Technical Updates

Posts Tagged ‘building envelope’

HVAC systems can contribute to IAQ problems in at least three ways:

• Inadequate building pressurization and dehumidification
• Intrusion of high-moisture outside air
• Inside surfaces of equipment that promotes or permits microbial growth

The HVAC system complements the building envelope by properly conditioning the building’s interior, including the building envelope, and pressurizing the building with dehumidified air (called exfiltration). When negative building pressurization occurs in humid climates, multimillion-dollar moisture and mold problems can result from intrusion and condensation of moist outside air.

HVAC systems that positively pressurize a building space by supplying unconditioned or only partially conditioned outside air will avoid infiltration of outside air through the building envelope. However, this same situation can result in moisture loads inside the building that exceed the dehumidification capabilities of the HVAC system. One of the most significant causes of moisture accumulation in existing buildings in hot, humid climates is an overemphasis on ventilation at the expense of proper dehumidification.

AC equipment is typically more efficient in cooling air than in dehumidifying it. As a result, unconditioned outside air brought into a building is often cooled to the desired temperate before it is properly dehumidified, creating elevated RH levels and microbial growth inside the building. Furthermore, because AC equipment is typically controlled by temperature (thermostat) instead of by humidity (humidistat), the equipment never senses the elevated moisture level within the building space and therefore never fully removes it.

In any climate, the normal functioning of standard AC units can result in microbial growth. Just downstream of the cooling coils, the air is at or near 100 percent RH during the cooling season. The interior surfaces of the AC unit and ductwork immediately downstream of the cooling coils are often lined with insulation, generally for acoustical purposes. Dirt and fungal spores are often trapped in the lining. This environment is conducive to microbial growth and can lead to IAQ complaints because the conditioned air (and any microorganisms it carries) is distributed inside the building.

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…

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…

Indoor air quality (IAQ) problems in facilities are overwhelmingly caused by moisture intrusion.  When moisture enters and remains in a building’s envelope and its occupied space, mold begins to grow – and so do complaints from occupants about their health and comfort.

Newly constructed buildings exhibit a disproportionate share of moisture and mold problems because significant errors made during the design, construction, or operation of a building often manifest themselves as moisture problems during the building’s first cooling season. We’ll address the errors that occur in new construction, but the concepts also apply to the remediation of problems in existing buildings.

Rather than following a topic-based organizational structure (for example, HVAC design and wall system design), these blog posts will be organized by design and construction phase. They’ll define the roles and responsibilities of the design team, the contractor, and the building owner. They’ll integrate moisture avoidance-related technology into the process of designing and constructing a building, and are oriented toward practitioners who are actively involved in the design and construction of commercial and institutional facilities. This organizational structure should make it easier for architects to solve mold-related problems, for which causes and therefore keys to prevention usually lie early in the design process.

To ensure that the recommendations in this blog do not impose undue financial or scheduling burdens on the development team, we selected only those recommendations that are most important in avoiding future moisture problems and that have proven themselves on our own projects over the past 12 years.

Decisions made daily during design and construction often occur without a full understanding of their ramifications. Beginning the process with a set of prescriptive guidelines, implementing strategic reviews at critical phases, and ending construction with sound HVAC system startup and performance verification procedures will improve architectural decision-making, and ultimately avoid future moisture-related building problems.

To be continued….