Best practices: Vapor retarders and air infiltrationbarriers
Vapor retarder overview
Editor's Note: This story is excerpted fromThe Rehab Guide: Exterior Walls -- one in a series of new guide booksproduced by the U.S. Department of Housing and Urban Development (HUD) to keepthe construction industry abreast of innovations and state-of-the-art materialsand practices in home construction & remodeling.
Vapor retarders first appeared in building construction in the 1920s. Earlytheories held that moisture vapor will migrate from a region of highconcentration towards a region of low concentration along a linear path. Theamount of moisture transfer is dependent on the differences in concentration andthe vapor permeability of the membrane separating the two regions.
This is the theory of vapor diffusion, which viewed the flow of moisturevapor directly analogous to the conductive flow of thermal energy. In thistheory, air movement, and the moisture propelled by it, were not considered tobe major factors. In the early 1950s, Canadian research found that air movementwas the primary mechanism of moisture vapor migration. Without active airinfiltration control, vapor retarder barriers become ineffective.
Current theory on vapor retarders indicates that both air infiltration anddirect diffusion play significant roles in the transfer of moisture vapor and,therefore, both must be accounted for. Effective vapor retarders must have awater vapor permeability not exceeding 1.0 grains per hour per square foot perinch of mercury vapor pressure difference (referred to as 1.0 perms), and mustbe 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 generallysettled, controversy still remains as to where to install it, if at all. Fromthis standpoint, the authority on the subject is the 1997 ASHRAE Handbook ofFundamentals, which has more to say on the topic than any of the model codes. Inwhat is defined as heating climates (4,000 heating degree days, base 65F, ormore), vapor retarders belong on the interior side of the insulation. In warm,humid, cooling climates (Florida and Gulf Coast) where moisture vapor transferconditions are effectively reversed, vapor retarders are best placed close tothe exterior.
In mixed climates (not fitting either of the above definitions), the vaporretarder should be placed to protect against the more serious condensationcondition, summer or winter. If in a mixed climate the winter indoor relativehumidity is kept below 35 percent, a vapor retarder at the interior side of theinsulation is usually not required, and an exterior vapor retarder strategy ismost effective. Where winter interior humidity is not controlled or if ahumidifier is used, an interior vapor retarder is most useful. Vapor retardersshould never be placed on both sides of a wall. Where a vapor retarder isemployed, the opposite wall surface must provide a permeable surface to allowdrying to occur. Thus, in hot, humid, cooling climates, where a vapor retarderis employed at the exterior, the interior wall surfaces should be permeable. Novapor retarder paints, kraft-faced insulation, or vinyl wall coverings should beused. 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 residentialrehabilitation is to minimize moisture vapor migration into a wall or roofassemly where it has the potential to deposit condensate when the dew point isreached. The resulting water in liquid form may cause decay in structural woodframing, wood-based sheathing materials, and interior gypsum board or plasterwall coverings. The prolonged presence of moisture will also encourage andfacilitate mold and mildew growth, raising potential serious health concerns forthe homes' occupants.
Vapor retarders can be classified into two major groups: flexible orcoatings. Metal foils, laminated foils, treated paper, and plastic films areflexible sheet goods, while paint, semi-fluid mastic, and hot melt are coatings.In typical residential construction and rehabilitation, the commonly usedmaterials are exterioror interior-applied plastic films, interior-appliedfoil-faced products, interior treated paper-faced products, and interior paintcoatings.
Vapor retarder techniques
Option: Apply a vapor retarder paint coating
A relatively new product on the market suitable for interior applications isvapor retarder paint. Produced by several manufacturers, includingSherwin-Williams and Glidden, vapor retarder paints are available as interiorlatex primers, typically with a perm rating of approximately 0.7. These primersare formulated to behave much like standard latex interior primers, in terms ofconsistency, coverage, and application. They are tintable and suitable for useover new gypsum board or previously painted surfaces. As with standard interiorprimers, normal prep work is needed, and stained areas will require astain-hiding primer prior to application. The cost per gallon of the vaporbarrier primers is generally competitive with standard interior primers.
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.
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 vaporbarriers usually take the form of kraft and foil-faced batt installation. In asituation where interior wall finish has been removed and new exterior wallinsulation is to be installed, kraft or foil-faced batts are cost-effective anddo provide an adequate to marginal vapor barrier.
The amount of unsealed edge is significant and does provide a path formoisture vapor migration. To improve effectiveness, the kraft or foil flangescan be installed over the face of the studs and lapped instead of stapled to theinner stud faces (Fig. 1). Convenient and cost-effective, kraft and foil battinsulation facings do have limitations and their use as a primary vapor barriershould be limited to applications where vapor barrier performance is notcritical, such as in mixed, non-humid climates. In heating climates with 4,000degree days or more, a more continuous vapor barrier surface should beconsidered.
