Environmental requirements

A wholesome building should provide an acceptable level of thermal comfort with minimal environmental impact.

Current building regulations require minimum thermal standards for cladding. The type of cladding adopted is a primary factor in the method through which thermal comfort will be achieved. Metal based cladding systems, for example, require separate layers of weathering and thermal protection. The ordering of these layers may in fact give rise to additional problems of thermal bridging and condensation. Generally most systems will require the inclusion of specific insulating layers.

The following table indicates the thermal performance of some standard cladding systems and materials.

Table: The thermal performance of some standard cladding systems and materials

Thermal bridging: Thermal bridges can reduce performance and cause condensation locally, and should therefore be avoided.

Whilst the specification of the particular layers may lead to an overall value of the thermal performance the details of the construction may generate thermal bridges which can reduce the thermal effectiveness of the cladding very considerably. Thermal bridges are caused by path ways for heat flow through the insulating layers produced by the inclusion of discrete components of higher thermal conductivity.

Thermal bridging will also lower the temperature locally and may result in the surface or interstitial condensation. Condensation occurs when the temperature of a surface drops below the dew point temperature of the surrounding air. Water vapour condenses into a liquid. Problems associated with condensation include: dampness, corrosion of steel components, damage to insulation layers and growth of mould. The risk of condensation increases with patterns of building usage that encourage the production of moist warm air, e.g. bathrooms, kitchens, swimming pools and industrial processes. Reducing the air permeability of buildings also increases the condensation risk as there is less opportunity for the removal of moisture laden air. Rainscreen systems often incorporate a vapour control layer on the inside layers of the construction. The vapour control layer is a effectively impermeable and endeavours to prevent the floe of warm moist air into the construction. In practice, whilst the material of the vapour control layer may be impermeable, it is difficult to ensure complete sealing at the joints. Some allowance for ventilation of the system is included to allow the air passing the Vapour control layer to be ventilated.

Cladding systems should be designed and specified as part of the fire response strategy of the building as a whole.

Many cladding systems have limited fire resistance. Where a wall provides a boundary to an adjacent building or a boundary to a protected escape route the wall will need a specified fire resistance (one or 2 hours) period. Some cladding materials, such as aluminium and plastic based panels have very little fire resistance compared with other materials such as concrete or masonry.

Additionally the cladding should not contribute to the spread or growth of a fire. Materials should be tested to ensure that they have limited combustibility

Test methods and index values for the ignition point and flame spread characteristics of materials are specified in BS 476 parts 6 and 7.

In multi-layer constructions the combustibility of the internal layers may be particularly important. Ventilated cavities can increase the rate at which fire spreads by a stack effect drawing smoke and flames upwards. The cavity should include incombustible barriers to prevent fire spread.

Durability of cladding should be considered at the design stage in relation to exposure, materials and finishes.

The durability of a particular system is dependent on many aspects including specification of materials, compatibility of different materials, climatic factors such as rainfall and exposure to solar radiation and detailing and workmanship. The consequence of inadequate durability is failure of the cladding systems. Weathering is the change in appearance of the cladding under the effect of climate and is primarily visual in its effect. Although it is self evident that buildings should be durable there are nevertheless some aspects of durability particular to cladding systems.

Compatibility of materials is important to avoid bi-metallic corrosion.

Bi-metallic corrosion can take place when two different metals connected together get wet. Is such circumstances differing metals should be separated using non conducting PVC tapes or washers. The risk of corrosion increases as the distance between materials on the galvanic series increases.

The galvanic series:

Electrolytic action increases the further apart two metals are in the series. The more anodic material will corrode preferentially to the more cathodic.

Finishes should be specified in relation to expected performance and appearance.

The CWCT guide to good practice recommends a minimum life of 25 years before there is an appreciable loss of performance or appearance. This applies to coatings on metal based panels. In the case of robust, porous claddings such as masonry and concrete this recommendation may not be relevant but care in the design of these elements is needed to control the adverse effects of premature staining due to weathering.

Uneven flow of water down the facade of porous cladding can result in premature staining. The staining is caused by the deposition of dust and other particles. Exposed areas of walls, particularly nearer the top will have receive greater quantities of rainfall runoff and may be self cleaning. Careful detailing of the facade can reduce the effects.