Climate Considerations to Help You Avoid a Mold & Moisture Problem
Construction of a large luxury resort located in a warm, humid climate was coming to a close during the summer. 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 both 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 (Figure 1). If these same design combinations had occurred in a more temperate climate, the problems would have been limited to increased energy consumption and possible 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 they contribute to IAQ and mold 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 ASHRAE as those that experience at least 4,000 heating degree days (HDD at 65 °F [18 °C ] 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) out to the drier 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.
Warm, Humid Climates
Hot, humid climates are also now referred to by ASHRAE as warm, humid climates. According to ASHRAE, a humid climate can be defined as one in which one or both of the following conditions occur:
A 67 °F [20 °C] or higher wet bulb temperature for 3,000 hours or more during the warmest six consecutive months of the year.
A 73 °F [23 °C] 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 (Figure 2) 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 problematic 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 adjusted 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. (See Figure 2.)
Comparing the latent and sensible loads for several major cities in different geographic regions (Peart and Cook, 1994) helps illustrate the new definition. Figure 3 shows the monthly average latent and sensible loads from outside air for Orlando, Florida; Atlanta, Georgia; and Columbus, Ohio. During the cooling season in Orlando (Figure 3A), the latent load far exceeds the sensible load of outside air. The effect of these conditions, which occur for more than half a year, is that any outside air drawn into the building envelope or occupied space will likely cause moisture accumulation and microbial growth problems. Furthermore, because this outside air is used for ventilating the building’s occupied spaces, it presents a huge dehumidification challenge for the makeup air system. Clearly, under these conditions, Orlando is highly susceptible to moisture intrusion problems.
As shown in Figure 3B, Atlanta is less susceptible to moisture intrusion problems than Orlando because, on average, the difference between sensible and latent load is small, particularly during the peak cooling months. Standard AC systems have a better chance of accounting for the latent load in Atlanta than in Orlando. Nevertheless, the latent load in Atlanta represents enough of a moisture accumulation risk that it belongs within the upper boundary of the humid zone. However, according to the ASHRAE-defined humid zone, Atlanta is outside the critical zone for humid conditions.
In the graph for Columbus (Figure 3C), the latent load from outside air is consistently less than the sensible load. The reversal of the load relationship explains why buildings in Columbus are not likely to develop moisture-related problems from outside air intrusion, because any outside air that infiltrates into buildings in Columbus will be adequately dehumidified before it is cooled.
The new definition also explains why, in certain areas of the country, building commissioning procedures are more critical than in others. For example, if the building exhaust systems are started before the AC and makeup air systems, as is typical, huge amounts of moisture may infiltrate the building, depending on the outdoor conditions.
In applying the new humid climate definition, two qualifications must be made:
As illustrated by the graphs in Figure 3, this definition is based on average climatological data. At certain times during the summer, the latent load of outside air can exceed the sensible load to a much greater extent than is reflected in the graphs. Such episodes of extreme high moisture entering the building can cause problems despite seemingly safe average conditions, and must be considered in problem prevention.
If the building envelope has an improperly located vapor retarder, moisture accumulation problems can occur, even if a favorable sensible/latent load relationship exists. Condensed moisture behind the vapor retarder will never reach the AC system for proper dehumidification but will accumulate in the wall system. Thus, architectural aspects of the building work in conjunction with outside conditions to create problems.
Typical Problems Caused by High-Moisture Conditions
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 (Figure 4). 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 interior moisture: outside air infiltration.
In areas like Charleston, 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.
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. Cold climates are just as susceptible to moisture problems as hot, humid climates, and building envelopes must be designed accordingly. 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. 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.
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 (Figure 5):
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, this 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, especially as it relates to the construction of vapor retarders in walls.
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) (Figure 6). In the hotel industry alone, fungi (mold and mildew) cause several hundreds of millions of dollars in repair costs annually (American Hotel and Motel Association, 1990). Unlike other types of indoor air contaminants, microbial growth (mold and mildew) is composed of living microorganisms. (For the purposes of this article, 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 (Figure 7) is a 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 have occurred, even a slight increase in moisture will cause the growth rate to rise exponentially (Figure 8).
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 (Figure 9) 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.
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 being 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. HVAC-induced moisture can appear to be, and be misdiagnosed as, rainwater leaks. 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. Additionally, HVAC-induced moisture can mask or obscure some rainwater leakage because HVAC-induced moisture is often an envelope-wide problem.
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 storm water and be designed to quickly shed water 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 secondary water-resistive barriers (WRBs) of fluid-applied membrane or 30-lb felt paper on the face of the interior wythe facing the cavity, with flashing and weep holes also installed. These walls are designed to drain water that either condenses in the cavity or gets through the relatively porous face brick or concrete masonry unit (CMU). Generally, if a small amount of moisture penetrates the WRB layer, little harm is done to the masonry in a multiple-wythe wall system.
With the widespread use of mold-prone, porous sheathing materials (such as exterior gypsum sheathing) and less water-tolerant wood sheathing materials (such as oriented strand board [OSB]), the selection of the WRB membrane in the drainage plane, and its interface with the flashing, require more careful thought. Even non-paper faced exterior gypsum sheathing products have their limits of water exposure.
Breaches in the WRB layer can easily result in wetting, degradation, and mold growth on the sheathing and other wall materials, including the interior drywall. Traditional WRB approaches, such as 15-lb or 30-lb felt paper, may not offer the full protection required for porous wall materials. Proprietary WRB systems that include accessories for terminations, transitions, and changes in plane offer better protection.
