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Procedures for Windows & Sliding Glass Doors. DTA, Inc., Mill Valley, CA. 1995. Connolly, J. "Humidity and Building Materials" in Proceed- ings: Bugs, Mold & Rot II (W. Rose and A. TenWolde, eds). National Institute of Building Sciences, Washington DC. 1993. Lstiburek, J. and J. Carmody. "Moisture Control Handbook: New, Low-rise, Residential Construction", prepared for U. S. Dept. of Energy. 1991. Trechsel, H. (ed.) "Moisture Control in Buildings" Ameri- can Society for Testing and Materials, ASTM MNL 18, West Conshohocken, PA, 1994. Timusk, J., Seskus, A., and K. Linger. 1992. A systems approach to extend the limit of envelope performance. In Proceedings: Thermal Performance of the Exterior Enve- lopes of Buildings V. Amer. Soc. of Heating, Refrigerat- ing, and Air-Conditioning Engineers (ASHRAE), Atlanta, GA. 3. Terminology 3.1 Standard Definitions—Refer to Terminologies C 168, C 717, D 1079, and E 631 for definitions of general terms. Three definitions from C 168 are reiterated (verbatim) in 3.1.1-3.1.3. 3.1.1 vapor retarder (barrier), n—a material or system that adequately impedes the transmission of water vapor under specified conditions. 3.1.1.1 Discussion—For low-rise residential construction, materials or components with a water vapor permeance not exceeding one perm are generally considered vapor retarders (see Practice C 755). 3.1.2 water vapor permeance, n—the time rate of water vapor transmission through unit area of fiat material or construction induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humid- ity conditions. 3.1.2.1 Discussion—Permeance is a performance evaluation and not a property of a material. 3.1.3 water vapor permeability, n—the time rate of water vapor transmission through unit area of flat material of unit thickness induced by unit vapor pressure difference between two specific surfaces, under specified temperature and humid- ity conditions. 3.1.3.1 Discussion—Permeability is a property of a mate- rial. Permeability is the arithmetic product of permeance and thickness. 3.2 Other definitions found in ASTM Standards: 3.2.1 air retarder, n—a material or system in building construction that is designed and installed to reduce air leakage either into or through an opaque wall or across a ceiling. NOTE 1—Source of this definition is ASTM D 1677. 3.2.2 opaque wall, n—exposed areas of a wall that enclose conditioned space, except openings for windows, doors and building service systems. NOTE 2—Source of this definition is ASTM D 1677. 3.3 Consensus Definitions from Other Sources: The follow- ing definitions are taken verbatim from the ASHRAE Hand- book of Fundamentals (1997). 3.3.1 ventilation, n—the intentional introduction of air from the outside of a building. 3.3.2 infiltration, n—the uncontrolled flow of outdoor air into a building through cracks and other unintentional openings and through the normal use of exterior doors for entrance and egress. 3.3.3 exfiltration, n—the uncontrolled flow of indoor air out of a building through cracks and other unintentional openings and through the normal use of exterior doors for entrance and egress. 3.4 Definitions of Terms Specific to This Standard: 3.4.1 air leakage, n—infiltration or exfiltration, in other words uncontrolled air flow into or out of a building through cracks and other unintentional openings and through normal use of exterior doors for entrance and egress. 3.4.2 building component, n—an inclusive term to collec- tively refer to building materials, products, or assemblies. 3.4.3 capillary break, n—a term applied to a material, most commonly a synthetic membrane material, used to limit liquid water transfer by diffusion or capillary suction from wet ground or from a wet or damp building component to another building component that can absorb liquid water. 3.4.3.1 Discussion—Capillary breaks may also be com- posed of corrosion-resistant sheet metal, asphalt impregnated and coated felt, or where lesser degrees of resistance to capillary transfer are required, asphalt-impregnated felt. 3.4.4 critical moisture content, n—a moisture condition parameter. This parameter is expressed as a moisture content level above which immediate or virtually immediate damage will occur to a building component at a given temperature, such that the level of damage is deemed unacceptable. 3.4.5 critical cumulative exposure time, n—a moisture con- dition parameter, this parameter is expressed as a time sum when moisture conditions are above a level that results in cumulative damage to a building component, such that the level of cumulative damage is deemed unacceptable. 3.4.5.1 Discussion—cumulative damage to a component may occur over a range of moisture and temperature combi- nations, and damage is frequently more rapid at some combi- nations than at others. The differing rate of damage accumu- lation at different sets of conditions is accounted for with intensity factors, which are discussed in Chapter 26 of ASTM MNL 18. 3.4.6 durability, n—in constructions, the capacity of a building component or a construction to remain serviceable as intended with usual and customary operation and maintenance during the designed service-life under anticipated internal and external environments. 3.4.7 flashing, n—a term applied to elements, most com- monly fabricated of sheet metal, but which may also be fabricated of synthetic materials, used at interruptions and terminations of water shedding systems of roofs and walls, and intended to prevent intrusion of liquid water at these points. 3.4.8 limit, v—to keep the value or level of some parameter, which is recognized as being problematic or potentially prob- lematic, below a value or level which is deemed to be objectionable. 3.4.9 limit state, n—a value which expresses a moisture E 241 2