Building Envelope, Moisture Management, and Health
Excess moisture or dampness and mold growth in buildings have been associated with some upper respiratory symptoms (nasal congestion, sneezing, runny or itchy nose) and respiratory diseases, especially asthma, in children and adults (IOM, 2000, 2004). Asthma affects 8 to 10 percent of the population and even larger proportions of children in certain cities or in poor urban populations. It is a common cause of absence from school and from the workplace as well; 14 million days of school loss were recorded in 1994-1996, 3.4 days per child with asthma (Cox-Ganser et al., 2005).
As long as a building is properly designed, sited, constructed, operated, and maintained, excess moisture can be managed effectively. However, excess water or moisture in a building can lead to structural failures and health problems when materials stay wet long enough for microbial growth, physical deterioration, or chemical reactions to occur (IOM, 2004).
Unfortunately, moisture problems in buildings are common in all climates of the United States. It is generally accepted that more than 75 percent of all building envelope (foundation, walls, windows, roof) problems are caused by excess moisture. Moisture also significantly affects the comfort and health of occupants (Lstiburek and Carmody, 1994; Achenbach, 1994; Tye, 1994; Deal et al., 1998).
Moisture in buildings comes from both outdoor and indoor sources: rain, snowmelt, groundwater, construction materials, plumbing systems, kitchens, shower rooms, swimming pools, and wet surfaces such as mopped floors (Figure 3.1). People also bring in rain and snow on their
clothing. The effects of excess moisture are manifested by mold, mildew, rotted wood, insect infestations, spalling of masonry, condensation on surfaces, stained finishes, peeling paint, and reduced service life of materials and systems.1
Typically, current green school guidelines address moisture management issues as they relate to the siting of a building (they discourage development on wetlands or below the 100-year floodplain), the placement of drainage and irrigation systems to prevent water accumulation in or near buildings, keeping construction materials dry, and using walk-off mats and grills to prevent the buildup of snow and rain brought into a building by people. The committee believes that moisture control over a school
building’s life cycle is critical and should have a more prominent place in future green school guidelines, for reasons outlined in this chapter.
EXCESS MOISTURE AND HEALTH
Recently concerns have been raised that indoor moisture, dampness, and mold growth can lead to a variety of health problems in adults and children. Scientific research indicates that the most consistent and convincing associations relate to respiratory disease, especially asthma.
Asthma is a disorder in which the airflow is obstructed. People with asthma are subject to episodic wheezing, coughing, and shortness of breath. Although these symptoms are common clinical features of asthma, they are common symptoms of other respiratory illnesses as well. Finding a widely accepted definition of this disease has proved problematic, but the following has been offered by the Institute of Medicine as the most acceptable:
Asthma is understood to be a chronic disease of the airways characterized by an inflammatory response involving many cell types. Both genetic and environmental factors appear to play important roles in the initiation and continuation of the inflammation. Although the inflammatory response may vary from one patient to another, the symptoms are often episodic and usually include wheezing, breathlessness, chest tightness, and coughing. Symptoms may occur at any time of the day, but are more commonly seen at night. These symptoms are associated with widespread airflow obstruction that is at least partially reversible with pharmacologic agent or time. Many persons with asthma also have varying degrees of bronchial hyperresponsiveness. Research has shown that after long periods of time this inflammation may cause a gradual alteration or remodeling of the architecture of the lungs that cannot be reversed with therapy (IOM, 2000, pp. 23-24).
Indoor environments are an important factor in chronic asthma symptoms and morbidity (the incidence of disease), whether these environments are in the home, workplace, or school. The Institute of Medicine (IOM) has issued two reports on the association between excess moisture or dampness and mold growth on the one hand and respiratory illness in building occupants on the other: Clearing the Air: Asthma and Indoor Air Exposures (IOM, 2000) and Damp Indoor Spaces and Health (IOM, 2004). Both concluded that damp, moldy buildings were associated with respiratory symptoms both in people suffering from chronic asthma and in the general population. To the extent that green schools can be designed to minimize the contribution of dampness to the incidence of asthma, they can have a positive impact on human health.
