A vapor retarder placed in an appropriate location, however, can control such condensation. For many years NRCA has maintained that vapor retarders should be considered when both of the following conditions occur: the outside average January temperature is below 40
F (4
C), and the expected indoor winter relative humidity is 45% or greater. However, these are very simple guidelines. Both the American Society of Heating, Refrigeration and Air Conditioning Engineers (ASHRAE) and the U.S. Army Corps of Engineers Cold Regions Research and Engineering Laboratory (CRREL) have developed recognized practices for determining the need for a vapor retarder in a roof system. These methods differ, and designers should choose the methodology which they deem most applicable for a given project. Situations more likely to require the inclusion of a vapor retarder are those where interior conditions of high humidity exist, such as in textile mills, laundries, canning factories, creameries, breweries, and indoor pools. Perm Ratings. The effectiveness of a vapor retarder is measured by its perm rating, which is a measure of porosity of material to passage of water vapor. Perm ratings are established by ASTM procedures. To be classified as a vapor retarder, the material should have a perm rating between 0.00 and 0.50 perms. A perm rating for a material is the number of grains of water vapor (7000 grains equal 1 lb) that will pass through 1 ft2 of the material in 1 hr when the vaporpressure differential between the two sides of the material equals 1 in of mercury

Fire-retardant chemicals may be impregnated in wood with recommended retentions to lower the rate of surface flame spread. After proper surface preparation, the surface is paintable. Such treatments are accepted under several specifications, including federal and military. They are recommended only for interior or dry-use service conditions or locations protected against leaching. These treatments are sometimes used to meet a specific flame-spread rating for interior finish, or as an alternative to code requirements for noncombustible construction. During fabrication and erection processes for wood construction, wood products should be handled and covered to prevent marring of the surfaces and moisture absorption. Overstressing of members and joints during handling and erection should be avoided. Competent inspectors should check materials and workmanship. 10.29.1 Fabrication of Structural Timber Fabrication consists of boring, cutting, sawing, trimming, dapping, routing, planing, and otherwise shaping, framing, and finishing wood units, sawn or laminated, including panels, to fit them for particular places in a final structure. Whether fabrication is performed in shop or field, the product must exhibit a high quality of

All-Purpose. A joint treatment material that can be used for bedding tape, finishing, laminating adhesive, and texturing. Joint. A cementitious material for covering joints, corners, and fasteners in finishing of gypsumboard installations, to produce a smooth surface. Setting-Type. A joint compound that hardens by chemical reactions before drying and that is used for shortening the time required for patching and completing joint finishing. Taping. A joint compound specially formulated for embedment of joint tape. Topping. A joint compound specially formulated to serve as the final finishing coat for a joint, but not intended for embedment of tape. Core. The gypsum structure between face and back papers of gypsumboard. Coreboard. A gypsumboard, usually 24 in wide and up to 1 in thick, with square, rounded or tongue-and-groove edges, and homogeneous or laminated. Corner Bead. A strip of sheet metal with flanges and a nosing at the junction of the flanges; used to protect arrises. Corner Cracks. Cracks in joint of intersecting walls or walls and ceilings. Corner Floating. (See Floating Angles.) Cornerite. Reinforcement for plaster at a reentrant corner. Cornice. A molding, with or without reinforcement. Cove. A curved concave, or vaulted, surface. Crown. A buildup of joint compound over a joint to conceal the tape over the