Fire Resistance

The NFPA 255 Standard Method of Test of Surface Burning Characteristics of Building Materials uses the ASTM E 84 Standard Test Method for Surface Burning Characteristics of Building Materials. ASTM E 84 demonstrates the rate of flame spread as the material in question burns. Often referred to as the “Tunnel Test,” it is conducted in a tunnel-like 25-foot-long by 20-inch-wide chamber. At one end, a gas burner is situated, and a draft created to fan the flame. At the other end of the tunnel, the exhaust end, a photometer, and light source measures the amount of smoke that occurs during the test. The test is run for 10 minutes and the distance the flame spreads is measured in comparison to an established standard. A Flame Spread Index is developed, indicating the flammability of the test material compared to that of an established red oak standard. The flame spread index takes into account both the distance and time of the test material's flame spread. As an example, a test material's flame spread index of 15 would mean that it has 15 percent of the standard's flame spread. Section 803.1.1 of the International Building Code defines three classes of materials based on ASTM E 84 test results, with a combustible material required to conform to the Class A requirements—0 to 25 Flame Spread Index, and 0-450 Smoke Developed Index.

Installation: Key to an Effective House Wrap

Obviously, inadequate sealing of gaps or seams will allow moisture intrusion. Improper handling by construction crews can damage the material, and if subject to long-term exposure to the elements during the construction process, the surface of the material can be damaged through tears that should be repaired to achieve anticipated performance levels. The house wrap should be stored in a clean, dry, covered area until installation. Ideally, the period between installation of house wrap and cladding should be kept to a minimum, but premium house wraps can be left exposed for up to 180 days without compromising the house wrap. Installers should be aware of the effect of ultraviolet (UV) exposure, appreciating the potential for deterioration in products that exceed their stated UV rating.

Total integration of house wraps with all building elements is essential to performance, with proper installation around windows, doors, decks, and other complex features often posing challenges. Consequently it is far preferable to employ qualified contractors with a firm grasp of the house wrap product and its installation requirements rather than general carpenters with little or no experience with the particular material. Manufacturers typically provide detailed installation guides and sometimes field support to maximize the performance of their product.

With textured house wraps, a sharp razor knife should be used for cutting and a good quality C920 Class 25 caulk and/or flashing tape with acrylic or butyl adhesive for sealing windows, doors, or penetrations. Because of the material's draining properties, it is recommended that all flashing tapes are rolled after installation with a spline roller to ensure proper bonding. Whenever there is an installation situation in which a product could bond to the house wrap, such as stucco, or cultured stone, building paper should be used.

Installation should be started at a corner and the material installed in shingle fashion, with the fabric side facing out. The house wrap should continue beyond the band joist and extends a minimum of 2 inches below the frame to the foundation intersection. Cap nails or cap staples should be used to attach the material, with fasteners installed no more than 24 inches OC, and 4 inches away from the sides and 12 inches away from the top of any window or door openings to leave room for flashing. Sections should overlap a minimum of 6 inches for horizontal joints and 12 inches for vertical joints. The material should be installed up beyond the attic and other unoccupied spaces to the roofline and under the trim ensuring that it properly integrates with all flashing. All seams should be sealed with an acrylic adhesive seam tape. It is not advisable for house wrap to be installed over saturated OSB unless there are provisions made for drying the structure, such as leaving windows open on dry days, dry heat, or dehumidification. If a seam falls within 16 inches of an outside corner, the overlapping should be extended at least 24 inches beyond the corner.

Penetrations should be flashed with the flashing product shingled under the house wrap, with the flashing tape applied with an acrylic or butyl adhesive to the vertical edges of the flap. Any small tears in the house wrap should be repaired with an acrylic adhesive seam tape, and tears longer than 6 inches repaired by cutting a flap in the house wrap material beyond the bottom and sides of the tear. A wider piece should be inserted under the flap, shingle style and taped on each side of the flap.

Manufacturer's instructions for installation should be followed closely, and failure to do so may result in invalidation of the warranty.

House Wraps: Controlling Moisture and Air Infiltration

Protecting a structure against moisture and air intrusion is critical to achieving today's goals of sustainable building and avoiding costly call backs, repairs, and remediation. Placed between the sheathing and the siding, house wraps have a long track record in meeting those goals. Breathable, next-generation products that offer protection against air and bulk liquid intrusion with a textured surface designed to encourage water that inevitably breaches the wall cavity to drain quickly offer architects an effective tool for moisture control. With a plethora of house wrap products on the market, however, due diligence is critical in selecting a material that exceeds the requirements of the prevailing testing standards and that stands up to the demands of the construction process. It is also important to note that regardless of which product is selected, a proper installation will mean the difference between moisture control and moisture intrusion.

Endnotes

1. http://plastics.americanchemistry.com/Plastics-Energy-and-Greenhouse-Gas-Savings-Using-Housewrap
2. http://energy.gov/energysaver/articles/air-sealing-new-home-construction
3. http://ww2.frost.com/news/press-releases/dynamic-building-energy-codes-and-directives-will-drive-profits-global-housewraps-market-finds-frost-sullivan/
4. http://www.astm.org/Standards/E2178.htm

 

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