A rainscreen façade is a cladding applied either during primary construction or as an over cladding to an existing structure. Rainscreen cladding consists of an outer weather-resistant decorative skin fixed to an underlying structure by means of a supporting grid, which maintains a ventilated and drained cavity between the façade and the structure.
Rainscreen façades are not normally sealed and a ventilation cavity of at least 25mm is allowed immediately behind the cladding panel. Insulation can be positioned within the cavity and openings at the top and bottom of clad areas allow for evaporation of moisture vapour and ventilation/drainage. A ventilated rainscreen incorporating insulation will allow the building fabric to breathe without the risk of interstitial condensation or structural decay. External wall insulation used in this way is superior in performance as it eliminates the condensation risks associated with internal or cavity wall insulation. This is particularly important for refurbishment schemes. In new construction the use of back ventilated rainscreen cladding provides the designer with the opportunity to use economical single skin load bearing blockwork for infill walls.
The need for complicated damp proof membrane detailing is eliminated and there will be less risk of cold bridging. The air gap provides ventilation and depending on the design of the rainscreen, in conjunction with ventilated panel joints, it may also assist in providing pressure equalisation across the outer skin. Fully pressurised and compartmentalised (zoned) systems control airflow locally and are usually used on high performance, multi-storey developments. Where lower performance is required, for example in low rise structures, then a similar cassette panel system, although not fully pressure equalised, performs well.
The pressure equalisation concept is simple:
When outside air pressure is transferred to an air space behind the exterior cladding, the air gap compartments must be small enough, the air barrier system must be airtight enough and the area of the venting through the rainscreen must be large enough to allow sufficient air to move in and out of the compartments under the applied air pressure. The strategy relies on the control of airflow within and through the wall assembly. In theory, pressure equalisation means a zero air pressure differential at all times across the rainscreen,resulting in a complete elimination of the driving force for pressure-induced water penetration.