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Despite a homebuilder's best efforts, some water occasionally will penetrate a home's first line of defense, the exterior finish (siding, brick, stone, or stucco), and may also enter through gaps around windows and doors under severe weather conditions. When wood framing gets wet and can't dry out, it can rot, resulting in structural damage such as settling or shifting. In addition, when wood framing is wet, mold can grow on it, causing problems with indoor air quality. What's more, visible damage from moisture (such as peeling or bubbling paint) isn't pretty, and it reduces the value of a home.
It's important in the framing stage to implement a drainage strategy to protect the vulnerable wood shell of the home by providing a barrier to water and draining the water away from the exterior of the home. The first part of this strategy includes a continuous roof and wall drainage plane.
Drainage-plane materials fall into four general categories:
- house wrap,
- felt building paper,
- thin structural sheathing, and
- rigid foam sheathing.
This Professional Building Tip will focus on housewrap.
Housewrap is a spun-plastic sheet material that is wrapped around a house to protect against moisture penetration. It performs like a shell for buildings; liquid water that has penetrated the exterior finish does not pass through, yet water vapor can escape. Generally, housewraps with higher permeability ratings allow more moisture to dry to the outside. Low-perm housewraps have a perm rating less than 15, while higher perm housewraps can have perm ratings up to 80 and still provide the water protection needed.
In northern climates, the dominant strategy involves these high-perm housewraps. In southern climates, however, the wall cavity dries both to the inside and outside, with most of the drying going to the inside, thanks to air conditioning. Therefore, high-perm housewraps in southern climates should be used behind non-absorptive exterior finishes such as vinyl or cement siding.
Behind brick, stone, and stucco, a lower perm housewrap should be used. Low-perm housewraps will have a perm rating less than 15, while higher perm housewraps can have perm ratings up to 80 and still provide the water protection needed.
New flexible window flashing materials also provide a continuous barrier against moisture around windows and have sticky backings that adhere directly to housewrap materials.
By keeping building materials dry, housewrap improves building durability, decreases maintenance costs, and reduces the risk of moisture-related problems such as bugs, mold, mildew, and rot. Housewraps can also reduce air infiltration, cutting utility costs and increasing comfort in the process.
For more information visit:
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This information is taken from Building America’s Builders and Buyers Handbook for Improving New Home Efficiency, Comfort, and Durability in the Hot and Humid Climate, one of a series of handbooks that focuses on best practices for all U.S. climate zones. Building America's system design recommendations and process improvements can help all builders interested in re-engineering their home designs to achieve high performance. This best practices handbook helps builders develop durable, comfortable homes that achieve 30% energy savings in space conditioning and water heating in hot and humid climates. The handbook contains chapters for every member of the builder's team—from managers to site planners to designers, site supervisors, the trades, and marketers. There is also a chapter to help homeowners select the best systems for their new home. View the entire guide at www.eere.energy.gov.
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