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.
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 ofthese materials, clear polyethylene, is also the most economical. Available in4-, 6-, and 10-mil thicknesses, it is best suited for interior wall applicationsover 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 contentcomes at a cost: its quality can be uneven and it generally has poor tear andpuncture resistance. Clear poly should never be used for exterior applicationsor applications with more than limited exposure to sunlight. Clear poly isavailable in widths of 4 to 32 feet in 100-foot long rolls. As with allpolyethylene vapor retarders, for horizontal application over wood framing,staples are most often used. For maximum effectiveness, joints should be kept toa minimum and seams should be lapped and taped.
Relatively inexpensive and easy to install. In more severeheating climates, the use of interior polyethylene films is most effective andis practical where interior finish surfaces are removed. Being transparent,attachments to framing members are simplified, as is the installation ofwallboard material over the polyethylene, because the studs are visible.
Limited tear and puncture resistance. Clear poly must beinstalled with care to avoid damage. All penetrations such as electricaljunction boxes must be taped and sealed to ensure effectiveness. Clear poly canbe 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 additionof carbon black to the composition as a Ultraviolet inhibitor. This permits theuse of the polyethylene where some limited exposure to sunlight is required,such as at exterior wall surfaces. Black polyethylene strength characteristicsare similar to clear poly, with low tear and puncture resistance.
For exterior wall surface applications in hot, humid, coolingclimates, black UV protected poly films can provide superior vapor retarderperformance.
Limited tear and puncture resistance. Unreinforced black polymust be installed with care to avoid damage. Its opaque nature makesinstallation more difficult by obscuring underlying framing, sheathing, andother components. Joints and seams must be lapped and taped for fulleffectiveness. Installation is limited to conditions where siding has been fullyremoved and attachment directly to exterior sheathing can be made.
Option: Install a cross-laminated polyethylene orfiber-reinforced polyethylene vapor barrier
Compared with standard polyethylene, high-density cross-laminated poly andfiber-reinforced poly are both specialty products manufactured for applicationswhere higher strength is required. For retrofitting over rough, irregularsurfaces, such as solid board sheathing, both products would be less susceptableto tearing or puncture by lifted nail heads, splinters, or exposed sharp corneredges. Either product would also be appropriate where rough handling and adversesite conditions are expected.
Stronger than standard poly, reinforced and laminated materialcan withstand more adverse site conditions and rough handling. The reinforcedand laminated products are typically rated for limited UV exposure for exterioruse and situations where the installation of siding and coverings is delayed.Black reinforced and laminated poly can be used as the required weather barrierunder exterior siding and cladding.
Higher initial cost compared to standard black poly.Application is limited to conditions where siding and exterior wall coveringshave been removed. Seams must be lapped and sealed for full effectiveness.
Air infiltration barriers: Overview
Air infiltration barriers, or "housewraps," as they are known inthe industry, have grown in popularity since their appearance in the 1970s inthe wake of the energy crisis. DuPont, one of the first companies to introducesuch a product, came out with Tyvek(tm) in the late 1970s. Today there is avariety of similar products that reduce air infiltration and improve energyperformance.
The primary attribute of housewraps is their ability to operate as airinfiltration barriers while not forming an impervious vapor barrier. When placedover the exterior surface of the wall sheathing, the material allows moisturevapor to escape from the frame wall cavity while reducing convective airmovement in the insulation, thereby helping to maintain the composite R-value ofthe wall. The greater the exterior air movement, the greater the benefit.
The ten biggest selling housewrap products fall into one of two basiccategories: perforated and nonperforated. Perforated products are either wovenpolyethylene, woven polypropylene, spun bonded polypropylene, or laminatedpolypropylene film. These materials are more impervious to moisture vapormigration than nonperforated wraps, thus are provided with"micro-perforations" to allow vapor migration and diminish their vaporretarding properties. With the exception of the polyethylene films, all theperforated housewraps are further coated with either polyethylene orpolypropylene for added air infiltration resistance.
