Architects must understand air, heat, and moisture flow to achieve better air quality 
 

The images are haunting: greenish-black fuzz advancing across ceilings and pink splotches blooming on vinyl wall coverings. Hotels close for months at a time to remove unsightly and destructive invasions, juries award hundreds of thousands of dollars to plaintiffs who believe toxins from such growth have seriously compromised their health, and property insurers have begun inserting language into owners’ policies to spell out what, if any, mold damage they cover. Has mold become the asbestos of our generation?
In the sense that it has the power to generate insurance claims, fuel lawsuits, and whip up general hysteria, perhaps yes. According to Robert P. Hartwig, senior vice president of the Insurance Information Institute in New York, “U.S. insurers paid out at least $3 billion in mold-related claims in 2002, more than double the $1.3 billion paid the previous year.” Although no building type is immune from mold, single and multi-unit residential structures, hotels, and schools seem to be of greatest concern.
According to Gita Dev, FAIA, an architect in San Francisco and chair of the AIA Housing Committee, members of the industry have become increasingly worried about mold, because its presence in a building could potentially make them liable for health-related problems. “That type of exposure is almost unlimited in dollars,” says Dev, “as opposed to construction defects, for which you can figure out how much it will cost to fix.”
 
Yet Victor O. Schinnerer & Company, the Chevy Chase, Maryland–based firm that underwrites professional liability insurance for architects and engineers nationwide, hasn’t seen an explosion of mold-related claims directed at these design professionals. Vice President Lorna Parsons reports that “over time, 3 percent of our claims have involved water intrusion, which is where you get mold from, and they are still running the same to date. So mold is not handicapping the A&E profession as it goes about getting insurance and pursuing business.”
Contractors, on the other hand, have been hit hard. As a result, since about 2002, a growing number of companies that provide comprehensive general liability insurance to contractors have begun to exclude mold coverage from new policies. Frank Musica, assistant vice president at Schinnerer, says that architects may one day feel the effects of this recent change. But he also thinks that the industry’s growing sensitivity to the issue has introduced better procedures and products that will temper the severity of such a situation.
 
Also, it’s still not clear that mold is the perilous medical threat sometimes portrayed on the evening news and in courtrooms. According to Damp Indoor Spaces and Health, a report released in May by the Institute of Medicine of the National Academies in Washington, D.C., current research suggests that indoor mold can be linked to coughing, wheezing, and upper respiratory tract symptoms in otherwise healthy people; to asthma symptoms in sensitized people; and hypersensitivity pneumonitis (a relatively rare immune-mediated condition) in susceptible people. The committee that authored the report goes on to state that, at the time they reviewed the existing scientific literature, research had not yet established a clear relationship between indoor dampness in general—or indoor mold in particular—and the most severe health conditions that some have attributed to mold.
The committee is careful to state that it found neither “sufficient evidence for a causal relationship” nor “limited or suggestive evidence of no association for the various illnesses considered.” In other words, more comprehensive studies must be undertaken to make a final determination one way or the other. The authors also stress that the “conclusions are not applicable to persons with compromised immune systems, who are at risk for fungal colonization and opportunistic infections.”
 
Although the jury is still out as to the degree to which mold causes health problems in otherwise healthy people, the committee stated that excessive indoor dampness—which is conducive not just to mold but also to bacteria, dust mites, cockroaches, and other larger organisms, and to chemical and particle emissions from building materials (all of which may have medical repercussions)—does pose a risk to public health. The authors emphasize the importance of educating building professionals on the causes and prevention of moisture problems.
So while there may be some relief to building professionals that, at least for now, there may not be sufficient evidence to prove beyond a shadow of a doubt that mold causes fatigue, cancer, or several other extremely serious maladies, mold growth must still be inhibited for the general health of occupants; to avoid the unpleasant conditions associated with it, such as odors and staining; and to maintain the structural integrity of furnishings and building materials.
Biology 101
To prevent mold, architects should first understand something about the creature. Molds are a type of fungus. Fungi occupy two kingdoms of the seven cited in the current classification system of biological organisms. Unlike organisms in the animal kingdom, which digest food internally, fungi secrete enzymes into the environment to break down material into smaller components that they can then absorb. This process serves the extremely important role of cleansing and recycling elements in nature.
The fuzzy-looking part of mold is called the mycelium, which is made up of many slender cells called hyphae. Digestion occurs at the ends of the hyphae. As the mold grows, the hyphae must continue to multiply and reach further out to gather more nutrients to satisfy the ever-larger organism. Different types of molds digest different types of foods. Unfortunately for the building industry, many molds like the cellulose in wood products; through this process, they can discolor and ultimately destroy the host material.
Molds reproduce through spores, which fly through the air with the greatest of ease. If conditions at landing are right, the spores will germinate and fungal growth will begin. Molds release microbial volatile organic compounds, which cause the musty smell, and produce allergens and, under certain conditions, toxins. The allergens and toxins are not airborne themselves but can be carried in flight with the spores. It is these allergens and toxins that are the potential medical culprits.
Spores, which are microscopic, are found virtually everywhere. There is no cost-effective way of removing them from all buildings. To germinate, they need oxygen, food, an acceptable temperature, and sufficient water. The typical indoor environment provides all these factors except possibly one: Mold requires a higher quantity of moisture (in the order of 70 percent relative humidity or higher) than is comfortable to humans (20 to 60 percent relative humidity). So, the building industry’s only viable defense against mold is moisture control.
 
