/June 2018).

However, the relatively light nature of rainscreen cladding means that it predominantly comprises many components that are synthetic and combustible, such as the insulation, vents, pipes and panels, and in particular the polyethylene core of aluminium composite material (ACM) panels.

The cavity formed between the external skin and supporting construction also creates a chimney effect by which fire can propagate rapidly if not controlled with properly installed fire-stopping, cavity barriers and controlling the supply of oxygen. The importance of effective compartmentation between floors, adjacent rooms, windows and penetrations cannot be overstated.

Depending on the design, specification and quality of installation, rainscreen systems can fail compliance tests because they do not satisfy the requirements of Approved Document B. In much the same way as buildings are required to be tested for air, all penetrations and gaps in the support wall should be sealed with intumescent caulking or sealant to ensure the construction will resist the spread of fire.

Central to a fully functioning rainscreen facade, including the structural integrity of the system, is the drive for an economic product, which is sometimes compromised by a general lack of knowledge about the requirements of the Building Regulations and their implicit requirement for the safety of the persons occupying or using the building.

Aluminium composite

ACM cladding is a versatile product, and in the past two decades has been used increasingly in high-rise properties throughout the world. It is essentially two thin skins of aluminium or other metals bonded to a plastic core sometimes referred to as filler, forming a relatively rigid sheet some 3–4mm thick.

Unless specified otherwise, the basic core material is highly flammable, with a heat potential comparable to that of petrol at more than 45MJ/kg, and it is ranked as class C- or D in the European Reaction to Fire classification system, it must therefore be used with absolute discretion, particularly on high-rise properties. The insulation, cavity barriers and even the wall construction behind it must be designed and installed carefully to comply with the appropriate parts of Approved Document B.

ACM and rainscreen cladding have their limitations which must be taken into account in terms of performance. But in the light of Grenfell Tower, compliance with the requirement and guidance in Approved Document B4 and the need for cladding materials to be of “limited combustibility” is very important.

This, together with the need for proper installation of cavity barriers, compartmentation and fire-stopping, is fundamental to good installation and compliance characteristics, which are ultimately designed to preserve life in the event of a fire.

The method of installing cladding panels and retention of cavity barriers and fire-stopping is equally important since ineffective fixings and loose materials can be injurious if they become detached in high winds, and worse if they are ablaze and start secondary fires wherever they land.

ACM panels are available with a fire-retardant core and usually have the suffix “FR”. Additives in the core can reduce the heat potential in such panels by about 30% to less than 13MJ/kg, putting it in class B. Meanwhile, the better type A2 ACM can have a heat potential of less than 10% when compared to the standard polyethylene core, at less than 3 MJ/kg.

Please note that FR means fire-retardant rather than fire-resistant. There are also additional classes for smoke development, designated s1, s2 or s3, and the amount of burning droplets emitted, d0, d1 or d3. Thus, an ACM panel may be designated A2-–s1, d0, for instance.

Cavity barriers

Such barriers are required because of the risk of fire spread in cavities behind rainscreen panels, which can occur rapidly, and out of sight, due to the chimney effect.

Note that cavity barriers are not fire-stops; fire-stops are located internally between the floor slab and the inner surface of the facade and are required to have the same fire rating as the compartment wall.

Cavity barriers are located in the cavity of the rainscreen and are both horizontal and vertical, although the horizontal barriers must include a 20mm gap to allow the cavity to be drained and ventilated. However, they must also intumesce and seal in the event of a fire.

The two criteria for cavity barrier performance are that the correct type is used in the facade and that they are installed correctly. Currently, there is only guidance on the requirement to inspect the presence and quality of installation of cavity barriers, including those in existing buildings.

Acrylic render also requires a mention since this and its backing material or insulation can be combustible and therefore non-compliant. Both the render and insulation are available in non-combustible form. The fixing methods for attaching the insulation and render to the substrate must be selected carefully, and the materials must be fitted correctly to avoid the entire render detaching from the construction, for example during high winds or if the fixings cannot sustain the weight of the construction when wet.

Diego Alves is CBRE Director and Head of Facade Consultancy.

For the latest information, visit the Ministry of Housing, Communities & Local Government

This article was first published in the Building Surveying Journal (July/August 2018).