Technical Updates

Posts Tagged ‘IAQ problems’

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.

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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.

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2) Interaction between the building envelope and the HVAC system – In hot, humid climates, the relationship between the building envelope and the building HVAC system is especially critical. Moisture and mold-related IAQ problems in humid climates are often misdiagnosed as caused either exclusively by envelope-related deficiencies or exclusively by HVAC-related deficiencies, because the complex relationship between the two systems is not clearly understood.

Once moisture problems occur, many investigators fail to account for the fact that, in a given cooling season, HVAC-induced moisture can equal or sometimes far exceed the amount of moisture attributable to rainwater leaks. Additionally, HVAC-induced moisture can mask or obscure rainwater leakage because it is often an envelope-wide problem. This misunderstanding can lead to misdiagnosis, which often results in expensive, unnecessary repairs to the building envelope when simply modifying the HVAC system would have been less expensive and more effective.

Building Envelope Considerations
Moisture-related IAQ problems can be avoided if the building envelope adequately retards moisture, liquid, vapor, or air movement into the building and allows any accumulated moisture to either drain to the exterior or evaporate.

In all climates, the building skin must be the primary defense against rain water and be designed to shed water quickly away from the building. Additionally, in most building envelope systems, a drainage plane and secondary barrier must be incorporated to deal with water that gets past the primary barrier. Traditional drainage planes in masonry cavity wall systems have consisted of liquid-applied waterproofing or felt paper on the face of the walls, with flashing and weep holes also installed. These walls are designed to drain water that gets through the relatively porous face brick or concrete masonry unit (CMU). Generally, if a small amount of moisture penetrates the waterproofing layer, little harm is done to the masonry.

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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.

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In the summer of 1988, construction of a large luxury resort was coming to a close. Because the vinyl wall covering on the interior side of the exterior walls had an impermeable finish, it functioned as a vapor retarder (also referred to as a vapor barrier). The HVAC system consisted of a continuous toilet exhaust and packaged terminal air-conditioner (PTAC) units. The outside air exchange rate in each guest room averaged six times an hour, all from infiltration. In this case, problems developed inside the building and inside the wall.

The combined effect of excessive outside air infiltration and an improperly located vapor retarder caused $5.5 million in moisture and mold damage, even before the facility was opened. If these same design combinations had occurred in a more temperate climate, the problems would have been limited to increased energy consumption and possibly to complaints about guest comfort.

This is one example of how hot, humid climates present unique challenges that are often overlooked by the design and construction community. However, challenges also occur for buildings located in other climates. Meeting these challenges depends on understanding a building’s local climate conditions and how those contribute to IAQ problems.

Cold climates offer challenges for moisture flow through the building envelope that are similar to those in hot, humid climes. Cold climates are defined by the American Society of Heating, Refrigerating, and Air Conditioning Engineers (ASHRAE) as those that experience at last 4,000 heating degree days (HDD at 65 degrees Fahrenheit [18 degrees Celsius] base) per year. Most problems occur during the winter, when the warm and relatively moist interior air is forced (due to high differential vapor pressures between indoors and outdoors) to the dryer and colder outdoor conditions. Moisture flow can be trapped and condensed on an improperly located vapor retarder. In addition, if the building is air-conditioned during the summer, the wall systems designed to address the heating condition can experience moisture damage inside the walls during the air-conditioned months. Therefore, few locations in the United States are completely free of potential moisture problems.

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Just months after occupying their new, multimillion-dollar government complex, employees of a Florida county began complaining of chronic sinus problems, allergy attacks, headaches, and asthma – classic signs of sick building syndrome and building-related illness. The architects, engineers, and microbiologists tasked with finding the cause of these symptoms identified a problem that is becoming widespread nationwide: severe microbial contamination of the building. Mold was growing unchecked throughout the building’s air-conditioning (AC) system and in many spaces within the building.

The mold was the direct result of excess moisture in the building, which was caused by a combination of rainwater leaks and an HVAC system that pulled moist outside air into the building during the hours when the cooling system had cycled off. Rainwater leaks caused mold growth to occur in exterior wall cavities. Once the HVAC system became infected with mold, it dispersed spores throughout the building. So, only a few years after opening its doors, the building underwent a major overhaul. The building’s exterior was removed to help correct the problems that allowed rainwater to invade the building envelope. The roof and the AC system were also extensively modified. Ultimately, repairs and other associated costs exceeded $20 million.

Unfortunately, the problem faced by this Florida county is not an isolated one. Rainwater leaks occur in every climate, and in this case, the leaks alone probably would have led to significant microbial contamination and building evacuation. However, the real devastation arose from the less obvious cause: improper interaction between the building envelope and the HVAC system.

Hot, humid, and rainy climates are particularly susceptible to interior mold problems. In these climates, new buildings are often hard hit with indoor air quality (IAQ) problems, mainly because of errors or compromises that occurred during design, construction, and initial operation. The costs of ignoring these problems are high in terms of deterioration of building structures and equipment, employee absenteeism, worker compensation claims, negative publicity, and potential liability.

Preventing IAQ problems in humid or rainy climates requires an understanding of certain key issues:

• Factors that contribute to the development of an IAQ problem
• Unique IAQ considerations for hot, humid, or rainy climates
• Reasons for failure in new buildings
• Steps that can be taken during design and construction to ensure future building success

To be continued….