 
 
Water, water everywhere
Buildings get wet: Some building materials are made with water; others are rained on during construction. Roofs and windows leak. Pipes break. And moisture-laden air finds the path of least resistance. Despite such realities, too many architects and builders design and construct as if water will never enter the building.
This was not a problem years ago, when construction systems were more robust. Traditional materials, most of which are vapor permeable, installed according to traditional methods, could easily store a reasonable amount of moisture and allow it to gradually dissipate, as atmospheric or other environmental conditions changed, without damaging the building assembly. But as construction practices evolved over the 20th century, the balance of moisture and materials that we had come to take for granted began to change.
 
Modern construction systems consist of many materials that are less permeable than traditional materials and so can neither store moisture vapor nor allow it to pass. Forensic engineer Joseph Lstiburek, a principal of Building Science Corporation in Westford, Massachusetts, estimates that, on average, the water-storage capacity of materials in a typical house has decreased from about 500 gallons a century ago to about 5 gallons today. And impermeable materials placed in the wrong location—like the vinyl wallpaper that has only too often been applied on the cooler interior walls of hotel rooms in hot, humid climates—can trap moisture where it doesn’t belong.
Newer, more processed materials—such as engineered woods and paper-faced gypsum board—offer mold a smorgasbord of more easily digestible food than do the traditional lumber and plaster that they replace. “To mold, plywood is like candy and paper is pablum,” says Lstiburek. So the now-wet paper on the gypsum board behind the vinyl wallpaper in that southern hotel room provides a veritable feast for the ever-present mold spores.
And our increased reliance on air-conditioning—even in northern states—means that the temperature gradient at an exterior wall can change dramatically over the year (in northern climates, colder air may be outside in winter but inside in summer), thus altering the dewpoint, or the position at which condensation forms, and the direction of vapor flow. Although it is still true that the vapor retarder, if needed, should go on the warm side of insulation, in some climates it becomes confusing as to which is the warmer side. Properly detailed, an air-conditioned building in Minneapolis is just as likely to have mold in the summer as one in Miami.
The scale was finally tipped after the oil crisis of the 1970s. In order to reduce fuel consumption, buildings were being better insulated. Insulation, however, reduces the ability of a wall to dry out. It can also shift the dew point within the wall to a point where, if not adequately drained or vented, mold growth and other water damage can occur.
 
Around the same time, buildings were being fitted with more sophisticated, energy-efficient glazings, lighting systems, and appliances. These measures succeeded in reducing the heat load in buildings. However, because air-conditioning systems are typically oversized, “the cooling mode does not come on often enough or long enough to allow proper dehumidification,” explains Michael Garrison, associate professor of architecture at the University of Texas at Austin.
In addition, greater efforts were being made to seal buildings and maintain positive interior air pressure with respect to the outside air to prevent unwanted infiltration through inevitable small cracks in the envelope. This legitimate strategy, however, was often unwittingly compromised by localized pockets of negative pressure created, for example, by a bathroom exhaust fan or rooms that are not adequately served by fully ducted return air vents. Although the overall building may have positive pressure, these localized pockets could inadvertently draw in moist air from the outside via poorly sealed portions of the building envelope, often traveling long and circuitous distances via the complex network of relatively hollow wall cavities and plenums common in most buildings.
Meanwhile, growing concerns over indoor air quality encouraged significant air-exchange requirements that are frequently satisfied by purposely drawing outside air into the building. Unless this air is dehumidified before mixing with the already conditioned air, the extra moisture from the outside is often too much for the air-conditioning to handle, especially in hot, humid climates. If the warm moist air hits a cooler surface, such as the interior gypsum board of an air-conditioned room or the cold-water supply pipe in a ceiling plenum, the vapor from the moist air will condense and mold will form on the wallboard or on the ceiling tiles below the pipe. In its effort to save energy, the industry forgot about the properties of moisture.
 
