Best practices: Vapor retarders and air infiltration barriers
Vapor retarder overview
Editor's Note: This story is excerpted from The Rehab Guide: Exterior Walls -- one in a series of new guide books produced by the U.S. Department of Housing and Urban Development (HUD) to keep the construction industry abreast of innovations and state-of-the-art materials and practices in home construction & remodeling.
Vapor retarders first appeared in building construction in the 1920s. Early theories held that moisture vapor will migrate from a region of high concentration towards a region of low concentration along a linear path. The amount of moisture transfer is dependent on the differences in concentration and the vapor permeability of the membrane separating the two regions.
This is the theory of vapor diffusion, which viewed the flow of moisture vapor directly analogous to the conductive flow of thermal energy. In this theory, air movement, and the moisture propelled by it, were not considered to be major factors. In the early 1950s, Canadian research found that air movement was the primary mechanism of moisture vapor migration. Without active air infiltration control, vapor retarder barriers become ineffective.
Current theory on vapor retarders indicates that both air infiltration and direct diffusion play significant roles in the transfer of moisture vapor and, therefore, both must be accounted for. Effective vapor retarders must have a water vapor permeability not exceeding 1.0 grains per hour per square foot per inch of mercury vapor pressure difference (referred to as 1.0 perms), and must be installed in such a manner as to prevent air leaks at joints and laps.
Although the issue of what makes a vapor retarder effective is generally settled, controversy still remains as to where to install it, if at all. From this standpoint, the authority on the subject is the 1997 ASHRAE Handbook of Fundamentals, which has more to say on the topic than any of the model codes. In what is defined as heating climates (4,000 heating degree days, base 65F, or more), vapor retarders belong on the interior side of the insulation. In warm, humid, cooling climates (Florida and Gulf Coast) where moisture vapor transfer conditions are effectively reversed, vapor retarders are best placed close to the exterior.
In mixed climates (not fitting either of the above definitions), the vapor retarder should be placed to protect against the more serious condensation condition, summer or winter. If in a mixed climate the winter indoor relative humidity is kept below 35 percent, a vapor retarder at the interior side of the insulation is usually not required, and an exterior vapor retarder strategy is most effective. Where winter interior humidity is not controlled or if a humidifier is used, an interior vapor retarder is most useful. Vapor retarders should never be placed on both sides of a wall. Where a vapor retarder is employed, the opposite wall surface must provide a permeable surface to allow drying to occur. Thus, in hot, humid, cooling climates, where a vapor retarder is employed at the exterior, the interior wall surfaces should be permeable. No vapor retarder paints, kraft-faced insulation, or vinyl wall coverings should be used. Conversely, in northern heating climates, with interior vapor retarders, the exterior wall coverings should be vapor permeable.
The primary purpose for installing a vapor retarder in residential rehabilitation is to minimize moisture vapor migration into a wall or roof assemly where it has the potential to deposit condensate when the dew point is reached. The resulting water in liquid form may cause decay in structural wood framing, wood-based sheathing materials, and interior gypsum board or plaster wall coverings. The prolonged presence of moisture will also encourage and facilitate mold and mildew growth, raising potential serious health concerns for the homes' occupants.
Vapor retarders can be classified into two major groups: flexible or coatings. Metal foils, laminated foils, treated paper, and plastic films are flexible sheet goods, while paint, semi-fluid mastic, and hot melt are coatings. In typical residential construction and rehabilitation, the commonly used materials are exterioror interior-applied plastic films, interior-applied foil-faced products, interior treated paper-faced products, and interior paint coatings.
Vapor retarder techniques
Option: Apply a vapor retarder paint coating
A relatively new product on the market suitable for interior applications is vapor retarder paint. Produced by several manufacturers, including Sherwin-Williams and Glidden, vapor retarder paints are available as interior latex primers, typically with a perm rating of approximately 0.7. These primers are formulated to behave much like standard latex interior primers, in terms of consistency, coverage, and application. They are tintable and suitable for use over new gypsum board or previously painted surfaces. As with standard interior primers, normal prep work is needed, and stained areas will require a stain-hiding primer prior to application. The cost per gallon of the vapor barrier primers is generally competitive with standard interior primers.
