blog-3By Herbert Slone, RA, and Art Fox

This is part three of a seven-part series of blog entries about the benefits of specifying and building with manufacturer-tested and warrantied wall systems compared to specifying individual components. Benefits include a much faster design and specification process, proven component compatibility, faster component installation and better performance, plus the peace of mind that comes from knowing all components are proven compatible and will perform as specified. Part three provides a look at vapor barriers.



Vapor barriers
Vapor barriers control the rate at which moisture moves in and out so the wall can dry. Many variables go into choosing and placing the correct barrier. For example, should it be located on the warm or cold side of the cavity? Since vapor will always move into the wall from the high-vapor-pressure (moister)
side of the wall, and migrate to the low-pressure (drier) side, the rule of thumb is the barrier always goes on the high-pressure side. This generally means the barrier goes on the interior or ‘heated’ side in northern locations, and on the exterior ‘high humidity’ side in the south. In the middle states, vapor barrier placement and the question of whether one should be used are a bit ambiguous. In such situations, further hygrothermal evaluation should be done by a qualified expert—often consultants or insulation manufacturers—using tools considering climate, building materials, HVAC systems, and building function.

In addition to placement, it is equally critical to decide between high- or low-perm barriers. Part of the vapor management consideration also involves the absorptive capability of the other components in the wall itself. All building materials absorb water, reservoir it, and then release it as conditions change, so one must account for these conditions as well.

A good place to start researching vapor barriers is the International Building Code (IBC) Section 1405.3, “Vapor Retarders,” which has definitions of and perm ratings for vapor barriers. The higher a material’s perm rating, the more permeable it is to water vapor. A Class I vapor barrier is a material with a perm rating of less than 0.1, which is at the level of polyethylenes or trilaminates like foil scrim kraft materials. Class II barriers have a permeance of greater than 0.1, but less than or equal to one, which is typical of fiberglass facers like a foil or kraft paper facer. Finally, there are the Class III barriers, which include all barriers with a perm rating greater than one and less than or equal to 10, such as common wall paint.

When it comes to placing the vapor barrier, IBC says a wall with continuous insulation is more tolerant of moisture because it stays warmer; therefore, condensation inside the wall becomes less of a possibility. If the cladding is back-ventilated, as it is in a masonry cavity wall, the wall can dry faster and more completely, which influences the vapor barrier choice. Given there are so many interdependent variables, and because each building and region creates a dynamic and unique set of conditions, a hydrothermal analysis, such as can be provided by WUFI software, is often helpful.

WUFI allows realistic calculation of the transient coupled one- and two-dimensional heat and moisture transport in walls and other multilayer building components exposed to natural weather, enabling a full understanding of how all the layers of the wall perform together to manage vapor and air movement under thermal conditions that vary by hour over years.

In addition to understanding the way vapor barriers handle moisture, it is necessary to consider their flame spread ratings. Typically, steel stud/brick veneer construction is classified by IBC as Type I or II construction and its insulation must use a facer with a flame spread less than or equal to 25 when tested in accordance with ASTM.

Be sure to come back to part four of this blog to learn about through-wall flashing, mortar dropping collectors and weep vents.

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