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Controlling rain is the single most important factor in the design and construction of durable buildings and in the control of mold. Drainage planes are used in the design and construction of building enclosures to control rain.
All exterior claddings pass some rainwater. As such, some control of this penetrating rainwater is required. In most walls, this penetrating rainwater is controlled by the drainage plane that directs the penetrating water downwards and outwards.
Drainage planes are water-repellant materials (building paper, housewrap, sheet membranes, etc.) that are located behind the cladding and are designed and constructed to drain water that passes through the cladding. They are interconnected with flashings, window and door openings, and other penetrations of the building enclosure to provide water drainage to the exterior of the building. The materials that form the drainage plane overlap each other shingle fashion or are sealed so that water drains down and out of the wall. The drainage plane also is known as the water-resistant barrier (WRB).
The most common drainage plane is tar paper or building paper (Photograph 1). More recently, the terms housewrap or building wrap have been introduced to describe building papers that are not asphalt-impregnated felts or coated papers such as polyethylene or polypropylene films (Photograph 2). Drainage planes also can be created by sealing or layering water-resistant sheathings such as a rigid insulation or coated structural sheathings (Photograph 3). Finally, fully adhered sheet membranes (Photograph 4), or trowel- and spray-applied coatings (Photograph 5) can act as drainage planes.
In commercial construction drainage planes are sometimes referred to as air barriers, but in my opinion, are misnamed, as their principle fundamental function is rainwater control, not air control. I prefer to refer to these types of commercial membranes as drainage planes that also act as air barriers and vapor barriers to emphasize their primary function. On drawings they should be referred to by their function-in order of importance. So if they also act as air barriers and vapor barriers, they should be noted as, "membrane drainage plane/air barrier/vapor barrier."
Drainage planes can be vapor permeable or vapor impermeable depending on climate, location within the building enclosure or required control function. Building papers and housewraps are typically vapor permeable (more than 10 perms), whereas...