Energy Vapor

[Charise Farag]

Inmost U.S. climates, vapor barriers, or -- more accurately -- vapor diffusion retarders (vapor retarders), should be part of a moisture control strategy for a home. A vapor retarder is a material that reduces the rate at which water vapor can move through a material. The older term "vapor barrier" is still used even though "vapor retarder" is more accurate.

The ability of a material to retard the diffusion of water vapor is measured in units known as "perms" or permeability. The International Residential Code describes three classes of water vapor retarders:

Effective moisture control in these areas and throughout a home must also include air-sealing gaps in the structure, not just the use of a vapor retarder. How, where, and whether you need a vapor retarder depends on the climate and the construction of your home.

Vapor retarders are typically available as membranes or coatings. Membranes are generally thin, flexible materials, but also include thicker sheet materials sometimes called "structural" vapor retarders. Materials such as rigid foam insulation, reinforced plastics, aluminum, and stainless steel are relatively resistant to water vapor diffusion. These types of vapor retarders are usually mechanically fastened and sealed at the joints.

Thinner membrane types come in rolls or as integral parts of building materials. Common examples include polyethylene sheeting and aluminum- or paper-faced fiberglass roll insulation. Another type is foil-backed wallboard. Most paint-like coatings also retard vapor diffusion.

In mild climates, materials like painted gypsum wallboard and plaster wall coatings may be enough to impede moisture diffusion. In more extreme climates, higher-perm vapor diffusion retarders are advisable for new construction. They perform best when installed closest to the warm side of a structural assembly -- toward the interior of the building in cold climates and toward the exterior in hot/wet climates.

Vapor retarder installation should be continuous and as close to perfect as possible. This is especially important in very cold climates and in hot and humid climates. Be sure to completely seal any tears, openings, or punctures that may occur during construction. Cover all appropriate surfaces or you risk moist air condensing within the cavity, which could lead to dampened insulation. The thermal resistance of wet insulation is dramatically decreased, and prolonged wet conditions will encourage mold and wood rot.

Except for extensive remodeling projects, it's difficult to add materials like sheet plastic as a vapor retarder to an existing home. Obtaining an energy assessment and thoroughly sealing any leaks it reveals is are very effective for slowing moisture movement in and out of your home.

Your home may not need a more effective vapor retarder than the numerous layers of paint on its walls and ceilings unless you live in extreme northern climates. "Vapor barrier" paints can be an effective option for existing homes in colder climates. If the perm rating of the paint is not indicated on the label, find the paint formula. The paint formula usually indicates the percent of pigment. To be a good vapor retarder, it should consist of a relatively high percent of solids and thickness in application. Glossy paints are generally more effective vapor retarders than flat paints, and acrylic paints are generally better than latex paints. When in doubt, apply more coats of paint. It's best to use paint labeled as a vapor diffusion retarder and follow the directions for applying it.

An air barrier/vapor retarder attempts to accomplish water vapor diffusion and air movement control with one material. This type of material is most appropriate for southern climates where keeping humid outdoor air from entering the building cavities is critical during the cooling season.

Water resistive barriers are generally placed around the perimeter of the building just under the exterior finish, or they may actually be the exterior finish. The key to making them work effectively is to permanently and carefully seal all of the seams and penetrations, including around windows, doors, electrical outlets, plumbing stacks, and vent fans.

Missed gaps of any size not only increase energy use, but also increase the risk of moisture damage to the house, especially during the cooling season. A water resistive barrier should also be carefully inspected after installation before it is covered by other work. If small holes are found, they may be repaired with caulk or polyethylene or foil tape. Areas with larger holes or tears should be removed and replaced. Patches should always be large enough to cover the damage and overlap any adjacent wood framing.

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Recent molecular dynamics simulations of the growth of [Ni0.8Fe0.2/Au] multilayers have revealed the formation of misfit-strain-reducing dislocation structures very similar to those observed experimentally. Here we report similar simulations showing the formation of edge dislocations near the interfaces of vapor-deposited (111) [NiFe/CoFe/Cu] multilayers. Unlike misfit dislocations that accommodate lattice mismatch, the dislocation structures observed here increase the mismatch strain energy. Stop-action observations of the dynamically evolving atomic structures indicate that during deposition on the (111) surface of a fcc lattice, adatoms may occupy either fcc sites or hcp sites. This results in the random formation of fcc and hcp domains, with dislocations at the domain boundaries. These dislocations enable atoms to undergo a shift from fcc to hcp sites, or vice versa. These shifts lead to missing atoms, and therefore a later deposited layer can have missing planes compared to a previously deposited layer. This dislocation formation mechanism can create tensile stress in fcc films. The probability that such dislocations are formed was found to quickly diminish under energetic deposition conditions.

FLUXNET is a global network of micrometeorological flux measurement sites that measure the exchanges of carbon dioxide, water vapor, and energy between the biosphere and atmosphere. At present over 140 sites are operating on a long-term and continuous basis. Vegetation under study includes temperate conifer and broadleaved (deciduous and evergreen) forests, tropical and boreal forests, crops, grasslands, chaparral, wetlands, and tundra. Sites exist on five continents and their latitudinal distribution ranges from 70N to 30S.

FLUXNET has several primary functions. First, it provides infrastructure for compiling, archiving, and distributing carbon, water, and energy flux measurement, and meteorological, plant, and soil data to the science community. (Data and site information are available online at the FLUXNET Web site, -eosdis.ornl.gov/FLUXNET/.) Second, the project supports calibration and flux intercomparison activities. This activity ensures that data from the regional networks are intercomparable. And third, FLUXNET supports the synthesis, discussion, and communication of ideas and data by supporting project scientists, workshops, and visiting scientists. The overarching goal is to provide information for validating computations of net primary productivity, evaporation, and energy absorption that are being generated by sensors mounted on the NASA Terra satellite.

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