Structural Moisture Control for Hot-Dry and Mixed-Dry Climates

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Annual precipitation in the hot-dry and mixed-dry climates is less than 20 inches. However, often the precipitation comes in heavy doses as part of intense storms. And moisture can come from other sources, such as landscape irrigation and indoor activities. In some regions, periodic humidity is also present. 

Two types of rain management systems have been identified: barriers and screens. Barriers rely on exterior cladding to drain water and are best used with water-resistant building materials, such as masonry block or concrete. Screens have multiple lines of defense against water entry and are used with wood, brick, and gypsum-based materials. Both barriers and screens rely on lapped flashings to direct water to the exterior at critical areas such as seams, windows, and penetrations. In both systems, it is essential that materials are lapped shingle fashion to direct water down and out, away from the wall assembly.

Best Practice: Roof and wall assemblies must contain surfaces that will drain water in a continuous manner over the entire area of the building. Water must have a path that will take it from its point of impact, around any elements such as windows, doors, and seams, all the way to the exterior ground, sloping away from the house.

Best Practice: In areas with potentially high winds and heavy rains install four inch to six inch “peel and seal” self adhering water-proofing strips over joints in roof decking before installing the roof underlayment and cover.

Water Leakage

One critical point of concern is water leakage around windows. The EEBA Water Management Guide offers examples of many window flashing applications. The window flashing examples here are taken from the Trades chapter. These examples are for homes with housewrap and plywood or OSB sheathing.

Best Practice: Specify that flashing be installed for all windows and doors. Window and door flashing details should be designed to match specific wall assemblies and claddings.

Flashing systems should be designed in accordance with the ASTM standard entitled Standard Practice for Installation of Exterior Windows, Doors, and Skylights (ASTM 2002). In addition to the standard and the EEBA guide, see DOE’s Technology Fact Sheet on Weather-Resistive Barriers (DOE 2000), available on the Web at www.eere.energy.gov/buildings/documents/pdfs/28600.pdf.

Extreme Weather

Parts of the hot-dry and mixed-dry climates can be vulnerable to catastrophic highwind, heavy-rain events, hurricanes and tornadoes. Parts of Oklahoma and Texas are part of “tornado alley,” a section of the country prone to tornados. Proper structural fastening and impact resistant windows, doors, and skylights are critical to surviving high winds. Proper use of roofing materials can help roofs withstand high winds and protect against severe rains. This document does not provide detailed information on

disaster survival but the following sources provide structural details and guidance and a listing of building materials acceptable for high wind areas.

·         Federal Emergency Management Agency. Building a Safe Room Inside Your Home. www.fema.gov

·         Federal Alliance for Safe Homes – FLASH, Inc. Designed primarily for Florida, this Web site contains generally-applicable information about building to resist high winds, wild fires, and floods. Blueprint for Safety. www.blueprintforsafety.org

·         Institute for Business and Home Safety. The IBHS has building guidelines and public information. www.ibhs.org

·         U.S. Department of Energy. A training program for home inspectors to identify hazards. www.eere.energy.gov/weatherization/hazard_workshop.html

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. Humid crawlspaces and attics are less of a problem in the hot-dry and mixed-dry climates than in other climates, but problems can arise under humid weather conditions or in areas with high ground moisture. The recommendations below are intended to help control both liquid water and vapor.

Best Practice: Do not install impermeable coverings, such as vinyl wallpaper, on exterior walls and do not install vapor retarders on the framing side of gypsum board or other wall coverings. Use unfaced insulation.

Best Practice: Water soaking through wood can carry with it contaminants that interfere with the ability of housewraps and building papers and felts to resist water. One step in the solution is to backprime all wood cladding to avoid water saturation and migration. This practice also makes the wood much more durable.

Best Practice: Creating an air space between the cladding and the drainage plane effectively increases the durability of both components. With some exterior finishes, such as bricks, an air space is even more 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. Information on housewrap and building paper performance behind brick and stucco can be found at www.buildingscience.com/resources/walls/brick_stucco_housewraps.pdf

Best Practice: If building paper or felt is used in areas 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.

Best Practice: 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. 

Additional information on moisture control can be obtained from:

·         DOE’s Technology Fact Sheet on Weather-Resistive Barriers, available on the Web at www.eere.energy.gov/buildings/info/documents/pdfs/28600.pdf

·         Building Science Consortium’s Web site at www.buildingscience.com/housesthatwork/ buildingmaterials  where you can compare wraps and other materials.

·         www.buildingscience.com/resources/walls/problems_with_housewraps.htm

·         The following article provides an overview of house wraps and was a key source for this section: Straube, John. 2001. “Wrapping it Up,“ Canadian Architect. May, 2001. Available at www.cdnarchitect.com

·         The National Association of Home Builders Research Center’s Moisture Protection of Wood Sheathing is available on the Web at www.nahbrc.org/docs/mainnav/moistureandleaks/792_moisture.pdf

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 (on the framing side of gypsum board) or impervious coatings, such as vinyl wallpaper.

A more recent and graphical reference on moisture control is the EEBA Water Management Guide (Lstiburek 2003), available for sale from the EEBA Bookstore, on the Web at www.eeba.org/bookstore.

For more information, please visit Building America or download the full reports below:

Cold and Very Cold Climates

Hot-Dry and Mixed-Dry Climates

Hot and Humid Climates

Marine Climates

Mixed-Humid Climates