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Indoor airPLUS

Technical Guidance to the Indoor airPLUS Construction Specifications

1. Moisture Control

Sections 1.5 - 1.6: Water-Managed Wall Assemblies

1.5 Install a Continuous Drainage Plane

Install a continuous drainage plane behind exterior wall cladding AND install flashing or an equivalent drainage system at the bottom of exterior walls to direct water away from the drainage plane and foundation. Drainage plane material shall overlap flashing and shall be fully sealed at all penetrations. Any of the following systems meets this requirement:

  • Monolithic weather-resistant barriers (i.e., house wrap) shingled at horizontal joints and sealed or taped at all joints, OR
  • Weather-resistant sheathings (e.g., faced rigid insulation), fully taped at all “butt” joints, OR
  • Lapped shingle-style building paper or felt.

Note: Include weep holes for masonry veneer and weep screed for stucco cladding systems, according to manufacturer’s specifications.

Detailed Illustrations

Click on the image for a full-page version

Drainage plane and drip-edge flashing with wood horizontal siding

Drainage plane and drip-edge flashing with wood horizontal siding

Two layers of building paper form drainage plane beneath stucco

Two layers of building paper form drainage plane beneath stucco

Masonry wall with drainage plane, flashing, and weep holes

Masonry wall with drainage plane, flashing, and weep holes

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Why

Exterior walls are covered with a cladding that is resistant to water damage, such as horizontal or vertical wood, fiber-cement or vinyl siding; panels of metal, fiber-cement or treated plywood; or one of a variety of stucco systems. Most of the rain that lands on exterior wall cladding drains down the exterior surface, although a small amount of the rainwater gets behind the cladding. The cladding may leak at joints between one piece of siding and another, between siding and trim or between trim and windows. It may leak at gaps around penetrations, such as windows and doors, dryer vents, electric outlets or utility entrances. Water can sometimes be driven by wind around cladding materials, and it can wick through porous claddings such as wood shingles. Once past the cladding, rain can wet sheathing, framing, insulation and other components, fostering growth of mold or wood-decaying fungi. A drainage plane serves as a second layer or back-up barrier to liquid water, protecting the wall sheathing and other moisture-sensitive components from water that penetrates the cladding. House wrap, weather-resistant sheathing, building paper or felt and other proprietary products are commonly used as drainage planes.

Water that reaches the drainage plane and moves down its surface must drain out at the bottom of the wall. Therefore, the overall wall system, its drainage plane, flashing and exterior cladding, must be designed to manage water that gets behind the cladding.

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How

Install drainage plane materials between the cladding and the sheathing. (With proper detailing, certain sheathing materials, such as rigid foam board, can be used as the drainage plane.) The wall’s drainage plane must be integrated with the foundation that it rests upon, so a base flashing material must first be installed at the base of the wall. This flashing will create a drip-edge and will direct any water that drips down the drainage plane away from the bottom of the wall system and out toward the face of the foundation wall to be drained away. (Each type of cladding will have a distinct flashing/drip edge detail that must also be included; for example, stucco requires a weep-screed material to be applied at the bottom of the wall under the first course of house wrap.) After this base flashing is installed, the first course of the drainage plane (e.g., house wrap or building paper) is applied to the sheathed wall. To shed water properly and keep moisture out of the wall, each additional course of house wrap must be lapped “roof-shingle style” over the preceding lower course. The tops of penetrations through the cladding are treated similarly.

In selecting a drainage plane system, care should be taken to avoid incompatible materials. Some construction materials contain chemicals such as surfactants that can degrade certain house wrap materials. Always check the cladding and house wrap manufacturers’ recommendations.

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References/Additional Information

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