Basics of Bulk Water Management through Rainscreens

Green Building

Basics of Bulk Water Management through Rainscreens

The building envelope must face a number of damage functions: bulk and capillary water, air-born water, vapor, radiation (ultraviolet degradation), pests, and people. But even in dry climates, water in all its forms perhaps poses the greatest challenge to building performance and longevity – and bulk water is public enemy number one.

In general, there are four basic approaches to bulk water management in a wall assembly:

1. Barrier

Barrier surfaces are designed to shed water without allowing any moisture penetration. Barrier-type exterior insulation finish systems (EIFS) exemplify the barrier system.

2. Mass

Traditional uses of concrete, masonry, timber structures, and other solid assemblies that are capable of serving as hygric buffers will shed most bulk water, absorb the remainder, and slowly release the absorbed moisture as a vapor.

3. Internal drainage plane

The plane located between the exterior cladding and supporting wall that serves as a redundant moisture barrier. Stucco or clapboard walls will often contain a dedicated internalized drainage plane.

4. Rainscreen

Rainscreens manage moisture through cladding, an air cavity, a drainage plane, and an airtight supporting wall system. These systems diminish the various forces of moisture-drive into a wall assembly.

In both teaching and practice, I’ve found rainscreens to be broadly misunderstood. I offer the following as a primer to differentiate between the basic types of rainscreens.

 

Two Basic Types of Rainscreens

The term rainscreen has been generalized in the building design and construction industry. In essence, there are two basic type to rainscreens:
  • Drained/Back Ventilated (D/BV)
  • Pressure Equalized Rainscreens (PERs)

 

Diagrams of a Drained/Back Vented (Open) rainscreen and a Pressure Equalized Rainscreen (PER). Illustration property of Daniel Overbey.

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