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FIRE SAFETY IS NOT YET ASSIMILATED INTO RAINSCREEN GUIDANCE

TECHNICAL & PRACTICE

The fourth in our series of NBS Shortcuts looks at rainscreen cladding.

Its modern incarnation was developed in the 1940s, and in the 1950s the UK's Building Research Station advocated drained and ventilated air spaces behind outer envelopes. However, it took 25 years to become commonplace.

The Centre for Window and Cladding Technology's (CWCT's) 'Standard for Systemised Building Envelopes' defines curtain walling as 'a predominantly vertical building envelope which supports no load other than its own weight and the environmental forces that act upon it.' Rainscreen cladding is a non-loadbearing external cladding assembly defined as 'a wall comprising an outer skin of panels and an airtight insulated backing wall separated by a ventilated cavity. Some water may penetrate the cavity but the rainscreen is intended to provide protection from direct rain.' The key distinction is that curtain walling is the whole envelope, while rainscreen cladding is the outer protective layer of the envelope.

As far back as the Second World War, British researchers were exploring the potential of a protective impervious cladding on the outer face of external walls with a drained and ventilated air gap behind it. Then in 1952 the Alcoa building in Pittsburgh - a 30-storey building clad in open-jointed aluminium bafe panels - became one of the earliest and best-known examples of rainscreen cladding, even though the phrase hadn't been invented at the time of its construction (the term 'rainscreen principle' was coined in the 1970s). It was in 1962 that the Norwegian Research Institute published a booklet which championed systems that ensure pressure equalisation of the air gap - that is, the air pressure in the gap separating the rainscreen cladding from the building/ inner leaf is the same as that of the outside conditions; this was shown to combat wind-generated air-pressure differences. Since then, the two systems - drained and ventilated and pressure equalised - comprising a notionally impervious and a permeable outer layer respectively, have been widely specified.

The acceptable degree of water penetration into the air gap defines the difference between the two systems. In a drained and back-ventilated system it is assumed that under adverse weather rain, snow or hail will be wind-driven into the air gap.

These systems are therefore detailed to prevent water crossing the gap and penetrating the insulation/backing wall.

In a pressure-equalised system, the relationship between the area of the open joint which gives access to the gap, the volume of the gap, and the air permeability of the air barrier is designed such that wind pressure acting on the face of the rainscreen is balanced by the pressure created at the joint. The air gap acts as a pressure cushion to prevent water from reaching the insulation and backing wall and, as such, the air-gap pressure is an essential component preventing excessive water passing through an open-jointed rainscreen. (NB: 'open joints' include bafed and labyrinth joints. ) As it happens, drained and ventilated rainscreens also achieve a certain degree of pressure equalisation.

Unfortunately, the term 'open-jointed rainscreen' has emerged to describe both systems and the understanding of the two technologies has become ill defined. Currently there is no specific British Standard for ventilated rainscreen walls. BS 8200, 'Code of Practice for Design of Non-Loadbearing External Vertical Structures' includes general details about the principles of drained and ventilated and pressure-equalised systems. Published in 1985, with no subsequent revisions, the standard is categorised by BSI as current but obsolescent, as it no longer represents current practice for new construction. A number of system manufacturers test to DIN 18516-1 'Cladding for external walls, ventilated at rear - Part 1: Requirements, Principles of Testing.' This standard recognises different joint types but does not include requirements for pressureequalised systems. National House Builders Council (NHBC) guidance only deals with pressure-equalised systems.

PRESSURE EQUALISATION The air gap and connecting interstices in the insulation will not act as a 'pressure cushion' unless there is a relatively impermeable layer behind the air gap and insulation. But variations in wind pressure mean no system ever achieves total pressure equalisation.

The CWCT states that the air gap should be at least 25mm, whereas the NHBC recommends at least 38mm for panels with rebated joints and a minimum 50mm for those with open joints. The CWCT states that open joints should be a minimum 6mm, but 'joints that are required to remain unblocked shall have a minimum opening of 10mm'. The NHBC insists on 10mm and that the air gap must be sufficient for any water passing through the joints to run down the back of the panel system without wetting the insulation or the backing wall. A breather membrane fitted to the front face of the insulation also helps.

The satisfactory pressure equalisation of the air gap is dependent, to some extent, on the permeability of the building structure, and in most tests this is taken to be 10 m 2, ie. the leakiest allowable under Approved Document L (ADL). Higher or lower permeability rates are acceptable but these must be factored in, especially in terms of U-value ratings and SAP calculations.

Where rainscreen systems are applied to (predominantly nondomestic) existing walls, the requirements of AD L2 will apply.

TESTING Testing must be carried out by an approved agency in conditions that replicate the site conditions in which the system will be fitted.

This means that air pressure, wind loads, rain incidence, fixing angles, etc, will need to be applied to the test panels and computer models to ensure that the rainscreen performs satisfactorily in situ.

The background fixings need to be such that the panels do not deect due to wind suction, sag or thermal movement.

Other issues affecting the system include the need for:

efficient drainage;

no thermal bridging;

fire stopping (risk assessed to ADB and designed to BR135 2003);

secure fixings;

an acoustically robust system;

panelised compartments that are earth-bonded as necessary; and a system designed to withstand the degree of possible impact scenarios specific to the site location (see drawing on page 41).

COMPARTMENT LAYOUT

Given that rainscreen systems tend to be applied to large buildings, the elevations need to be isolated into compartments suitable for the prevention of the spread of smoke and fire and these barriers must not compromise the pressure equalisation air ow behind the panels. At the time of writing, the consequences of AD B - Fire Safety haven't been fully assimilated into current rainscreen guidance, but perforated intumescent cavity barriers ensure fire compartmentation without restricting air ows.

Each system must have cavity barriers at storey heights and at least 6m horizontal centres. At junctions, corners, abutments, etc, these centres need to be reduced to take account of increased wind pressure. Barriers need to start 1.5m from the edge, although the requirement for barriers at 1.5m centres seems excessive.

A rainscreen cannot be totally pressure equalised, but any system can perform satisfactorily if its air permeability is more than 10 times the air permeability of the air barrier and if each compartment numerically has a vent area (in m 2) greater than 0.0125 of the volume (in m 3) of its compartment it serves.

Austin Williams is the author and illustrator of NBS Shortcuts. For more information visit www. thebuildingregs. com

REFERENCES: 'Standard for Systemised Building Envelopes', (2006) Centre for Window and Cladding Technology BS EN 14019 (2004) Curtain walling. Impact resistance BS 8200 (1985) Code of Practice for the Design of Non-Loadbearing External Vertical Building Enclosures BS 8118-2 (1991) Structural Use of Aluminium - Part 2: Materials, Workmanship and Protection BS 8297 (200), Code of Practice for Design and Installation of Non-Loadbearing Concrete Cladding BS EN 13022-1-4 (under development) Glass in Building. Structural Sealant Glazing - Part 1: Actions, Requirements and Terminology BS EN 13116 (2001) Curtain Walling - Resistance to Wind Load - Performance Requirements Curtain Walling and Cladding Technology, TN 41 Site Testing for Watertightness' Centre for Window and Cladding Technology, CWCT BR135 (2003) 'Fire Performance of External Thermal Insulation for Walls of Multi-Storey Buildings', BRE RECOMMENDED READING:

Curtain Walling and Cladding Technology, Technical Note No 52 (2006) 'Impact Performance of Cladding' Centre for Window and Cladding Technology, CWCT, www. cwct. co. uk

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