Weather tightness of the facade

One of the earliest requirements of any building was shelter, protection from the wind and rain. The cladding should resist both water penetration and air penetration.

The design of cladding systems to resist rain and wind penetration requires an understanding of the mechanisms of water penetration.

For water to penetrate a facade three conditions must be present simultaneously.

• Water on the surface of the facade.
• Openings in the facade
• Force to drive the water through the cladding

In the absence of wind, rain would fall vertically and consequently very little rain would land on the vertical surfaces of the wall. The combination of wind and rain causes the wall to get wet. The annual weather patterns vary across the country resulting in varying conditions of driving rain. The driving rain index is a measure of the intensity of wind driven rain fall and is defined as the annual rainfall x the average wind speed. The driving rain index provides a guide to the prevailing climatic conditions across the country. The pattern of wind flow around a building can be quite complex as the building itself affects the wind flow. The intensity of wind driven rain will vary across the elevation of a building depending on the overall shape of the building, the degree of modelling of the facade and the presence of design details that influence the wind flow across the surface of the building. The top and sides of the building tend to receive a greater intensity of rainfall than other areas.

Once the rainfall hits the walls for penetration to occur there must be openings in the wall. Water will collect and run off down the facade. The rate of run off is dependent on the degree of absorption of the cladding material. More porous materials such as masonry and concrete will absorb some rainfall before runoff starts, whereas impermeable materials such a glass and metal claddings have greater rates of runoff and may concentrate water flow at particularly sensitive junctions and seals. When unintentional water penetration does occur it is may be due to gaps or cracks in masonry walls, failed sealants in panel systems or gaps caused by lack of fit or relative movement between the structure and the cladding.

Water is now running down the walls. When it reaches an opening there must be a force to push the water through the opening in the wall.

There are various mechanisms by which water can penetrate through walls.

Understanding these mechanisms plays a crucial role in the detailed design of joints and seals. By far the most important of these is differential wind pressure. Pressure differential between the outside and inside of the wall will result in air flow through gaps. The air flow can then carry water droplets through gaps in the facade.

Drained and ventilated systems form a multi-stage barrier.

This approach to weathertightness recognises the difficulty of achieving a fully face sealed envelope and adopts a multi-stage barrier to weathertightness. It can be applied to both framed curtain wall system and rainscreen systems. In the case of curtain walling, the glazing may be sealed to the frame but should water penetrate the front seal an internal drainage path to the outside is provided.

Effective drainage requires openings to the outside of the system. These openings also provide ventilation into the cavity of the framing therefore assisting in the removal of any moisture present. The openings in the system must also restrict the flow of water into the cavity. The methods used to resist the various mechanisms of water penetration are highlighted. The inner seal has two particular functions; to bed the glazing onto the support frame and to provide an internal air seal. In the particular diagram drainage occurs through the horizontal transoms of the section. In other systems drainage takes place through the vertical mullions of the system.