- ADVANTAGES
Vapor retarder primers are the simplest application in situations where existing wallboard or plaster surfaces are not to be significantly disturbed. Where interior primers are used, the vapor retarder function comes at virtually no additional cost. Can effectively upgrade the vapor transmission performance of an exterior frame wall with no more effort and cost than a new primer and finish coat paint application. - DISADVANTAGES
Appropriate for interior wall surface applications only. With the vapor retarder at the inside surface of the wall assembly, damage to the paint can compromise retarding ability. If required prepriming prep work is inadequate, the primer coat vapor retarder effectiveness will be diminished. To be fully effective, all penetrations and material intersections at the interior surface of the wall must be caulked or otherwise sealed.
Option: Install treated paper or foil vapor retarders
For residential rehabilitation purposes, treated paper and foil vapor barriers usually take the form of kraft and foil-faced batt installation. In a situation where interior wall finish has been removed and new exterior wall insulation is to be installed, kraft or foil-faced batts are cost-effective and do provide an adequate to marginal vapor barrier.
The amount of unsealed edge is significant and does provide a path for moisture vapor migration. To improve effectiveness, the kraft or foil flanges can be installed over the face of the studs and lapped instead of stapled to the inner stud faces (Fig. 1). Convenient and cost-effective, kraft and foil batt insulation facings do have limitations and their use as a primary vapor barrier should be limited to applications where vapor barrier performance is not critical, such as in mixed, non-humid climates. In heating climates with 4,000 degree days or more, a more continuous vapor barrier surface should be considered.
- ADVANTAGES
The most cost-effective interior vapor retarder strategy where exterior wall framing is exposed and new insulation is to be installed. Saves labor costs as fiberglass batt insulation and vapor retarder are installed in one step. - DISADVANTAGES
Installation requires that walls are stripped to rough framing and that fiberglass batt insulation be installed. The number of joints and edges inherent in this system make for a functionally marginal vapor retarder, but sufficient for mixed climates or where indoor humidity is controlled in heating climates. Performance can be improved by installing faced batts with flanges attached to narrow face of studs and lapped.
Option: Install a clear polyethylene vapor retarder
Most plastic barrier films are either clear polyethylene, black polyethylene, cross-laminated polyethylene, or reinforced polyethylene. The most basic of these materials, clear polyethylene, is also the most economical. Available in 4-, 6-, and 10-mil thicknesses, it is best suited for interior wall applications over framing and insulation.
As clear poly's content is up to 80 percent "reprocessed" material, it is also an environmentally sustainable choice. The high recycled content comes at a cost: its quality can be uneven and it generally has poor tear and puncture resistance. Clear poly should never be used for exterior applications or applications with more than limited exposure to sunlight. Clear poly is available in widths of 4 to 32 feet in 100-foot long rolls. As with all polyethylene vapor retarders, for horizontal application over wood framing, staples are most often used. For maximum effectiveness, joints should be kept to a minimum and seams should be lapped and taped.
ADVANTAGES
Relatively inexpensive and easy to install. In more severe heating climates, the use of interior polyethylene films is most effective and is practical where interior finish surfaces are removed. Being transparent, attachments to framing members are simplified, as is the installation of wallboard material over the polyethylene, because the studs are visible.
DISADVANTAGES
Limited tear and puncture resistance. Clear poly must be installed with care to avoid damage. All penetrations such as electrical junction boxes must be taped and sealed to ensure effectiveness. Clear poly can be used only in instances where wall finishes and surfaces have been removed, fully exposing wall framing.
Option: Install a black polyethylene vapor retarder
Black polyethylene is nearly identical to clear poly, except for the addition of carbon black to the composition as a Ultraviolet inhibitor. This permits the use of the polyethylene where some limited exposure to sunlight is required, such as at exterior wall surfaces. Black polyethylene strength characteristics are similar to clear poly, with low tear and puncture resistance.
