Gypsum Sheathing. This type is used as fire protection and bracing of exterior frame walls. It must be protected from the weather by an exterior facing. Sheathing should conform with ASTM C79. It comes 24 to 48 in wide, 6 to 12 ft long, and 3/8, 4/10, 1/2, and 5/8 in thick. Gypsum Formboard. This type is used as a permanent form in the casting of gypsum-concrete roof decks. (Architect Data BookConstruction Products and Systems, Gold Bond Building Products, a National Gypsum Division, 2001 Rexford Road, Charlotte, NC 28211; Gypsum Products Design Data, Gypsum Association, 1603 Orrington Ave., Evanston, IL 60201; Gypsum Construction Handbook, United States Gypsum, 101 South Wacker Drive, Chicago, IL 60606.) Gypsum lath is similar to gypsumboard in that it consists of a core of set gypsum surfaced with paper. The paper for gypsumboard, however, is produced so that it is ready to receive paint or paper, while that for gypsum lath is specially designed or treated so that plaster will bond tightly to the paper. In addition, some lath provides perforations or other mechanical keying to assist in holding the plaster firmly on the lath. It is also available with reflective foil backing (see also Art. Gypsum lath should conform with ASTM C37. It comes in 16-, 161/2-, 24-, and 32-in widths, lengths of 32, 36, and 48 in, and 3/8- and 1/2-in widths. Veneers plasters, special proprietary compositions for thin plaster surfaces, are best applied over veneer plaster base, similar to gypsum lath, but produced to accommodate the veneer plaster compositions. Both gypsum lath and veneer base are made as regular, X-rated (fire-retardant), and insulating (foil-backed) types. These bases should conform with ASTM G588. They come 48 in wide, 6 to 16 ft long, and 3/8, 1/2, and 5/8 in thick. Gypsum sheathing boards are similar in construction to gypsumboard (Art. 4.26), except that they are provided with a water-repellent paper surface. They are commonly made 3/4 to 5/8 in thick, 6 to 12 ft long, and with a nominal width of 24 or 48 in in conformance with ASTM C79. They are made with either square edges or with V tongue-and-groove edges. Sheathing boards also are available with a waterrepellent core or fire-resistant Type X. Gypsum tiles or blocks are used for non-load-bearing partition walls and for protection of columns, elevator shafts, etc., against fire. They have been essentially replaced by dry-wall systems. A precast gypsum product used particularly for roof construction is composed of a core of gypsum cast in the form of a plank, with wire-fabric reinforcement and usually with tongue-and-groove metal edges and ends. The planks are available in two thicknessesa 2-in plank, which is 15 in wide and 10 ft long, and a 3-in plank which is 12 in wide and 30 in long. (See ASTM C377.) Glass is so widely used for decorative and utilitarian purposes in modern construction that it would require an encyclopedia to list all the varieties available. Clear glass for windows and doors is made in varying thicknesses or strengths, also in double layers to obtain additional thermal insulation. Safety glass, laminated from sheets of glass and plastic, or made with embedded wire reinforcement, is available for locations where breakage might be hazardous. For ornamental work, glass is available in a wide range of textures, colors, finishes, and shapes. Various types and grades of glass are used for glazing: Clear Window Glass. This is the most extensively used type for windows in all classes of buildings. A range of grades, as established by Federal Government Standard DD-G-451c, classifies quality according to defects. The more commonly used grades are A and B. A is used for the better class of buildings where appearance is important, and B is used for industrial buildings, some low-cost residences,

Certain manufacturing processes call for close control of temperature and relative humidity. A typical control system is shown in Fig. 13.45. The humidistat may be of the single-pole, double-throw type. On a rise in room humidity, the refrigeration compressor cuts in for the purpose of dehumidification. Since the sensible capacity of the cooling coil usually is much higher than the sensible load, whereas the latent capacity does not differ too much from the latent load, undercooling results. The room thermostat will then send enough steam or hot water to the reheat coil to maintain proper room temperature. A small drop in room relative humidity will cut off the compressor. A further drop in room relative humidity will energize the water or steam solenoid valve to add moisture to the air. The thermostat is also arranged to cut off all steam from the reheat coil when the room temperature reaches the thermostat setting. However, there may be periods when the humidistat will not call for the operation of the refrigeration compressor and the room temperature will climb above the thermostat setting. An arrangement is provided for the thermostat to cut in the compressor to counteract the increase in room temperature. When calculating the load for an industrial system, designers should allow for the thermal capacity and moisture content of the manufactured product entering and leaving the room. Often, this part of the load is a considerable percentage of

The preceding articles present the basic knowledge necessary for accurate determination of the heat losses of a building. Such procedures make possible sizing and selection of heating equipment that will provide reliable and satisfactory service. The basic procedure used in sizing a heating plant is as follows: We isolate the part of the structure to be heated. To estimate the amount of heat to be supplied to that part, we must first decide on the design indoor and outdoor temperatures. For if we maintain a temperature of, say, 70
F inside the structure and the outside temperature is, say, 0
F, then heat will be conducted and radiated to the outside at a rate that can be computed from this 70
temperature difference. If we are to maintain the design inside temperature, we must add heat to the interior by some means at the same rate that it is lost to the exterior. Recommended design inside temperatures are given in Table 13.10. Recommended design outdoor temperatures for a few cities are given in Table 13.5, p. 13.27. (More extensive data are given in the ASHRAE HandbookFundamentals, American Society of Heating, Refrigerating and Air-Conditioning Engineers.) Note that the recommended design outdoor winter temperatures are not the lowest temperatures ever attained in each region. For example, the lowest temperature on record in New York City is
14
F, whereas the design temperature is 0
F. If the design indoor temperature is 70
F, we would be designing for (70
0) /(70  14), or 83.3%, of the capacity we would need for the short period that a record cold of
14
F would last. Once we have established for design purposes a temperature gradient (indoor design temperature minus outdoor design temperature) across the building exterior, we obtain the heat-transmission coefficients of the various building materials in the exterior construction for computation of the heat flow per square foot. These coefficients may be obtained from the manufacturers of the materials or from tables, such as those in the ASHRAE HandbookFundamentals. Next, we have to take off from the plans the areas of exposed walls, windows, roof, etc., to determine the total heat flow, which is obtained by adding the sum of the products of the area, temperature gradient, and heat-transmission coefficient for each item (see Art. 13.3). TABLE 13.10 Recommended Design Indoor Winter Temperatures Type of building Temp,
F