Curtain walling

Patterns of wind and rain exposure

All openings in a facade are a potential source of water penetration. However, the geometry of the building, its height and orientation all affect the performance of the facade in terms of air and water leakage.

Walls facing the wind are subjected to greater driving rain, while leeward walls are more sheltered. However, variations in pressure occur between the sides, centre and top of a wall. As an air current meets a building, it splits to travel round and over it.

This parting of the air creates a cushion of high pressure but relatively still air at the centre of the wall, which tends to reduce the amount of rain hitting it at that location. By contrast, the amount of rain hitting the wall at the corners or top of the building can be as much as 20-50 times that at the centre, since the movement of air accelerates around these features. A tall, narrow building will suffer more from this type of problem than a wider, more squat building.

Illustration of rainfall effect on a facade

The diagram above shows the effects of rainfall on the facade of a tall building. After about 10 minutes the migration of wetting commences, gradually extending down the building and down the corners. With impervious claddings, such as metal and glass curtain walls, water simply flows down the wall surface and the accumulated flow can be significant by the time it reaches the bottom of a tall building. Wind flow around corners and parapets can also draw water laterally and even upwards.

Aside from appearance and climate control, the key function of a curtain walling system is to exclude air and water. Indeed, most failures are related to water leakage in one way or another and a proper understanding of the mechanisms of water entry and the main ways of preventing this will assist in the diagnosis of leakage problems.

There are 5 mechanisms of water ingress:

• kinetic energy;
• surface tension;
• capillarity;
• gravity; and
• air pressure differentials.

Essentially, the methods of prevention of water ingress are:

• to use sealants;
• to design joints in such a way as to prevent water leakage;
• to use mechanical seals (gaskets); and
• to use a combination of two or more of the above.

Although it is theoretically possible to construct a curtain wall using only one of the above methods, in practical terms it is usual to employ more than one. Different options are possible:

• fully sealed or front sealed systems (usually where a flat plane is required);
• drained and back ventilated systems; and
• pressure equalised rainscreen systems.

Kinetic energy (figure A)

A rain drop possesses translational kinetic energy; a function of the mass of the raindrop and its speed. When driven by wind, drops of rain may have sufficient forward momentum to be carried through small joints or openings in the cladding system. On hitting a surface the drops can shatter, with the resultant smaller drops carrying further into the cladding system. To counteract this, joints are designed with a baffle so as to provide a labyrinthine structure.

Studies by Choi (Velocity and impact direction of wind driven rain on building faces) on the effect of wind driven rain showed that drops of small diameter (0.5mm) are capable of moving upwards, particularly in strong wind. Although the upward movement of larger drops (10mm in diameter) is unlikely, the horizontal longitudinal velocity component can be quite large and raindrops can move in a direction that is horizontal and sideways parallel to the building. This means that not only can raindrops penetrate into the depth of a cavity in a cladding system, they can also travel sideways within it.

Surface tension (figure B)

Surface tension is the property of a liquid to behave as if it were covered with a weak elastic skin and is caused by the cohesive forces of molecules at the surface. Water has a particularly strong surface tension, which enables it to run vertically down a surface and then back horizontally over an overhanging edge. By this mechanism, water can be drawn into a joint that is not exposed to direct rainfall.

To counteract the effects of surface tension, it is usual to provide a small drip on the underside edge of a projection.

Capillarity (figure C)

Like surface tension, capillarity is due to forces at molecular level and an unbalanced attraction between the molecules of the liquid and those of a narrow tube or space. If the molecules of the liquid are more attracted to the tube than other liquid molecules, the liquid will rise. For this to happen, the walls of the tube or enclosure must be sufficiently close together. But capillarity by itself can never result in leakage, simply because water can never rise beyond the faces of the enclosure.

However, wind-assisted capillarity can result in water penetration, as the force of the wind can push water further up the walls of the enclosure and beyond. To counteract capillarity, joints can incorporate an air space to break the surface tension and prevent the water rising at the interfaces.

Capillaries of less than about 0.01mm draw and hold a small volume of water with such high suction that they seldom contribute to rain penetration. A greater volume of water, however, is held by the lower suction in large capillaries such as unbonded interfaces. Large capillaries are important contributors to water penetration when an additional force of even low magnitude is added.

Gravity (figure D)

The force of gravity plays an important role in water penetration as it can pull water down and in through an opening that leads inwards and slightly downwards. While wider joints can prevent capillarity occurring, air within the space coupled with the effects of gravity can result in water travelling vertically until it reaches a horizontal surface.

Air pressure differential (figure E)

Of the 5 mechanisms of water entry described, air pressure differentials are the most important and are often the most significant source of water ingress. Air will move from an area of high pressure to an area of low pressure. This movement of air can carry water with it, so that water can be drawn into a joint, glazing rebate or even to the interior of a building.

Wind blowing against or around a building creates large pressure differentials. The leakage of air through a wall system prevents the outside pressure from equalising across the cladding - rather like trying to inflate a balloon with a hole at both ends, the air leaks away. The key to dealing with air pressure differentials is to aim to equalise pressures as quickly as possible (see Pressure equalised systems under Methods for preventing water ingress). This is accomplished by providing a rigid air barrier on the inside and suitable gaps on the outside to allow air movement.