The report Damp Indoor Spaces and Health considered separately the common respiratory symptoms (wheeze, cough, shortness of breath) and the diagnosis of asthma (usually based on reported physician diagnosis, reversible obstruction measured by lung function tests, or the respondent’s use of appropriate medication). In addition, the report distinguished asthma development (the appearance of asthma for the first time) from asthma exacerbations (asthma symptoms in persons with chronic asthma). The report found sufficient evidence of an association between indoor dampness and several respiratory health outcomes, including asthma. That is, an association has been observed between indoor dampness and respiratory health outcomes in studies in which chance, bias, and confounding can be ruled out with reasonable confidence. However, the evidence was not strong enough to say there was a causal relationship, i.e., that dampness directly caused the respiratory health outcomes.
There are at least two distinct variants of asthma: an extrinsic, allergic variant that occurs in the context of immunoglobulin E (IgE)-mediated sensitization to environmental allergens and an intrinsic, nonallergic variant with no detectable sensitization and low IgE concentrations. In both variants, the airways are strikingly hyperresponsive, and symptoms may also be mediated by irritant responses. In those studies that evaluated asthmatic patients for IgE-mediated sensitization, the association was stronger in sensitized individuals; thus the IOM study concluded that the association was strongest in sensitized individuals. In addition, studies of the general population consistently found an association between dampness and mold and the symptoms of cough or wheeze. Because asthma has been diagnosed in only 8 to 10 percent of the population, it was unlikely that this relationship could be accounted for in these studies by asthma alone. Thus, the 2004 IOM study concluded that moisture and mold were also associated with cough and wheeze in the general population.
Not enough studies were found to support an association between dampness and mold and the development of asthma. Nine of the 10 studies available found an association with moisture, mold, or both. Only 1 (Jaakola et al., 2002) found that the association was insignificant. Particularly important were three birth cohort studies (Belanger et al., 2003; Slezak et al., 1998; Maier et al., 1997) in which infants and children who were genetically at risk of developing asthma were observed for several years. Stark et al. (2003) reported on a birth cohort of 849 infants less than 1 year old who had at least one sister or brother with physician-diagnosed asthma; they found that wheeze and persistent cough were associated with airborne concentrations of Penicillium and Cladosporium, two types of mold commonly found in indoor air samples.
Finally, upper respiratory symptoms (nasal congestion, sneezing, runny or itchy nose) were also associated with damp indoor environ-
ments and mold. Like asthma, chronic rhinitis (inflammation of the inner lining of the nose) has allergic and nonallergic variants. The allergic variant occurs in the context of IgE-mediated sensitization to environmental allergens. In the studies included in the 2004 IOM report, upper respiratory symptoms were associated with dampness and mold in persons with self-identified allergic rhinitis as well as in the general population. Other studies reported that the frequency of “colds,” that is, of acute viral infectious rhinitis, was associated with dampness and mold. Because the cause of upper respiratory symptoms could not be identified, the committee concluded that the symptoms, but not a specific illness, were associated with dampness and mold.
The mechanisms by which damp indoor spaces and mold are associated with respiratory illness are not clear, but there are several possibilities. First, many people with asthma demonstrate IgE-mediated sensitization to mold, so the symptoms could be related to specific immune mechanisms. Mold produces a number of materials, such as peptidoglycans and polysaccharides, that induce inflammation through the innate immune pathways. Other materials such as volatile organic compounds2 and toxins may have direct effects because asthmatic airways are excessively responsive to exposures to irritants. Other organisms such as gram-negative or gram-positive bacteria might coexist with mold in damp environments; endotoxin or lipoteichoic acid from these organisms might induce airway symptoms. The interaction of moisture and mold with building materials may produce metabolites that have direct irritant effects on asthmatic airways.
The findings of key relevance to this report from Damp Indoor Spaces and Health are summarized in Box 3.1.
Health effects from excess moisture are mediated by increased indoor organisms or by deteriorated building materials that produce bioaerosols, defined as contaminants that come from living organisms and are airborne. Health effects may arise from a wide variety of mechanisms, including direct irritation of the eye or respiratory mucosa, immunologic bacteria, direct inflammation induced by toxic effects of bacterial products such as endotoxin or by VOCs, immunologic sensitization and inflammation, and the direct effects of fungal exotoxins. The multiplicity of possible mechanisms illustrates that the pathways to respiratory effects will be complex, but all of these mechanisms are plausible consequences of excess indoor moisture. Maintaining structures that are dry (i.e., without excess moisture) could prevent all of these effects.
