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condition parameter, generally a critical moisture content or a critical cumulative exposure time, that is deemed to be at the border of what is acceptable, and beyond which an unaccept- able level of damage to a building component may be expected. 3.4.10 perm, n—the time rate of water vapor migration by diffusion through a material or component equal to 1 grain per hour, square foot, inch of mercury vapor pressure difference. In SI units, one perm is 57.2 ng/(Pabsbm 2 ). 3.4.11 serviceability, n—in a construction, the capacity of a building component or a construction to perform the func- tion(s) for which it was designed and constructed. 3.4.12 water or moisture, n—water as liquid, vapor, or solid (ice, frost, or snow) in any combination or in transition. 4. Significance and Use 4.1 Moisture degradation is frequently a significant factor that either limits the useful life of a building or necessitates costly repairs. Examples of moisture degradation include: 1) decay of wood-based materials, 2) spalling of masonry caused by freeze-thaw cycles, 3) damage to gypsum plasters by dissolution, 4) corrosion of metals, 5) damage due to expansion of materials or components (by swelling due to moisture pickup, or by expansion due to corrosion, hydration, or delayed ettringite formation), 6) spalling and degradation caused by salt migration, 7) failure of finishes and 8) creep deformation and reduction in strength or stiffness. 4.1.1 Moisture accumulation within construction compo- nents or constructions may adversely affect serviceability of a building, without necessarily causing immediate and serious degradation of the construction components. Examples of such serviceability issues are: 1) indoor air quality, 2) electrical safety, 3) degradation of thermal performance of insulations and 4) decline in physical appearance. Mold or mildew growth can influence indoor air quality and physical appearance. With some components, in particular interior surface finishes, mold or mildew growth may limit service life of the component. Moisture conditions that affect serviceability issues can fre- quently be expected, unless corrected, to eventually result in degradation of the building or its components. This guide does not attempt however to address serviceability issues that could be corrected by cleaning and change in building operation, and that would not require repair or replacement of components to return the building (or portions or components of the building) to serviceability. 4.2 Prevention of water-induced damage must be consid- ered throughout the construction process including the various stages of the design process, construction, and building com- missioning. It must also be considered in building operation and maintenance, and when the building is renovated, rehabili- tated or undergoes a change in use. 4.3 This guide is intended to alert designers and builders, and also building owners and managers, to potential damages that may be induced by water, regardless of its source. This guide discusses moisture sources and moisture migration. Limit states (or specific moisture conditions that are likely to impact construction or component durability), and design methods are also cursorily discussed. Examples of practices that enhance durability are listed and discussed, as are ex- amples of constructions or circumstances to avoid. The ex- amples listed are not all-inclusive. Lastly, field check lists are given. The checklists are not intended for use as is, but as guides for development of checklists which may vary with specific building designs and climates. 5. Moisture Sources and Migration 5.1 Moisture sources for buildings can be broadly classified as follows: (1) surface runoff of precipitation from land areas, (2) ground water or wet soil, (3) precipitation or irrigation water that falls on the building, (4) indoor humidity, (5) outdoor humidity, (6) moisture from use of wet building materials or construction under wet conditions and (7) errors, accidents and maintenance problems associated with indoor plumbing. At a given instant of time the categories are distinct from each other. Water can change phase and can be trans- ported over space by various mechanisms. Water may therefore be expected to move between categories over time, blurring the distinctions between categories. Chapter 8 of ASTM MNL 18 provides quantitative estimates of potential moisture load from various sources. 5.1.1 High indoor humidity during winter is often a major cause of moisture problems in cold or temperate climates. Moisture-induced damage may be expected unless the building is designed to tolerate the levels of indoor humidity that occur in use. Conversely, moisture induced damage may be expected unless indoor humidity is kept within limits that the building will tolerate. Buildings should be designed and built so as to tolerate indoor humidity levels commensurate with their in- tended use. For some buildings, (for example: those intended for habitation by persons with certain medical conditions or those housing swimming pools or textile production equip- ment), the levels of indoor humidity which the building should be expected to tolerate are moderately high, even if the building is located in a cold climate. Conversely however, most buildings are not designed nor built to tolerate high indoor humidities during winter. It is therefore unreasonable to expect such buildings to perform adequately if operated at high indoor humidities during winter. 5.1.1.1 The potential for indoor humidity to cause damage depends on the local climate. Occupant density, that is number of occupants per given unit of space, and occupant activities frequently have a large influence on indoor humidity levels. Among occupant activities that influence indoor humidity, cooking, bathing and laundry activities, and use of unvented combustion appliances are those most likely to be significant. Air exchange between the living space and the exterior can significantly lower indoor humidity levels during winter in temperate climates. Control of indoor humidity is discussed in greater detail in 8.3 and its subsections. 5.1.1.2 Mathematical evaluation tools (see 7.1.2 and 7.1.3) can be used to identify if a given building design in a given climate will tolerate a given level of indoor humidity, or alternatively, to estimate tolerable indoor relative humidities for a given building design and climate. 5.1.2 Although use of dry building materials is preferable, wet building materials are commonly used. With some building materials (for example cast-in-place concrete) a wet initial condition is an inherent characteristic of the material, and thus E 241 3