This complex combination of multidisciplinary and sometimes competing interests and seemingly invisible forces unintentionally created a situation in which greater amounts of moisture were being trapped in inappropriate places for longer periods of time and coming into contact with more vulnerable materials, leading to mold and other health and construction concerns. And because HVAC systems provide such efficient distribution pathways, conditioned air that has become contaminated with mold has sufficient opportunity to spread the spores, odors, and potential toxins around for people to inhale, causing problems to multiply.
To make matters worse, certain standards and codes have been written and adopted that, in some regions, unintentionally exacerbate the moisture problem. According to Lstiburek, the state of Minnesota requires insulation and a vapor barrier on the interior of a basement wall, blocking the only direction in which the subterranean wall could possibly dry out; and in Miami, where the air in the attic is typically drier than the outdoors, ventilation systems originally designed to take moisture out of a humid attic in the north are inappropriately required. Lstiburek also believes that the amount of fresh air that is often brought into commercial buildings (currently stipulated at 20 cfm/person by ASHRAE Standard 62.1, “Ventilation and Acceptable Indoor Air Quality”) is too high: In hot, humid climates it actually brings too much moisture inside, thus causing the very problems that the standard intended to prevent. By stating that “some existing codes may inadvertently promote dampness,” the Institute of Medicine’s report supports Lstiburek’s contention that, at least in some regions, the moisture problem has become institutionalized.
An integrated solution
Fortunately, the conditions that lead to dampness and mold are generally understood by building scientists who study these phenomena, and strategies to prevent such problems are available (see resource table for helpful organizations and Web sites on page 178).
For architects to apply many of these strategies, it is critical that they become familiar with the natural flow patterns of air, heat, and moisture, which follow from basic laws of physics: Heat flows from warm to cold; moisture flows from warm to cold and from more to less (but, Lstiburek adds, “if these two are different, which is highly unusual, ‘more to less’ beats ‘warm to cold’ ”); air flows from higher to lower pressure; and gravity acts down.
Susan Doll, an associate at Environmental Health & Engineering, in Newton, Massachusetts, encourages architects to think about moisture dynamics: “Where does it come from—outside climate or interior activities (such as cooking, bathing, or even breathing)? And where does it go—into materials, air, or a condensation pan?”
To prevent mold from becoming a building problem, let alone a potential health menace, architects must minimize the amount of water—both in liquid and vapor form—entering a building, provide methods or details for it to be removed if and when it does enter, and specify materials appropriate to the moisture conditions. Appropriate strategies range from the obvious, such as sloping grade away from the structure and properly flashing all envelope penetrations, to the more obscure, such as thoroughly sealing the envelope with air barriers and creating consistent positive pressure throughout the building—even within wall cavities, ceiling plenums, and areas that house mechanical equipment—so outside air is not drawn in accidentally by the HVAC system.
Recognizing the multidisciplinary, multiphase, and regional variations of the moisture and mold problem, some clients who repeatedly build in different parts of the country are beginning to enlist moisture experts to monitor all facets of a project, from building envelope to mechanical systems, that affect building moisture at the various phases of design and construction. David Odom, senior consultant with Liberty Building Diagnostics Group, a building-forensics firm in Orlando, Florida, has worked with the Walt Disney Company, the U.S. Army, and now Harrah’s Entertainment, which is headquartered in Las Vegas but operates casinos around the country. As an independent consultant, Odom conducts design peer reviews that focus on building performance rather than initial cost, schedule, or constructability, which are already being considered by many other team members. He looks for problem areas and standard-of-care issues, as would an expert witness in a lawsuit. “We lead the team through that process when it is not adversarial,” says Odom. “By doing so, it forces everyone to better define how the building will likely operate once it’s constructed.”
And for those projects that don’t have the budget for this added layer of scrutiny, Gita Dev suggests that architects be more careful about which mechanical engineers they work with and pay more attention to construction administration. “The architect must be extremely demanding that details be met, and point out potential issues to the owner,” says Dev.
  The mold story may, strangely enough, have a happy ending: Armed with knowledge of mold and its prevention, architects may be in a better position to convince building owners, who are increasingly fearful of the fuzzy fungi, to spend a little more on quality design, construction, and on-site observation to prevent a future invasion of the nasty gunk. The result could lead to better quality design and construction overall.
http://archrecord.construction.com/features/green/archives/0409edit-1.asp
 
 

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