Findings from the Institute of Medicine Relevant to This Study
Sufficient Evidence of an Association Between Dampness and Respiratory Health: Evidence is sufficient to conclude that there is an association.That is, an association between the agent and the outcome has been observed in studies in which chance, bias, and confounding can be ruled out with reasonable confidence.
Limited or Suggestive Evidence of an Association: Evidence is suggestive of an association between the agent and the outcome but is limited because chance, bias, and confounding could not be ruled out with confidence. For example, at least one high-quality study shows a positive association, but the results of other studies are inconsistent.
Sufficient Evidence of an Association Between Mold or Other Agents and Respiratory Health
Limited or Suggestive Evidence of an Association
SOURCE: IOM (2004).
BUILDING ENVELOPE AND MOISTURE MANAGEMENT
The foundation, walls, windows, and roof of a building make up an “envelope” intended to shelter people, equipment, and furnishings from the weather and from natural and manmade hazards. Windows and doors allow outside air, light, people, equipment, and supplies to enter or exit a building. Skylights allow in natural light or daylight. Building envelopes can be designed for natural ventilation, for mechanically conditioned air systems, or for some combination of these. Whether planned or not, buildings have multiple openings that allow the penetration and internal movement of air, water, and contaminants.
Building assemblies exist in a dynamic environment. Some materials have the ability to store moisture and subsequently dry without harmful affects. Excess moisture can be controlled when a healthy balance or equilibrium is maintained between the rates of entry and removal. For example, masonry construction incorporating a drain screen in the walls can provide effective moisture control through a balance of storage capacity and high drying potential. Problems with building assemblies arise only when assemblies accumulate moisture faster than their ability to store and/or dry without associated degradation of performance: Because steel framing and gypsum wallboard have virtually no storage capacity, a small leak can quickly become a large problem.
A complex set of moisture-transport processes related to climate, building design, construction, operation, and maintenance determine whether a building will have excess moisture that could influence the health of the occupants. The approach used by many building scientists to understand and diagnose moisture transport is termed “source–path–driving force” analysis: For any particular case, there is a source of moisture, a pathway moisture follows, and a force that drives moisture along that pathway. If a building designer is able to control at least one of the three elements in this chain, moisture can be effectively controlled. Controlling more than one element provides for a valuable redundancy.
Effective moisture management considers the potential damage and degree of risk associated with each of the following four transport mechanisms (from most to least potent):
Air transport, and
Bulk transport is the liquid flow of rain, snowmelt, or groundwater into a building envelope under the influence of pressure differences exerted by gravity, hydrostatic pressure, wind, or air pressure. It is the
most significant moisture transport mechanism that must be addressed by designers.
Capillarity is the wicking of liquid through the pore structure of a material (Lstiburek and Carmody, 1994; Straube, 2002). Wood, concrete, brick, and mortar are able to draw water into their porous structures in a manner similar to a sponge. Below-grade building assemblies like foundation walls, footings, and slabs are particularly sensitive because they are in contact with wet soil and standing water. Water drawn through these assemblies evaporates into the inside space, elevating interior humidity levels. Above-grade components are at capillarity risk too. Rain and splashback on exterior walls can be drawn into the envelope through capillary pathways, such as overlaps in siding, pores in wood and masonry materials, and joints between otherwise nonporous materials.
Air transport is the transfer of water vapor through the movement of air. It poses a threat roughly equal to that posed by capillarity. Air moves moisture into and through building assemblies both from within the conditioned space and from the outside (Lstiburek and Carmody, 1994; Straube, 2002; Rousseau, 2003). Water is simply one of the many gases found in air. As air moves, so does the water vapor. Airborne moisture moves under the influence of air pressure differentials created by wind, mechanical equipment, and the stack effect.3 The relationship between air transport of moisture and heating, ventilation, and air-conditioning (HVAC) systems is discussed in Chapter 4.
Vapor diffusion is the least powerful moisture transport mechanism (Lstiburek and Carmody, 1994; Straube, 2002; Achenbach, 1994) and is often confused with air transport because it too deals with invisible water vapor. The primary difference is that diffusion is the movement of water vapor through the actual structural matrix of a material, not through holes and cracks in an assembly, as is the case with air movement. Moreover, diffusion is driven by vapor pressure, not air pressure. As a result the process is slow and relatively weak when compared with air transport of water vapor. The directional vector is typically from the warmest side of the building envelope toward a colder side. The rate of diffusion is a function of the vapor permeability of a material and the driving force, vapor pressure.
