02.02 Overcladding
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Introduction
Where a new or existing wall of concrete, brickwork or blockwork forms the inner leaf of the wall, the rainscreen cladding is called overcladding. The rainscreen may be supported by the inner wall as shown here, however the rails supporting the rainscreen panels may also be fixed to the structure at floor level independently of the inner wall.
Loads
The inner wall may or may not carry vertical building loads but will be required to resist the applied wind which will normally be transferred to the building frame.
Where the rainscreen is supported on cladding rails fixed to the building frame, the loading on the inner wall will be largely unaffected by the method of fixing the rainscreen as the inner wall will still be required to resist the full wind load.
Where the rainscreen is supported by the inner wall, the wall will be required to support the dead load of the cladding. The distribution of the wind load will also be affected. The wind load applied to the inner wall through the cladding may be applied as point loads at the fixings whereas the wind load on the unclad wall will be a uniformly distributed load. Although the rainscreen and the inner wall may both be designed to resist the full external wind load this load can only be applied to the overall structure once hence the total load on the wall is unaffected.
New construction
In new construction the designer may choose whether the rainscreen is used as overcladding or forms part of a curtain wall as most rainscreen panels can be used in both situations. The use of overcladding with a masonry or concrete inner leaf reduces the need for framing members and allows the benefits of thermal mass of the inner leaf to be obtained. The strength and solidity of masonry or concrete may also be preferred to an internal lining supported by framing members. Supporting the rainscreen panels on rails spanning between floors gives fewer points of contact between the cladding and the structural frame and may simplify the accommodation of tolerances.
Existing structures
Many existing concrete and masonry structures are poorly insulated leading to high energy use and condensation. This is often accompanied by lack of weathertightness, deterioration of the building fabric and poor appearance. In such situations overcladding provides a means of refurbishment which can overcome all these problems by
- Providing additional insulation in the cavity reducing heat loss and condensation risk
- Improving weathertightness by protecting the structure from rain
- Reducing deterioration of the fabric by keeping it dry
- Changing the external appearance
Overcladding may be more cost effective and cause less disruption to building occupants than the alternative of demolition and rebuilding however it will not always be suitable and each situation needs to be evaluated on its merits.