Overcladding

The cladding may fail or deteriorate to such an extent that extensive replacement or renewal is needed. Overcladding is the renovation of a facade by the addition of new skin over the original external fabric.

Overcladding has been used extensively on medium and high rise framed buildings in the UK and Europe, most often in residential apartments. Many of these, 25-30 years old, suffer from a combination of poor thermal performance and physical performance, for example, spalling concrete, defective seals and rotting or corroded window frames.

The cost of overcladding can be offset by savings such as improved insulation and extended life.

Below is an example of how a building can be improved by overcladding.

                 Before                                                        After

Overcladding can be fairly expensive and is usually justified by a combination of a number of factors:

• Savings due to improved insulation
• Increase in the effective life span of the building.
• Increased income by improving the appearance and overall amenity value of the building
• More cost effective than demolition and rebuild.
• A substantial reduction in future maintenance costs by a reduction in condensation risk, thermal bridging and deterioration of the existing fabric.

Overcladding is generally designed as a ventilated rainscreen.

Most systems are designed as ventilated rainscreen and typically include the following layers:

• External weathering skin
• Support structure
• Ventilated cavity
• Insulation layer.

The systems usually rely on the existing wall to act as the air barrier, which may have to be repaired to ensure sufficient air-tightness.

A sub frame structure, aluminium or galvanised stick frame is fixed to the existing structure. There has to be sufficient adjustment in the system to allow the skin to be installed to an accurate line and level. A separate layer of insulation is then attached to the original facade and finally the rainscreen is attached to the subframe. The cavity is vented and drained. Windows can by formed by the insertion of a prefabricated pod that straddles and weathers the cavity, providing a suitable receiver for a replacement window.

In some applications steel strong backs have been used, for example Huse Gladesaxe in Copenhagen. Other projects have included over-roofing or the addition of new balconies and toilets.

An example of an overclad building is Huse Gladsaxe.

At Huse Gladsaxe, four concrete blocks, 16 storeys high were overclad. The existing south facing balconies were enclosed using a prefabricated steel strong back to support infill panels and sliding glass windows.

Given the increasing complexity of buildings envelopes, wide range of available materials and improving performance standards, a new discipline of facade engineering is emerging that treats the facade with the same care as services and structure.

Traditionally responsibility for the cladding often fell between the engineers and the architect, reflecting, one could say, that which it neither entirely technical nor entirely decorative. The performance requirements of buildings are changing as the design process responds to life cycle costing and environmental needs, ultimately leading to increased complexity and rapid technological development. Given this complexity and a large casebook of failures it is not surprising that this new discipline emerges.