Newly constructed buildings give off significant amounts of moisture during the first 2 years of use. Materials like concrete, masonry, lumber, plaster, and various surface coatings hold large quantities of water that evaporate into the indoor air.4 These “wet” assemblies can be designed to dry to the outdoors to reduce the loading of internal air. Alternatively, the moisture contained in the indoor air can be diluted through ventilation or dehumidification (Lstiburek and Carmody, 1994).
SOLUTIONS/DESIGN REQUIREMENTS FOR MOISTURE MANAGEMENT
Designing for moisture management is complicated. Architects must incorporate design features to control the entry of large amounts of rain and ground water from the outside, block capillary transport through and within the structure, and prevent excessive transport of water vapor through air movement and vapor diffusion. Compounding the challenge, designers must concurrently provide a healthy indoor environment, minimize energy use, and control construction costs (Lstiburek and Carmody, 1994; Achenbach, 1994; Powell, 1994). In addition, moisture control must be placed in the context of structural design, operation, maintenance, and use of the building as it relates to external conditions including climate, soil, and microclimates (IOM, 2004; Powell, 1994; Lstiburek and Carmody, 1994).
A central role for the above-grade building envelope is to keep rainwater out. If rainwater is allowed to penetrate the envelope, it will overwhelm the impact of all other interior moisture sources—for example, moisture from occupants, plants, bathrooms, kitchens, and evaporation from wet surfaces (Christian, 1994). Architects can provide for redundant layers of protection, include a drainage plane within the envelope, and attempt to control air pressure differentials across the exterior cladding with a vented rain screen (Lstiburek and Carmody, 1994).
Water originating above-grade from rain, surface runoff, or snowmelt is typically drawn down through the soil by gravity. Migrating surface water and subsurface water from a high water table can enter below-grade building envelopes through the cracks, joints, holes, or pores in a structure. Subsurface transport is driven by hydrostatic pressure, gravity, and/or capillary transport (Lstiburek and Carmody, 1994; Christian, 1994; Straube, 2002). Proper site selection, the use of appropriate drainage systems, and application of effective barrier membranes are the most successful bulk-moisture control strategies for below-grade assemblies.
In general, the easiest way to control capillary transport is to reduce the availability of moisture, seal the pores, make the pores too large to support capillarity and/or provide a receptor (Lstiburek and Carmody, 1994). Capillary transfer of below-grade moisture into concrete and masonry walls and floors can be controlled by the use of damp proofing, waterproofing membranes, and non-capillary-conducting drainage materials (Lstiburek and Carmody, 1994; ORNL, 1988; Tye, 1994).
One large source of moisture in building enclosures is the migration of moisture from the surrounding soil into foundations and basements. This moisture ultimately moves by air transport when wet surfaces evaporate into the air of conditioned spaces (Christian, 1994). Blocking this liquid source from entry before it becomes an airborne problem is a smart design choice. Installation of air-barrier systems at the interior and/or exterior surfaces of a building shell can help to reduce the amount of air and airborne moisture transported.
Moist air also leaks into basements through floor and slab perimeters, wall joints, cracks, windows, and around drains. Winter stack forces can act to depressurize the basement area, drawing moist air into the basement through these pathways. Moist air is subsequently directed to the upper regions of a structure. Since below-grade moisture is one of the most powerful sources of moisture in conditioned spaces, building dry foundation assemblies can be one of the most effective strategies used to control interior airborne moisture (Lstiburek and Carmody, 1994; ORNL, 1988; Christian, 1994).
The easiest way to control diffusion is by installing vapor impermeable materials on the side of the assembly with the highest vapor pressure (Rousseau, 2003). Building codes consider materials with a permeability rating of ≤1 to be vapor barriers. As a general rule, designers should position vapor barriers toward the inside surface in heating-dominated climates and toward the outside surface in cooling-dominated climates. Many building codes and architectural standards require seams and holes in vapor barriers to be sealed to form a continuous, uninterrupted line of protection. However, effectiveness depends on a material’s vapor permeability and surface area covered. In other words, if 95 percent of an envelope surface is covered with a vapor barrier, the barrier is 95 percent effective as a vapor diffusion retarder (Lstiburek and Carmody, 1994). This relationship provides designers and builders with some leeway, suggesting as it does that diffusion barriers need not be installed perfectly. However, in order for air barriers to be effective, they must be continuous and durable.