ADVANTAGES
For exterior wall surface applications in hot, humid, cooling climates, black UV protected poly films can provide superior vapor retarder performance.
DISADVANTAGES
Limited tear and puncture resistance. Unreinforced black poly must be installed with care to avoid damage. Its opaque nature makes installation more difficult by obscuring underlying framing, sheathing, and other components. Joints and seams must be lapped and taped for full effectiveness. Installation is limited to conditions where siding has been fully removed and attachment directly to exterior sheathing can be made.
Option: Install a cross-laminated polyethylene or fiber-reinforced polyethylene vapor barrier
Compared with standard polyethylene, high-density cross-laminated poly and fiber-reinforced poly are both specialty products manufactured for applications where higher strength is required. For retrofitting over rough, irregular surfaces, such as solid board sheathing, both products would be less susceptable to tearing or puncture by lifted nail heads, splinters, or exposed sharp corner edges. Either product would also be appropriate where rough handling and adverse site conditions are expected.
ADVANTAGES
Stronger than standard poly, reinforced and laminated material can withstand more adverse site conditions and rough handling. The reinforced and laminated products are typically rated for limited UV exposure for exterior use and situations where the installation of siding and coverings is delayed. Black reinforced and laminated poly can be used as the required weather barrier under exterior siding and cladding.
DISADVANTAGES
Higher initial cost compared to standard black poly. Application is limited to conditions where siding and exterior wall coverings have been removed. Seams must be lapped and sealed for full effectiveness.
Air infiltration barriers: Overview
Air infiltration barriers, or "housewraps," as they are known in the industry, have grown in popularity since their appearance in the 1970s in the wake of the energy crisis. DuPont, one of the first companies to introduce such a product, came out with Tyvek(tm) in the late 1970s. Today there is a variety of similar products that reduce air infiltration and improve energy performance.
The primary attribute of housewraps is their ability to operate as air infiltration barriers while not forming an impervious vapor barrier. When placed over the exterior surface of the wall sheathing, the material allows moisture vapor to escape from the frame wall cavity while reducing convective air movement in the insulation, thereby helping to maintain the composite R-value of the wall. The greater the exterior air movement, the greater the benefit.
The ten biggest selling housewrap products fall into one of two basic categories: perforated and nonperforated. Perforated products are either woven polyethylene, woven polypropylene, spun bonded polypropylene, or laminated polypropylene film. These materials are more impervious to moisture vapor migration than nonperforated wraps, thus are provided with "micro-perforations" to allow vapor migration and diminish their vapor retarding properties. With the exception of the polyethylene films, all the perforated housewraps are further coated with either polyethylene or polypropylene for added air infiltration resistance.
In contrast, nonperforated housewraps are either spun bonded polyethylene or fiber-mesh-reinforced polyolefin. The structure of these materials allows water vapor to pass through, while inhibiting air infiltration. In addition to their primary functions as air infiltration barriers and water vapor transmitters, some (but not all) of the major housewrap brands are code approved as substitutes for required moisture protection barriers. To gain national code approval as a substitute for No. 15 felt, the product manufacturer must apply to each of the three major model building codes, or CABO, and supply specific testing data on water penetration resistance. With code recognition, the product can be used under all siding applications, including stucco and masonry veneer. Currently, at least four products are listed by all three model codes as acceptable moisture protection barriers: Amowrap, Pinkwrap, R-Wrap, and Tyvek. Tyvek also produces a product, StuccoWrap(tm), that is specifically intended for use with traditional and synthetic stucco, and is code listed for that application. Other housewraps are acceptable to some codes as weather resistant barriers. Before using a particular product as a weather barrier, its approval should be verified with the governing code.
