SYSTEMS/PRECAST
The pressure-equalized rain screen approach can also be applied to the whole wall system by
incorporating a cavity behind the precast panel. Vents equalize the cavity pressure to the outdoor
pressure, decreasing the pressure-driven rain penetration of the cavity. An air barrier within the
wall is essential to achieving pressure equalization. Ideally this air barrier is located behind the
insulation, protecting the air barrier and associated seals from outdoor temperature swings. The
cavity must be well drained to the outside in order to remove any water that does penetrate. This
design approach is discussed further in the section Air Leakage and Water Vapor Control.
Along with the control of water leakage into and through the envelope, the flow of water over the
precast facade is important. Changes in facade appearance over time caused by dirt and pollutants
in surface runoff water, so-called weathering, does not impact thermal and air-tightness
performance. However, controlling runoff is important to lessen the demands on water leakage
control elements and is another design factor to consider in joint design. Architectural Precast
Concrete (PCI) contains a very thorough discussion of weathering and its control through the use of
water drips to prevent water from running over the entire height of the building.
PAGE 4.4-5
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