In some cases diffusion can drive moisture through an envelope from the inside and from the outside. A structure built in a locality where a balanced mix of heating and cooling is required is one example. A foundation
assembly where significant amounts of moisture exist inside and outside the envelope is another example of a situation where moisture can diffuse in both directions. And porous claddings like masonry and wood that become rain soaked and subsequently exposed to solar radiation give rise to vapor driving inward, even in a heating-dominated climate (Lstiburek and Carmody, 1994). This type of diffusion will affect the design of vapor barriers. Buildings should be designed such that moisture can dry toward both the inside and the outside.
Box 3.2 lists design measures that could be incorporated into green school guidelines to ensure appropriate moisture management as it relates to a building’s envelope. Excellent resources for proper moisture control design include The Moisture Control Handbook: Principles and Practices for Residential and Small Commercial Buildings by Joseph Lstiburek and John
Design Strategies for Moisture Control
Carmody (1994), The Building Foundation Design Handbook (ORNL, 1988), and Moisture Control in Buildings (ASTM, 1994). Building commissioning can also be an effective way to identify and preempt potential moisture problems in schools. Building commissioning is discussed in greater detail in Chapter 9.
In addition to bringing potential health benefits, designing for effective moisture management will probably have benefits for the building itself. The more durable a building is, the longer its components will last (Deal et al., 1998). Materials in long-lived building assemblies are replaced less frequently than those in nondurable structures. This makes dry structures resource- and energy-efficient, because no replacement materials need be harvested, mined, or produced, nor is energy used to make, transport, or assemble the replacement components. Dry buildings also require fewer
resources and money for repair and maintenance. For example, damp surfaces cause stains and peeling paint, which necessitate frequent repainting and cleaning. For these reasons, dry buildings may have lower life-cycle costs, in addition to offering potential health benefits for occupants.
More research is needed on the moisture resistance and durability of materials used in school construction. Such research should also investigate other properties of these materials, such as their generation of bioaerosols and indoor pollutants as well as the environmental impacts of producing and disposing of them.
CURRENT GREEN SCHOOL GUIDELINES
Green school guidelines and standards endeavor to prevent rain and snow from entering a building in a number of ways. These include discouraging development on sites below the 100-year flood plain or in wetlands; keeping site irrigation to a minimum; designing all drainage systems and HVAC condensate drainage systems to prevent the accumulation of water under, in, or near buildings; using walk-off grills and mats to prevent the buildup of moisture from rain and snow brought in by occupants; keeping all construction materials dry and well ventilated; discarding materials that have been wet for more than 24 hours; and requiring a maintenance plan that specifies the staff time and materials that will be dedicated to HVAC, plumbing, and roof systems.
FINDINGS AND RECOMMENDATIONS
Finding 3a: There is sufficient scientific evidence to establish an association between excess moisture, dampness, and mold in buildings and adverse health outcomes, particularly asthma and respiratory symptoms, among children and adults.
Finding 3b: Excess moisture in buildings can lead to structural damage, degraded performance of building systems and components, and cosmetic damage, all of which may result in increased maintenance and repair costs.
Finding 3c: Well-designed, -constructed, and -maintained building envelopes are critical to the control and prevention of excess moisture and molds. Designing for effective moisture management may also have benefits for the building, such as lower life-cycle costs.
Finding 3d: Current green school guidelines typically do not adequately address the design detailing, construction, and long-term maintenance of
buildings to ensure that excess moisture is controlled and a building is kept dry during its service life.
Recommendation 3a: Future green school guidelines should emphasize the control of excess moisture, dampness, and mold to protect the health of children and adults in schools and to protect a building’s structural integrity. Such guidelines should specifically address moisture control as it relates to the design, construction, operation, and maintenance of a school building’s envelope (foundations, walls, windows, and roofs) and related items such as siting and landscaping.
Recommendation 3b: Research should be conducted on the moisture resistance and durability of materials used in school construction. Such research should also investigate other properties of these materials such as the generation of bioaerosols and indoor pollutants as well as the environmental impacts of producing and disposing of these materials.