Technical Updates

Posts Tagged ‘high-moisture problems’

With the widespread use of mold-prone, porous sheathing materials (such as exterior gypsum sheathing), the selection of the waterproofing membrane in the drainage plane and its interface with the flashing requires more careful thought. Breaches in the waterproofing layer can easily result in wetting, degradation, and mold growth on the sheathing and other wall materials, including the interior drywall.

Failures of exterior insulation and finish systems (EIFS) installed in the 1980s and 1990s have been widely reported. The early uses of this European system in the United States often failed because they relied entirely on the primary weather barrier of the synthetic stucco. When this stucco failed, often where it joined other building components such as windows, water penetrating behind the insulation could not drain out. The porous sheathing materials (most likely gypsum or oriented strand board [OSB]) absorbed the water, degraded, and failed. Newer EIFS designs require drainage planes in the wall system, which reduce the likelihood of such water drainage problems.

To control air and moisture flow through the wall, any air barrier or vapor retarder must have the proper air resistance or moisture permeability and must be installed at the correct location within the walls. The presence of multiple vapor retarders within a wall system is a common problem, and many architects do not recognize that common construction materials act as effective barriers. For example, exterior grade plywood is a relatively low-permeability material that can function as a vapor retarder.

Condensation tends to occur where cool surfaces meet warm, moist air. If moisture-laden outside air is retarded before it meets the first cool surface inside the building envelope (often called the “first plane of condensation”), then few problems will result. If this moisture is allowed to further enter a wall system, it will condense. That is when moisture and microbial growth problems threaten. If the cool surfaces and moist air meet within the occupied space, then moisture problems can occur throughout the building, resulting in widespread mold odors and complaints from occupants.

To be continued…

Shortly after construction was completed, a seven-story, four-star hotel in Charleston, South Carolina, developed severe moisture and mold problems. The investigators attributed the problems to rainwater intrusion through the hotel’s exterior brick veneer. Following that diagnosis, the hotel owner spent more than $10 million on renovations, including a completely redesigned and reconstructed building envelope.

The summer after the renovations were completed, the moisture and mold problems returned. While focusing on the envelope leaks, the investigators had overlooked the significant secondary source of moisture: outside air infiltration.

In areas like South Carolina, where hot, humid conditions persist, IAQ problems are largely due to a combination of high ambient moisture, improper interaction between the building envelope and the HVAC system, and misapplication of design and operation principles.

1) High ambient moisture – Given the high ambient moisture levels in humid climates during the summer months and the dehumidification limitations of many AC systems, excessive moisture accumulation within buildings and the resulting microbial growth are understandably major problems. Microbial-related IAQ problems in buildings can also occur in temperate climates, although more serious errors in the design, construction, or operation of a building normally must occur for such problems to develop in these areas. Cold climates are just as susceptible to moisture problems as hot, humid climates, and building envelopes must be designed accordingly. Many microbial problems in temperate climates are more commonly a result of water intrusion (rainwater and subsurface water) through breaches in the building envelope system, including subsurface envelope systems.

In all climates, anything that elevates the indoor RH or results in damp materials (leaky pipes, for example) for an extended period can cause microbial IAQ problems. Landscape irrigation systems, indoor swimming pools, and building humidification systems can provide enough moisture to create microclimates and microbial growth problems, even in dry climates. Buildings in Boise, Idaho; Denver, Colorado; and Kona, Hawaii have all been hit with severe IAQ problems from microbial growth as a result of introduced moisture, despite the fact that they are considered arid climates.

To be continued…