Technical Updates

Posts Tagged ‘moisture prevention’

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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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.

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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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Problems from excess moisture can be controlled if proper humidity levels are maintained in a building. (ASHRAE recommends a range between 40 and 60 percent RH.) Architects usually do not calculate or estimate quantities of moisture expected from the above sources as they design buildings. Fortunately, however, the amount of moisture from the four possible sources combined is usually insufficient to cause problems.

Microbial growth is the number one indoor air contaminant, according to a 700-building, 10-year survey (Business Council on Indoor Air 1991). In the hotel industry alone, fungi (mold and mildew) cause several hundreds of millions of dollars in repair costs annually (American Hotel and Lodging Association 1990). Unlike other types of indoor air contaminants, microbial growth (mold and mildew) is composed of living microorganisms. (For the purposes of this blog, the term mold will hereafter refer to mildew, mold, fungi, and other similar forms of microbial growth.)

ASHRAE’s moisture threshold for space conditions of 60 percent RH is commonly accepted design practice, but using RH alone as the index for microbial growth overlooks the critical interrelationships between mold growth rates, elevated RH, and ambient temperature. According to Brundrett (1990), once the threshold moisture conditions for germination of mold spores has occurred, even a slight increase in moisture will cause the growth rate to rise exponentially. Furthermore, the moisture level at which germination begins is species-specific. For example, Stachybotrys chartarum (formerly called Stachybotrys atra) requires significantly higher amounts of moisture for initial germination than many other mold species (that is, more than 90 percent RH, compared to 70 to 80 percent RH for many other species).

Understanding this difference in moisture germination requirements is especially useful in pinpointing the source of moisture in a building. For example, the high level of moisture required for Stachybotrys chartarum is usually the result of plumbing leaks or rainwater leaks through the building envelope, not just high RH.

Because of its growth characteristics, simply removing mold from affected materials and equipment will not resolve a mold problem. Mold will grow back, and the problems associated with it will reoccur. The real key is to modify the environmental conditions within the building to eliminate one or more of the five conditions required for microbial growth. The condition most easily controlled is excess moisture.

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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.

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According to ASHRAE, a humid climate can be defined as one in which one or both of the following conditions occur:

1) A 67 degrees Fahrenheit [20 degrees Celsius] or higher wet bulb temperature for 3,000 hours or more during the warmest six consecutive months of the year.

2) A 73 degrees Fahrenheit [23 degrees Celsius] or higher wet bulb temperature for 1,500 hours or more during the warmest six consecutive months of the year.

This definition is somewhat problematic. First, it is difficult to interpret and apply to problem solving. Second, high dew-point conditions can also indicate areas where moisture problems occur. Atlanta, Georgia, for example, does not qualify as a humid climate under the ASHRAE definition, but high dew points are experienced in this area and problem buildings are often found there.

Industry experience with building failures suggests the need for a new definition of humid climates that more clearly identifies the geography where problem buildings are more likely to be found, and better explains why these problems occur at all. This new definition is based on observations about latent and sensible load: A humid climate is defined as one where the average monthly latent load of outside air meets or exceeds the average monthly sensible load for any month during the cooling season. (Latent load is the moisture in outside air that is brought into the building and requires removal via dehumidification. Sensible load is the air temperature that is sensed and addressed by the HVAC system, either by heating or cooling the air, to reach the established set point.)

Infiltration of air with a high latent load will cause moisture to accumulate in building materials such as gypsum wallboard, with subsequent material degradation and mold growth. This infiltration may also exceed the ability of the HVAC system to remove moisture from the supply air. On any given day in many temperate areas, the latent load may be greater than the sensible load without causing problems; however, when these conditions persist for a longer period (a month, for example), the resulting moisture accumulation is sufficient to cause building failure.

The occurrence of a high latent load during the cooling season is a critical factor in building failure. Thus, defining hot, humid climates in terms of the relationship of sensible to latent load in ambient air expands the ASHRAE humid climate zone to include other parts of the  United States that are highly susceptible to moisture-related building failures.

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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.

To be continued…