Fluid Applied Air/Moisture Barriers for Moisture Control and Mold Prevention in Wall Construction

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p class="mainboldBlue">Water Penetration Resistance

The traditional moisture protection used in wall construction is asphalt-saturated felt or kraft waterproof building paper. The terms weather-resistive barrier or moisture barrier are often used to describe these components in wall construction. They are generally installed over sheathing by lapping them shingle-style and fastening with nails, screws or staples to the sheathing. Their general purpose in walls is to protect against ingress of incidental water into the building and to protect moisture-sensitive components like gypsum sheathing in the event of a breach in the outer wall covering, such as a crack in stucco. Building wraps are often used in place of asphalt felt in wall construction, often with the same perceived purpose. The water resistance, air infiltration resistance, and vapor permeability characteristics of building wraps vary widely, depending on the brand of wrap selected. (See references, PHRC Report No. 59). Seamless fluid applied moisture protection provides a significant improvement over traditional moisture protection and building wraps.In fact, they can be 10 times more resistant to water penetration than building wraps and nearly 200 times more resistant to air leakage than asphalt felt .

Water Vapor Permeability

A fluid applied air/moisture barrier may or may not be a vapor-retarding material. The generally accepted definition of a vapor-retarding material is one that has a water vapor permeance of 57.4 ng/(Pa·s·m2) [1.0 perms] or less. In Table 1, the fluid applied air/moisture barrier components are not vapor retarders. The joint treatment has a vapor permeance of 994 ng/(Pa·s·m2) [17.3 perms] and the waterproof coating has a vapor permeance of 327 ng/(Pa·s·m2) [5.7 perms], about the same as Type 15 building felt.

Building Material Water Vapor
Permeance
(Perma)
Water Vapor
Permeance
ng(Pa-s-m2)
4 mil Polyethylene 0.08 4.60
6 mm (1/4 inch) Plywood3 (ext glue) 0.7 40.2
101mm (4 inch) Brick3 0.8 46.0
203mm (8 inch) Concrete Block3 2.4 138
25mm (1 inch) Expanded Polystyrene1 5 287
Type 15 Building Felt2 5.6 322
Fluid Applied Air Moisture Barrier Waterproof Coating 5.7 327
19mm (3/4 inch) Plaster on Metal Lath3 15 862
Fluid Applied Air Moisture Barrier Joint Treatment 17.3 994
9.5mm (3/8 inch) Gypsum Wallboard3 50 2873
Table 1: Water vapor permeance of fluid applied air/moisture barrier materials
and common building materials. Check online material for Table 1 notes.

The purpose of a vapor retarder in wall construction is to minimize water vapor diffusion through the wall assembly and thus reduce the risk and the amount of condensation on cold surfaces in the wall assembly. Whether or not a vapor retarder should be placed in a wall assembly and where it should be placed must be carefully evaluated in relation to climate, the physical characteristics of other components of the wall assembly, and interior relative humidity conditions. In cold climates the predominant water vapor diffusion direction through most of the year is from the inside to the outside, as warm, humid air from the interior environment moves in the direction of cold, dry outside air. Conversely, in hot, humid climates, the predominant water vapor diffusion direction through most of the year is from the warm, humid outside environment towards the cooler, dryer, air-conditioned interior environment. Based on these general conditions, a vapor retarder is customarily placed on the interior of wall construction in cold climates and on the exterior in hot, humid climates. A vapor retarder should not be placed on the interior in hot, humid climates, since it will potentially cause condensation by restricting vapor diffusion to the interior. The use of interior vapor retarders has been shown to be a contributing cause in many cases of moisture problems and IAQ problems in buildings in hot, humid climates. One tool that is available to assist in making decisions about whether a vapor retarder is needed and where to place it in the wall assembly is a water vapor transmission analysis that can be performed manually (see ASHRAE Handbook-Fundamentals, chapters 21 and 22) or by computer (Trechsel, Moisture Analysis and Condensation Control in Building Envelopes).

 

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Originally published in Architectural Record.
Originally published in October 2005

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