In this exclusive extract from NBS Shortcuts externallink, written by noted industry figure Austin Williams, we introduce rainscreen cladding and its use in the UK.

Rainscreen cladding has been around, in various guises, for centuries, but its modern, scientifically-validated incarnation was developed in Scandinavia during the 1940s. During the '50s, UK's Building Research Station declared the advantages of drained and ventilated air spaces behind impervious outer envelopes. However, it took a further 25 years for it to become commonplace.

The Centre for Window and Cladding Technology (CWCT) Standard for systemised building envelopes defines curtain walling as 'a form of predominantly vertical building envelope which supports no load other than its own weight and the environmental forces which act upon it'. Similarly, 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 into the cavity but the rainscreen is intended to provide protection from direct rain'. So, the key distinction is that curtain walling is usually the whole envelope, while rainscreen cladding is the outer protective layer of the envelope.

As far back as the Second World War, UK researchers had been exploring the potential of a protective impervious cladding applied to the outer face of external walls with a drained and ventilated air gap behind it. Then, built in 1952, the Alcoa building in Pittsburgh - a 30-storey building clad in open-jointed aluminium baffle panels - became one of the earliest and best known examples of rainscreen cladding, even though the phrase hadn't yet been invented at the time of its construction (the term 'rainscreen principle' was coined in the 70s). 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-driven rain infiltration. 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 so as 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 baffled 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 the 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 revisions, the standard is categorized 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 pressure equalised systems. The National House Builders Confederation (NHBC) guidance on the other hand, only deals with pressure equalised systems.

Pressure equalisation

The air gap and any connecting interstices within the insulation will not act as a 'pressure cushion' unless there is a relatively impermeable layer behind the air gap and any included insulation. However, variations in wind speeds, wind direction and hence wind pressure caused by the prevailing conditions, as well as the specific exposure rating of the wall, all have the effect of ensuring that no system can ever achieve total pressure equalisation.

The CWCT state that the air gap should be at least 25 mm, whereas the NHBC recommends at least 38 mm for panels with rebated joints and a minimum 50 mm for those with open joints. The CWCT states that open joints should be a minimum 6 mm, but 'joints that are required to remain unblocked shall have a minimum opening of 10 mm'. Suffice to say that the NHBC insist on 10 mm 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 m3/hm2, i.e. the leakiest allowable under Approved Document L (AD L) externallink . 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 non-domestic) existing walls, the requirements of AD L2 externallink will apply.