Nearly 70 percent of construction litigation is related to moisture infiltration, according to a 2008 paper by University of Florida researchers. Critical to reining in these problems are building wraps. These barriers help buildings control the uncontrollable: water, air, and heat, in all of their forms. Choosing the right wrap, though, can be as tricky as predicting the weather. This piece will focus on building wraps installed on the exterior face of structural sheathing.
Terminology
Building wraps can protect sheathing from precipitation that infiltrates the exterior cladding, enable water vapor to exit the building envelope, and regulate airflow between the outdoors and indoors. They can also serves as drainage planes, directing rainwater behind the cladding away from the structure. Other names for building wraps include water-, air-, or vapor-control layers, depending on the product’s functionality, and “mechanically attached flexible membranes,” says Laverne Dalgleish, executive director of the Air Barrier Association of America, in Walpole, Mass.
Materials
Since building wraps must survive the life of a building, materials matter. Typical building wraps include asphalt-saturated felt or kraft paper, woven or nonwoven polymeric, and liquid-applied polymer-asphalt membranes. Felt wraps comprise loosely laid softwood and recycled short fibers. Since the shortness makes them brittle, felt wraps must be thicker than other building wrap materials to achieve the necessary strength. The 2018 International Building Code requires at least “one layer of No. 15 asphalt felt” in compliance with ASTM D226, Standard Specification for Asphalt-Saturated Organic Felt Used in Roofing and Waterproofing when used as a continuous water-resistive barrier behind the exterior finish, which equates to a minimum heft of 11.5 pounds per 100 square feet.
Compared to felt, kraft wrap contains less recycled fiber and more softwood fibers, which are longer and thus enable a thinner wrap that is less likely to tear. Ideal for use behind stucco, masonry, and brick walls, this paper-based product comes in varieties able to resist water for 10, 20, 30, 60, and even 150 minutes.
Woven polymeric wraps are made of crisscrossing plastic tapes coated with a waterproof film and may be micro-perforated to allow vapor to pass. Ideal for use behind fiber cement siding, wood, and vinyl, the film can also include UV inhibitors if prolonged exposure, up to about four months, is expected.
Nonwoven polymeric is made of spun-bonded polymer resins; the fibers are melted and flattened to form a fabric-like material. Nonwoven wraps can be specified with surfactant resistance to construction chemicals that might be sprayed, such as power washing solutions and detergents.
Liquid-applied wraps come in a bucket and are applied with a roller, trowel, or spray rig. They then need to cure for about four hours to form a flexible, rubbery coating.
Hybrid wraps combining two material types are also available. Since no wrap is 100% perfect, these products’ integrated backup can bring peace of mind. For example, asphalt-based wraps can be backed by a spun-bonded polymeric layer.
Application
Wraps are either mechanically attached, self-adhered, liquid- or fluid-applied, or pre-attached to a composite wall product. Always refer to the applicable building code for fastening requirements for wraps intended for mechanical attachment. An example fastening pattern could be 1-inch plastic caps, or 1-inch crowned staples, at 24 inches on center maximum.
Self-adhesive wraps can be impermeable or permeable, the latter of which may be appropriate for wood-based construction projects. Less than ideal weather conditions—wind and water—during installation can affect the seal.
Liquid-applied wraps can also be permeable or impermeable. They are the most expensive type of wraps, offering a seamless finish as well as continuous adhesion to the substrate. “If water gets through your resistant fluid-applied barrier, it can’t continue to migrate behind it,” says Andrew Dunlap, AIA, building technology studio leader at SmithGroup and president of AIA Detroit. However, any building expansion or movement might affect their longevity.
Compatibility Comparison
A building wrap must also work in concert with compatible materials, such as sheathing tape, flashing, and sealants. Sheathing tape aims to prevent water intrusion at seams and reduce wind damage to the wrap. Flashing is necessary at penetrations through the building wrap and to manage transitions between different materials. Primer can help when other chemicals are present, such as at joint and window materials. For example, Dunlap says, “If you put in a window and want to use a sealant joint, you have verify the compatibility of the wrap with the sealant joint.” To be certain of compatibility, “you have to ask the manufacturers and get a letter in writing,” Dalgleish says. “There’s no easy way around it.”
Climate Requirements
In regions experiencing four seasons, the building wrap should allow humid indoor air to escape through the wall outside while also acting as a drainage plane, Dunlap says. “Most people think that building wraps are just for keeping the water out. But often, most people are designing for that building wrap to be both a water and air barrier, and not all products will perform both functions.” Mixed climates—specifically zones 4C (marine), 5, 6, 7, and 8—require vapor barriers because otherwise the winter heating months will send interior humidity right into the wall, ceiling, or floor. Construction schedule is another factor too: most materials can stay weather tight when exposed for a 60- to 90-day window, but for any longer, Dunlap says, architects should specify products with a suitable UV exposure.
Performance Comparison
So what is the best material? Like most decisions, the answer depends on several factors: expected building life, construction budget, climate zone, and more. The most important thing is that they’ve been evaluated against a material specification, Dagliesh says. Two applicable ASTMs include ASTM E96, Standard Test Methods for Water Vapor Transmission of Materials and ASTM D779, Standard Test Method for Determining the Water Vapor Resistance of Sheet Materials in Contact with Liquid Water by the Dry Indicator Method.
