CWCT FACETS

02.08 Fire Performance

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Categories: Rainscreens

Introduction
By its nature a rainscreen system is a multi-layer wall with each layer providing a separate function and consequently having different fire performance requirements.  This Section deals with aspects of fire performance that are specific to rainscreen walls.

Approved Document B of the Building Regulations provides guidance on the fire performance and surface spread of flame required for this type of external wall construction.
 


The air barrier
The air barrier should provide the necessary level of fire resistance required by the specifier which should be greater than the minimum provisions of the Building Regulations.  Due regard has to be taken of the fire resistance requirement in respect of building usage, height and distance from the relevant boundary.

Because the cavity is ventilated, its bounding components - the rainscreen, the background wall, insulation and air barriers will behave as if they are fully exposed outside the building or possibly worse if a flue effect develops within the cavity.

A number of concerns are:

  • The air barriers could ignite and spread the fire from inside the building into the  cavity
  • The overall fire and smoke resistance of the wall may not be maintained by the air barrier
  • Disproportionate damage may result if fire or smoke spread widely around the cavity, away from the point of ignition.



The rainscreen cladding
The outer skin of a wall of 'rainscreen' construction (with a drained-and-ventilated or pressure equalised cavity) should have a surface spread of flame of Class 'O' rating on both faces.  The two faces are the external (visible/decorated) panel surface and the internal (inward or cavity facing) panel surface which could be impinged upon by fire spreading in the cavity.

The controlling factors on external wall surface classification are the distance from relevant boundaries, building height and usage.
 


Ventilated cavity (including system framework)
The cavity behind the rainscreen and in front of the air barrier should not include materials which can significantly promote flame spread within the unseen cavity.  In general this zone may contain a timber, aluminium or other metal vertical framework and an appropriate non-combustible insulation.

The use of any combustible material for the cladding framework and insulation within the cavity may need to be carefully considered as the building height increases.  A clear distinction exists between one and two storey buildings of essentially domestic construction and larger buildings.  It is usually necessary, for the purpose of preventing spread of flame, to sub-divide the cavity into compartments each six storeys high or less.
 


Design principles
It is recommended that designers and rainscreen systems suppliers adopt the following principles for the external fire performance of the rainscreen:

  • Aim to localise the vertical damage to within three storeys or 10m of the fire incident.
  • Aim to minimise lateral spread of the flames on the rainscreen surface and within the rear cavity.
  • Incorporate fire barriers and fire resistant details to prevent or reduce the risk of the re-entry of flames to any window or opening adjacent to the fire incident (vertically and laterally).
  • In no case shall design be such that the damage could be disproportionate to the original incident.



Testing
Full scale simulation of an intense 'flat' fire with different building configurations, panel composition, cavity depth and associated fire barrier configurations is possible.  Fire Note 13 published by the BRE is currently (2000) being developed as a British Standard.
 


Fixings and fasteners
The designer should take reasonable care in the selection of all primary fixings, panel fasteners and insulation retainers to ensure that a fire does not result in premature system detachment.

By preventing premature failure of a connecting component the occupants and fire fighters should not be exposed to unnecessary hazards.  Failure of primary anchors, attaching the system framework, can cause significant areas of cladding and materials to detach; the use of well fitting insulation (when specified) around the fixings can help to protect them from heat.
 


Insulation in the cavity
Where it is necessary to provide insulation in the cavity fixed to the outer face of the air barrier for the purposes of thermal insulation and condensation control then the insulation should normally be of a non-combustible type.

In certain conditions insulation with appropriate surface spread of flame characteristics may be considered.  However, the use of a separate fire barrier, not incorporating the combustible insulation, should then be specified.
 


Insulation fixings
Where the insulation is attached to the wall, a minimum of one extra non-combustible fixing per square metre or per batten, whichever is the lesser, should be provided to prevent detachment of the insulation in the event of fire.

There is an opinion that the early detachment of insulation mats in the cavity can afford some restriction to flame spread.  Consequently the use of non metallic insulation retainers can be considered if:

  • The insulation is not part of the fire resisting element of the whole wall (i.e. thermal use only)
  • The random collapse of the insulation is only considered to be fortuitous in restricting fire in the cavity - the appropriate cavity fire barriers should always be incorporated.



Compartmentation
The cavity behind the rainscreen should normally be sub-divided with horizontal and vertical cavity fire barriers at centres not exceeding 20m or less as required by the Building Regulations.  However, this condition may be relaxed by the specifier in agreement with the Building Control Officer where there are no penetrations such as windows or doors.
 


Cavity fire barriers
The cavity fire barriers must provide the requisite period of fire resistance as set out in the Building Regulations.

The cavity fire barriers should be constructed of materials commensurate with their location within the cavity and the design life of the building.  Note that the cavity behind the rainscreen is deemed a moist zone.  Materials selected for the fire barrier must not corrode or otherwise deteriorate so as to affect the performance of the fire barrier during the design life of the barrier.

There is always a conflict between the need for ventilation and the provision of the fire barrier.  Intumescent fire barriers allow ventilation until the openings in them close under the effect of heat.

Cavity fire barriers should be positively fixed in position so that they will retain their integrity in the event of a fire.  The fixing must secure the barrier in position for a period at least equal to the design fire resistance period of the cavity fire barrier.  Note that some fire barrier materials may absorb water, and the fixings should be sufficient to retain the fire barrier even when waterlogged.
 


Penetrations
Penetrations through the rainscreen such as for windows, doors, electrical and mechanical services etc. must be constructed so as not to reduce the fire performance of any elements of the wall through which they pass as follows:

  • Closers around doors and windows that penetrate through the cladding should maintain the required level of fire resistance of the air barrier.
  • The materials used to form such penetrations shall be of at least the minimum standard for the elements of the rainscreen as set out above.

Aluminium and aluminium alloys show a reduction in strength at temperatures much above 100° C and at 300° C most of the strength is lost.  For this reason aluminium can only be used as a closure around windows and doors and not as a fire stop.  Additional fire stops may be needed.
 


Electrical and mechanical services
All electrical and mechanical services should pass directly through the wall, and they should be sealed at the air barrier.  Exhaust into or intake from the cavity should not be permitted.
 


Base of cavity
Consideration should be given to the protection of the base of the cladding so as to prevent direct access of fire or smoke. This may be achieved by such methods as stopping the cladding at first floor and building the lowermost course in masonry to deflect smoke or fire away.  The risk at all higher storeys is related to fire within the building or dwelling and not external circumstances.

The risks associated with the possibilities of bonfires being built against the sides of the cladding or parked vehicles being set alight need to be recognised when decisions about the extent of the cladding are made.  Due care and attention shall be paid to refuse and refuse storage areas wherever they occur on the building - either at ground or intermediate floors - as these provide areas of high risk for vandalism.  All cladding into or around these areas should take due account of the risks involved.