In contrast, nonperforated housewraps are either spun bonded polyethylene orfiber-mesh-reinforced polyolefin. The structure of these materials allows watervapor to pass through, while inhibiting air infiltration. In addition to theirprimary functions as air infiltration barriers and water vapor transmitters,some (but not all) of the major housewrap brands are code approved assubstitutes for required moisture protection barriers. To gain national codeapproval as a substitute for No. 15 felt, the product manufacturer must apply toeach of the three major model building codes, or CABO, and supply specifictesting data on water penetration resistance. With code recognition, the productcan be used under all siding applications, including stucco and masonry veneer.Currently, at least four products are listed by all three model codes asacceptable moisture protection barriers: Amowrap, Pinkwrap, R-Wrap, and Tyvek.Tyvek also produces a product, StuccoWrap(tm), that is specifically intended foruse with traditional and synthetic stucco, and is code listed for thatapplication. Other housewraps are acceptable to some codes as weather resistantbarriers. Before using a particular product as a weather barrier, its approvalshould be verified with the governing code.
In addition to air leakage resistance, permeance, and moisture resistance,two other material characteristics are worth considering: UV sunlightresistance, and strength. All major housewrap brands have a manufacturer's ratedUV exposure time ranging from 120 days to more than 1 year. Some products aremanufactured with antioxidants and UV stabilizers, while others are naturallymore resistant by their composition. In the field, however, covering thehousewrap as quickly as practicable is recommended, as some UV degradation willoccur even over a short period, and other unrelated damage to the membrane canbe avoided.
Strength of the housewrap can be critical, as wind conditions or adverse jobsite handling can tear or puncture the material during and after installation.Even small holes can negatively affect overall performance. The inherentstrengths of housewrap can be judged on three levels: tensile strength, tearstrength, and burst strength. Respectively, these are the material's ability towithstand damage from pulling and stretching; withstand tearing at nail andstaple locations; and to withstand separation of material fibers, fabrics, orfilms. Unfortunately, testing procedures and standards vary betweenmanufacturers, so product comparison is difficult. Generally, the spun bondedproducts have good tensile and burst strength but tear easily; woven andfiber-reinforced have good tear and burst strength, but are susceptible todiagonal tensile loading; laminated film products tend to be weakest of all andcan lose strength significantly, making a tight installation more difficult.
Although the wide variety of housewrap products with varying performancecharacteristics may appear confusing, they offer a wide selection for anyparticular job. In northern heating climates, where interior vapor barriers arethe norm, a highly moisture vapor permeable housewrap may be required. In hot,humid, cooling climates, where an interior vapor barrier is not required, ahousewrap with a low air leakage rate may be preferred. In low-windenvironments, a low-strength material may be selected. A particularlycost-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 existingsolid board sheathing, plywood, or OSB, or over new plywood or OSB where theexisting sheathing needs replacement. Housewraps come in rolls of varyingwidths, with 9 feet being the standard. Other widths are available, depending onthe manufacturer, including 1 foot 6 inches, 3 feet, and 4 feet 6 inches. Rolllengths 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 unrollthe material across the face of the sheathing for a short distance. Make surethe roll remains plumb and that the bottom edge of the housewrap extends overthe foundation by two inches. The application should start at an outside cornerextending around the starting point corner by six inches (below).
Manufacturers specify acceptable fasteners, typically large head nails, nailswith plastic washers, or large crown staples. Fastener edge and field spacingpatterns are also specified. Housewrap sheets are installed shingle-style, fromthe bottom up. Horizontal laps should be a minimum of 2 inches; vertical laps of6 inches are acceptable (above). To be fully effective in their primary role asair infiltration barriers, all seams and edges must be taped or caulked. Whilesome manufacturers market products for this purpose, others provide informationoutlining the performance requirements for approved products.
Relatively low cost, lightweight, easily installed energyconservation and moisture control product. Especially effective in mixed andnorthern heating climates where unchecked air infiltration can significantlydegrade house energy performance and occupant comfort. Beneficial in limitingairborne moisture vapor transmission into the wall cavity by limiting airmovement, while allowing moisture in the cavity to be expelled. Some productscan be used as a code-approved substitute for building felt.
Slightly more in initial cost than building felt. Availabilityof some products may be limited. Inferior performance as a weather barriercompared with building felt. Nail penetrations in housewrap are notself-sealing, as they tend to be in felts. Housewraps are not selective vaporpermeable membranes: moisture vapor will pass through in both directions. Aswater-absorptive siding materials such as wood and brick veneer dry, moisture invapor form can be forced through housewrap into sheathing and insulation. Lessvapor-permeable building felt can better withstand reverse vapor migration.
Somerecent studies appear to indicate that surfactants, a class of substances foundin wood, stucco, soap and detergents, can decrease the natural surface tensionof water and allow it to pass through housewraps wetting the underlyingmaterials. According to anectdotal field observations, this process is mostlikely to occur in regions with heavy rainfall and when unprimed wood siding isplaced in direct contact with the housewrap.
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