Buyers's guide to housewraps & vapor or moisture or rain barrier products: this article describes choices and properties of moisture barrier or housewrap products. Page top photo: DuPont Tyvek HomeWrap® house wrap is shown during the removal of a problematic fiber cement siding installation, courtesy Galow Homes.
This 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.
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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
The old standby, asphalt felt, has a perm rating of around 5 and moderately good water resistance, making it suitable for use as a sheathing wrap. However, unlike plastic housewraps, asphalt felt will absorb water when wet.
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 Wrap
Grade 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.
Guide to Buying & Using Plastic Housewrap on buildings
Some synethetic house wrap products have perforations to let water vapor pass through and the others are designed to let water vapor diffuse through the fabric itself.
Photo at left: two brands of housewrap (DuPont Tyvek HomeWrap® and Owens Corning PinkWrap®) are exposed during the removal of a failed fiber cement siding installation job. Photo provided courtesy of the repair contractor, Galow Homes .
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 Housewraps
In 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.
-- Adapted with permission from Best Practices Guide to Residential Construction.
Producers of Plastic or Other Housewrap Products
Grade D Building Paper
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