Failures of exterior insulation and finish systems (EIFS), first installed in the 1980s and 1990s, are now widely known. 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 synthetic 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 (usually OSB or gypsum) absorbed the water, degraded, and failed. Newer EIFS designs include drainage planes in the wall system, which reduce the likelihood of such water damage problems. However, EIFS with drainage planes or the traditional “classic” EIFS still require careful attention to detail during installation to act as a proper weather barrier and building skin.
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, 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 reach a point where it will condense. That is when moisture and microbial growth problems threaten. If the cool surfaces and moist air meet within the occupied space, or on either side of the interior drywall, then moisture problems can occur throughout the building, resulting in widespread mold odors and complaints from occupants.
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 self-adhered sheet membranes (SASM), also known as “peel-and-stick.” A common SASM product is self-adhering composite membranes of rubberized asphalt bonded to polyethylene film. That SASM product is often installed in masonry wall cavities or directly behind envelope finish materials, such as fiber-cement siding or portland cement-based plaster (stucco) on lath. There are a number of “peel-and-stick” products available, as well as fluid-applied versions. The fluid-applied versions can offer a seamless continuity.
In temperate climates, 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 also 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 problems contribute to significant moisture-related IAQ/mold 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 and subsurface water 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, leading to mold growth.
Architects need to be aware of the various functions, proper placement, interrelationships, strengths, and weaknesses of building envelope barriers: WRBs, vapor retarders, and air barriers. The efficacy of these barriers relies in such knowledge when specifying and detailing, as well as careful installation. As energy efficiency demands increase, especially in green buildings, the need for proper air barriers will also increase. Proper understanding of air barrier dynamics will be necessary to meet the goals of energy conservation without leading to unwanted moisture and mold problems.
HVAC Systems Considerations
HVAC systems can contribute to IAQ problems in at least four ways:
Improper building pressurization for the climate
Lack of adequate dehumidification capability
Intrusion of high-moisture-content outside air
Equipment surfaces that promote or permit microbial growth
The HVAC system complements the building envelope by properly conditioning the building’s interior, including the building envelope, and in humid climates 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 within the wall assembly.
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 condition 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.
Air conditioning (AC) equipment is more commonly sized for cooling air rather than dehumidifying it. As a result, unconditioned outside air brought into a building is often cooled to the desired temperature 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, 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, 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.
Misapplication of Traditional Design Principles
Design practices appropriate for temperate climates are sometimes applied in humid climates—with devastating results. For example, good design practice dictates providing certain levels of outside ventilation air in occupied spaces based on occupancy type and density. However, in hot, humid climates, the highest priority is to maintain proper humidity levels at all times—during both occupied and unoccupied periods.
In spite of the proliferation of IAQ/mold information, confusion still exists within the design community about envelope performance in hot, humid and other climates. Critical issues include: the integrity requirements of air barriers, weather barriers/WRBs, and vapor retarders; the way all three barriers/retarders can be incorporated into one membrane; the location of these features within the building envelope; the effects of using multiple vapor retarders; and even the need for air barriers and vapor retarders in every facility, especially to meet green building energy efficiency demands.
Critical to the success of building envelope barriers is clear and complete detailing as well as careful and proper installation. Usually installation in the field of the barrier materials is relatively easy. Failures usually occur at terminations of materials (especially at fenestration elements), transitions to other materials (since some barrier materials are chemically incompatible with other barrier materials), and changes in plane (since some barrier materials have difficulty with abrupt, closely-spaced corners). Sometimes 3-dimensional, sequenced, and/or exploded details are necessary to show the proper way to install barriers.
This confusion about differences in design, construction, and operational practices between humid and dry climates accounts for many moisture and microbial growth problems. Even as early as 1984, and continuing today, ASHRAE Fundamentals has cautioned that different climates present different problems and buildings should be designed and operated accordingly. Although this is an obvious requirement, it is often not met.
Misunderstanding also exists about proper building pressurization or depressurization relative to outside conditions. Even architects who appreciate the need to achieve a continuously pressurized building in warm, humid climates may not understand the level to which a building must be pressurized and how these pressures should be measured. Furthermore, the general belief is that as long as the volume of makeup air being supplied exceeds the amount of air being exhausted, the building is properly pressurized. This does not take into account the partially connected interstitial spaces and the potential for uneven distribution of makeup air within the building. Failure to account for an uneven distribution has resulted in devastating moisture intrusion problems.
The Florida Solar Energy Center (FSEC) has found that building pressures as low as +1 pascal (Pa) (1 Pascal equals 1/250 of-an-inch water column) relative to outside conditions is sufficient to prevent outside air infiltration problems. On the other hand, even a slightly depressurized building (-1 Pa relative to normal outside conditions) in warm, humid climates can develop serious moisture and microbial growth problems when the building envelope traps this moisture.
Technically, South Florida is the only part of the continental U.S. that meets the current ASHRAE definition of a hot, humid climate. However, the design requirements for warm, humid climates are generally the same as for hot, humid climates when it comes to mold and moisture prevention.
The irony of the pressurization/depressurization issue is that despite the need for great accuracy in measuring building pressurization in hot, humid climates, measuring tools and methods are surprisingly crude. Even though more advanced tools are prevalent in the industry, they have not yet gained widespread use by the day-to-day contractor. As a result, the first sign of a problem is often health complaints by the building occupants or severe degradation of building materials.