DESIGN/RAIN PENETRATlON
Rain Penetration Mechanisms and Control
There are four forces that move water through walls: gravity, capillary action, kinetic energy and air
pressure differences (Brand). Gravity will move water through any opening or along any element,
such as a brick tie, that slopes downward. Capillary action draws water into small cracks and pores
in building materials and can account for the leakage of large amounts of water, particularly in
masonry construction. Kinetic energy refers to water leakage into and through walls due to the
force of wind-driven raindrops impinging on openings in the wall. Water will also penetrate a wall
when there is an air pressure difference between the wetted side of the wall and the opposite side.
There are two basic approaches to controlling rain penetration, eliminating the openings and
controlling the forces acting across these openings. Both approaches are used in different
systems, but before considering either approach it is important to stress the control of rain water
that flows down the facade of a building. Even the most well designed and carefully constructed
system will have trouble preventing water leakage if the facade is constantly exposed to a stream of
rain water runoff. In order to keep rain water off the face of the building, the facade must have
properly designed drips at copings, ledges, sills, balconies, window and door heads, and other
facade features. The design of drips is covered in many construction guides including Architectural
Precast Concrete from PCI. Robinson and Baker also present a thorough discussion of wind-driven
rain and the control of runoff.
The control of rain penetration by plugging the holes on the facade is sometimes referred to as the
face-seal approach. This involves the use of various sealants at panel joints and other interfaces.
The problem with this technique is that the sealant is exposed to severe conditions of sunlight and
ultraviolet radiation, temperature cycling, water exposure and the differential movement of facade
components. These conditions place very severe material requirements on the sealant and the
technique of the sealant installer. For these reasons it is very difficult, some would say impossible,
to achieve long-term success with the face seal approach without significant maintenance efforts.
Any gaps or holes that arise over time, or occur at the time of installation, will leak water since no
effort is made to control the forces transporting water across the facade. The first costs may be
lower than in other approaches, but the costs to maintain performance can be high.
Controlling the forces that cause rain penetration involves designing and constructing joints and
other envelope elements to deal with each of the four mechanisms referred to above (Brand).
Gravity is controlled by sloping all openings to the outside so that water runs out instead of into the
envelope. This is essentially the approach taken with flashing and weepholes in masonry
construction and the use of sloping joint designs in precast concrete panels. Capillary action is
generally more of a problem in masonry systems than in other systems and can be controlled by
obtained a good bond at the unit-mortar interface as discussed in the section Systems/Masonry. To
control capillary action, intentional openings should be wide, at least 10 mm (3/8 inch). Rain
penetration due to kinetic energy can be controlled by shielding openings with cover battens,
splines and internal baffles. Air pressures across the envelope can be controlled by designing
openings into the facade such that the cavity behind the facade is equalized to the outside surface
pressure. The so-called pressure-equalized rain screen approach has been advocated for many
years (Garden) and is used in different forms in many wall systems, as discussed below.
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