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Buyers's guide to housewraps & vapor or moisture or rain barrier products: this article describes choices and properties of moisture barrier or housewrap products. Green links show where you are. © Copyright 2013 InspectAPedia.com, All Rights Reserved. Author Daniel Friedman. Building Sheathing Wrap Materials & ChoicesThis article series discusses best practices construction details for building exteriors, including water and air barriers, building flashing products & installation, wood siding material choices & installation, vinyl siding, stucco exteriors, building trim, exterior caulks and sealants, exterior building adhesives, and choices and application of exterior finishes on buildings: paints, stains. This article series includes excerpts or adaptations from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.
In addition to our main page on this topic, VAPOR BARRIERS & CONDENSATION in buildings also see VAPOR BARRIERS & HOUSEWRAP, where the need for sheathing wrap is explained, and HOUSEWRAP PRODUCT CHOICES, as well as HOUSEWRAP INSTALLATION DETAILS and HOUSEWRAP at SILLS, SOLES, TOP PLATES for details. Also see AIR SEALING STRATEGIES for details about this topic. Readers should also be sure to see AIR LEAK MINIMIZATION and also AIR BYPASS LEAKS, and be sure to read ENERGY SAVINGS RETROFIT LEAK SEALING GUIDE. See BLOWER DOORS & AIR INFILTRATION and see HEAT LOSS DETECTION TOOLS for more sophisticated and accurate methods of detecting points of un-wanted building heat loss or heat gain. At THERMAL TRACKING & HEAT LOSS we describe other visual clues that can help spot points of significant air (and heat) leakage in buildings. Contact us to suggest text changes and additions and, if you wish, to receive online listing and credit for that contribution. Installed carefully, any of the sheathing wraps can perform well and keep water out of walls. The three main choices are traditional asphalt felt, Grade D building paper, and the newer plastic housewraps. The optimal product will depend upon the siding choice, building details, and climate. With any sheathing wrap material, however, the key to good performance is to carefully lap the material to shed water. This job has been made easier by the introduction of a number of peel-and-stick membranes for use around windows, doors, and other trouble spots. General performance characteristics of sheathing wraps are summarized in Table 1-1 below. Using Asphalt Felt as Building Wrap
Once wet, its permeability jumps from around 5 to as high as 60. In the event of water leaking into the wall, asphalt felt may help store some of the water, and its high permeability when wet will promote drying to the exterior. Housewrap, in contrast, tends to trap any liquid water that gets behind it. Some contractors find felt easier to install and weave into flashings because of its rigidity and narrow roll width. Felt, however, tends to get brittle and deteriorate under long-term exposure to UV radiation [not that there is much UV radiation exposure of felt when it is located underneath building siding-DF] and is more prone to tear during installation than plastic housewraps. For situations where prolonged exposure is expected, plastic housewraps are better suited. Otherwise, asphalt building felt remains a valid choice for modern homes. Although traditional 15-pound rag felt weighed 15 pounds per 100 square feet, the material sold today as No. 15 felt is made of recycled cardboard and sawdust and actually weighs only 7 to 8 pounds per square. Most of the lightweight building paper sold has no ASTM rating. ASTM-rated No. 15 felt is either a minimum of 7.6 pounds per square (ASTM D4689) or 11.5 pounds per square (ASTM D226). Similarly, the unrated variety of No. 30 felt typically weighs only 15 to 20 pounds per square versus 26 to 27 pounds for rated Type 2 felt (ASTM D226). Using Grade D Building Paper for Building WrapGrade D building paper is an asphalt-impregnated kraft-type paper, similar to the backing on fiberglass insulation. Unlike asphalt felt, it is made from new wood pulp, rather than recycled material. Its most common use is under stucco in the western United States. The vapor permeance of Grade D paper is similar to asphalt felt. Its liquid water resistance ratings range from 20 to 60 minutes, as measured by using the boat test (see Water Resistance in "Water Resistive Barriers on Building Exterior Walls"). Because Grade D paper tends to deteriorate under prolonged wetting, the trend in three-coat stucco is to use two layers of 30-minute paper. Because the paper tends to wrinkle, the two layers tend to form a small air space, creating a rain-screen effect. Details about using roofing felt or 15 pound felt as a housewrap or vapor barrier are at FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER Guide to Buying & Using Plastic Housewrap on buildingsThere are a wide range of plastic housewraps on the market. Most are nonwoven fabrics made from either polyethylene or polypropylene. Some have perforations to let water vapor pass through and the others are designed to let water vapor diffuse through the fabric itself. Because there is no single testing standard for plastic housewrap performance, it is difficult to make apples-to-apples comparisons. However, published performance data and limited field studies suggest the following:
Guide to Choosing & Installing Draining HousewrapsIn the last few years, manufacturers have responded to the need for an air space and drainage plane with a variety of housewrap products that are either wrinkled or corrugated to provide an integrated air space. These include products intended primarily for stucco, such as DuPont’s StuccoWrap®, and others developed for siding, such as Raindrop Housewrap, which is a plastic drainage mat from Pactiv, Inc. (see “Resources,” page 47). The air space created by these products is minimal, ranging from 0.02 inch thick for StuccoWrap to 0.008 for RainDrop®. Although these materials may allow for some drainage, it is unlikely that they will provide any measurable airflow to promote drying. A more promising approach is a 1/4-inch nylon matrix, called HomeSlicker®, which has vertical drainage channels and installs between the sheathing wrap and siding. The material is rigid and thick enough to resist compression by the siding but thin enough that windows, doors, and trim can be installed without furring. Sheathing Wrap Installation Guidelines & StandardsThe primary function of the sheathing wrap, whether building felt or plastic housewrap, is to protect against water leakage. It is critical, therefore, to cover the entire shell from roof to foundation, including gable ends and band joists, and always to lap upper layers over lower layers to shed water. It is also critical to integrate the sheathing wrap with all window, door, and other wall flashings if the weather barrier is to be successful. The IRC requires asphalt felt to be minimum 14 pounds per square (ASTM D226), overlapped a minimum of 6 inches at vertical joints and 2 inches at horizontal laps. Plastic-housewrap manufacturers recommend 6 to 12 inches of overlap at vertical seams and 4 inches at horizontal laps, with all joints taped. It is good practice to wrap corners at least 6 inches each way. If the walls are sheathed and wrapped before being raised, leave a 6- to 12-inch overlap at one side of each corner, and leave a 12-inch, unstapled flap at the bottom to cover the band joist area after the sheathing is nailed off. Wide staples with a minimum 1-inch crown are recommended every 12 to 18 inches for plastic housewraps. -- Adapted with permission from Best Practices Guide to Residential Construction. More details about housewrap products, inspection, failures, and diagnosis of problems can be found at HOUSEWRAP AIR & VAPOR BARRIERS and VAPOR CONDENSATION & BUILDING SHEATHING. List of Producers of Plastic or Other Housewrap Products
Grade D Building Paper
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