Technical Updates

Posts Tagged ‘vapor diffusion’

In hot, humid climates, one membrane can often act as the secondary weather barrier, air barrier, and vapor retarder. The most common of these membranes is “peel-and-stick” bituthene membrane (self-adhering composite membranes of rubberized asphalt bonded to polyethylene film) installed in masonry wall cavities or directly behind envelope finish materials, such as fiber-cement siding or stucco on lath.

In temperate climates, such condensation can easily occur in the winter, wetting the wall components. Even with low indoor RH levels, the wide temperature differential through the wall generally ensures that a first plane of condensation will be within the wall. Not only does condensation in such conditions cause mold growth, but the wetting of insulation reduces the wall’s thermal effectiveness.

Thus, the building envelope plays a vital role in minimizing uncontrolled moisture and air movement into a building and in preventing moisture entrapment within the wall. Although the building envelope contributes to moisture-related IAQ problems in hot, humid climates, infiltration of humid outside air and vapor diffusion through the envelope is not usually as great a factor in more temperate climates.

However, in temperate climates, the building envelope plays an important role in minimizing rainwater intrusion into the building, and in avoiding the subsequent mold growth that can result from such intrusion. In very cold climates, vapor diffusion or exfiltration of humid indoor air during colder months can also be a problem in wall cavities.

(To be continued…)

A five-year study of 5,000 construction claims by the Design Professional Insurance Company (DPIC) found that the most prevalent building problems – corrosion, building material degradation, and mold – were moisture-related (Engineering News-Record 1991). Moisture comes from four sources, which have different priorities depending on climate.

• Rainwater intrusion. Moisture present in building materials and on the site during construction can be a source of problems. Significant amounts of moisture can also result from water leaks within building systems or through the building envelope. In both hot, humid and temperate climates, rainwater leaks are a major source of building moisture and microbial growth problems.

• Infiltration of outside moisture-laden air. Whether introduced by wind or through the HVAC system, air infiltration can cause condensation on interior surfaces, including inside building cavities. Condensation and high RH are important factors in creating an environment conducive to mold growth and are the primary problems in hot, humid climates.

• Internally generated moisture. After construction, occupant activities and routine housekeeping procedures can generate additional moisture, contributing to the mold problem. Normally, if no other significant sources exist, well-designed and properly operating AC systems can adequately remove this moisture.

• Vapor diffusion through the building envelope. Differential vapor pressure, which can cause water vapor to diffuse through the building envelope, is a less significant cause of moisture problems in buildings. Nevertheless, it is a mechanism to consider in building design and construction, particularly in cold climates and in hot, humid climates, and especially as it relates to the construction of vapor retarders in walls.

To be continued…