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Structural Moisture Control

Managing moisture in both liquid and vapor forms is critical in every home. Since air conditioning is installed in many new homes, cold surfaces are present where condensation can occur. Controlling the infiltration of this moisture-laden air into the building envelope and keeping moisture away from cold surfaces are very important. The following best practices cover moisture management in the roof, walls and window openings of the building envelope.

Extreme Weather

Each area of the country has slightly different extreme weather concerns, but everywhere is vulnerable to high wind and heavy rain events. Proper structural fastening and impact resistant windows, doors, and skylights are critical if a house is to survive. Proper use of roofing materials can help roofs withstand high winds and protest against severe rains or snowfalls. The following sources provide structural details and guidance as well as a listing of building materials acceptable for high wind areas.

• Federal Emergency Management Agency. Building a Safe Room Inside Your Home.

• Federal Alliance for Safe Homes – FLASH, Inc. Provides building guidelines for hazardous areas in Florida. Blueprint for Safety.

• Institute for Business and Home Safety. Provides building guidelines and public information.

• Miami-Dade County Building Code Compliance Office. Includes a searchable database of building materials approved for high-wind locations.

• U.S. Department of Energy. Offers a training program for home inspectors to identify hazards.

Vapor Management

Water has many guises, and water in its liquid state is not the only problem. Water can also cause problems as vapor. The problem is especially bad when vapor gets trapped within an assembly, such as a wall, turns to its liquid form (condenses), and wets structural assemblies. Condensation can also form in and on ductwork, especially when air-conditioning cools duct surfaces that come in contact with humid air, such as in an attic or crawlspace. The recommendations below are intended to control both liquid water and vapor.

• Do not install vapor diffusion retarders or impermeable coverings on the interior and exterior of wall assemblies to block moisture entry from both directions. This approach could be very dangerous if moisture becomes trapped inside the wall.

• Back-prime all wood cladding to avoid water saturation and migration. Water soaking through wood can carry with it contaminants that interfere with the ability of house wraps and building papers and felts to resist water. This practice also makes the wood much more durable.

• Create an air space between the cladding and the drainage plane to increase the durability of both components. With some exterior finishes, such as bricks, an air space is especially important. An air space stops the capillary movement of moisture, stops the contamination of the drainage plane via contact with the cladding, and allows for better drying.

• If building paper or felt is being used in an area prone to severe rain, install two layers. The use of two layers was once common and provides better than twice the performance of one layer. By providing a double drainage plane, they offer increased resistance to leakage at fasteners and allow for more flexible installation.

• Installation is key for all types of housewraps. The sheets must be lapped, shingle-style, especially over and around windows, doors, and other penetrations (and their flashing systems). Use manufacturer-specified fasteners and space them closely enough to provide required support.

• Roof and wall assemblies must contain elements that, individually and in combination, permit drying of spaces inside of walls and other assemblies. Exterior housewraps, including building paper, will allow vapor to pass through and should be installed on the exterior of sheathing. Interior walls should be painted but not covered with plastic vapor retarders, such as polyethylene, (on the framing side of gypsum board) or impervious coatings, such as vinyl wallpaper.

More information on moisture control can be obtained from the Energy and Environmental Building Association (EEBA) Water Management Guide (Lstiburek 2003), available for sale from the EEBA Bookstore, and from the BuildingScience.com section on Moisture Control.