In addition to air leakage resistance, permeance, and moisture resistance, two other material characteristics are worth considering: UV sunlight resistance, and strength. All major housewrap brands have a manufacturer's rated UV exposure time ranging from 120 days to more than 1 year. Some products are manufactured with antioxidants and UV stabilizers, while others are naturally more resistant by their composition. In the field, however, covering the housewrap as quickly as practicable is recommended, as some UV degradation will occur even over a short period, and other unrelated damage to the membrane can be avoided.
Strength of the housewrap can be critical, as wind conditions or adverse job site handling can tear or puncture the material during and after installation. Even small holes can negatively affect overall performance. The inherent strengths of housewrap can be judged on three levels: tensile strength, tear strength, and burst strength. Respectively, these are the material's ability to withstand damage from pulling and stretching; withstand tearing at nail and staple locations; and to withstand separation of material fibers, fabrics, or films. Unfortunately, testing procedures and standards vary between manufacturers, so product comparison is difficult. Generally, the spun bonded products have good tensile and burst strength but tear easily; woven and fiber-reinforced have good tear and burst strength, but are susceptible to diagonal tensile loading; laminated film products tend to be weakest of all and can lose strength significantly, making a tight installation more difficult.
Although the wide variety of housewrap products with varying performance characteristics may appear confusing, they offer a wide selection for any particular job. In northern heating climates, where interior vapor barriers are the norm, a highly moisture vapor permeable housewrap may be required. In hot, humid, cooling climates, where an interior vapor barrier is not required, a housewrap with a low air leakage rate may be preferred. In low-wind environments, a low-strength material may be selected. A particularly cost-conscious choice would be laminated film.
Air infiltration techniques
Install housewrap over new or existing sheathing
For rehab applications, housewraps will generally be placed over existing solid board sheathing, plywood, or OSB, or over new plywood or OSB where the existing sheathing needs replacement. Housewraps come in rolls of varying widths, with 9 feet being the standard. Other widths are available, depending on the manufacturer, including 1 foot 6 inches, 3 feet, and 4 feet 6 inches. Roll lengths vary from 60 to 200 feet. Some custom sizes and lengths are available. Material thickness varies somewhat, but is irrelevant in terms of application. Beginning at an outside corner, hold the roll of housewrap vertically and unroll the material across the face of the sheathing for a short distance. Make sure the roll remains plumb and that the bottom edge of the housewrap extends over the foundation by two inches. The application should start at an outside corner extending around the starting point corner by six inches (below).
Manufacturers specify acceptable fasteners, typically large head nails, nails with plastic washers, or large crown staples. Fastener edge and field spacing patterns are also specified. Housewrap sheets are installed shingle-style, from the bottom up. Horizontal laps should be a minimum of 2 inches; vertical laps of 6 inches are acceptable (above). To be fully effective in their primary role as air infiltration barriers, all seams and edges must be taped or caulked. While some manufacturers market products for this purpose, others provide information outlining the performance requirements for approved products.
ADVANTAGES
Relatively low cost, lightweight, easily installed energy conservation and moisture control product. Especially effective in mixed and northern heating climates where unchecked air infiltration can significantly degrade house energy performance and occupant comfort. Beneficial in limiting airborne moisture vapor transmission into the wall cavity by limiting air movement, while allowing moisture in the cavity to be expelled. Some products can be used as a code-approved substitute for building felt.
DISADVANTAGES
Slightly more in initial cost than building felt. Availability of some products may be limited. Inferior performance as a weather barrier compared with building felt. Nail penetrations in housewrap are not self-sealing, as they tend to be in felts. Housewraps are not selective vapor permeable membranes: moisture vapor will pass through in both directions. As water-absorptive siding materials such as wood and brick veneer dry, moisture in vapor form can be forced through housewrap into sheathing and insulation. Less vapor-permeable building felt can better withstand reverse vapor migration.
Some recent studies appear to indicate that surfactants, a class of substances found in wood, stucco, soap and detergents, can decrease the natural surface tension of water and allow it to pass through housewraps wetting the underlying materials. According to anectdotal field observations, this process is most likely to occur in regions with heavy rainfall and when unprimed wood siding is placed in direct contact with the housewrap.
