October 2011

Volume 20, Number 10

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How Rainscreens Work

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Ventilated Rainscreen

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Illustration: Steve Baczek

If you have ever gone camping, you have probably used a simple rainscreen—a tent fly. While there are many ways for a tent to get wet inside, a fly at least keeps rain out; it can do so because the frame keeps the fly separate from the tent fabric.

A rainscreen system for a building works similarly, keeping the cladding separate from the building through the use of furring strips or a specialized rainscreen product. Though they come in three main forms, all rainscreen systems rely on a space between the cladding and the exterior wall to prevent water from penetrating the structural wall assembly.

A ventilated rainscreen system features a continuous air space, open at top and bottom, to encourage airflow and convective drying when water gets between the cladding and the exterior wall of the building—whether due to wind-driven rain, bulk water shedding, or capillary action through porous cladding. This cavity must be protected with overhangs and screens so that bulk water and insects cannot get in. To further ensure that moisture will not penetrate the building envelope, a properly designed rainscreen system also needs a continuous moisture barrier over the sheathing to create a drainage plane on the outer face of the exterior wall. Many ventilated systems also include water-resistant rigid insulation on the exterior wall to improve energy performance and reduce the likelihood of condensation within the wall assembly.

Ventilated systems are most common in homes and small commercial buildings. They should not be confused with vented systems, which are open only at the bottom; these systems encourage drainage but do not have sufficient airflow to enable convective drying.

Pressure-Equalized Rainscreen

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Both vented and ventilated systems assume that moisture will inevitably get behind the cladding. In contrast, pressure-equalized rainscreen systems, used primarily in tall commercial buildings, are designed to prevent moisture from entering in the first place. They also allow drainage—but they cannot be open at the top, as this would prevent pressure equalization.

Instead of relying on a continuous ventilation cavity, a pressure-equalized system is compartmentalized into discrete chambers, taller than they are wide. These ventilation cavities respond to constantly changing wind pressure. When wind-driven air enters openings in the bottom of the cladding and finds no way to exit, the air pressure inside the cavity matches the wind’s pressure and “pushes back” against it, preventing wind-driven rain from entering. If a small amount of rain is driven into the cavity, the same openings allow the water to drain.

All rainscreen systems require careful design and detailing, making them more expensive than other cladding options; however, they are essential to occupant health and building longevity in wet climates and are advisable in all but the driest regions.

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October 5, 2011