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210 CHAPTER 9 1957 Guide U = 1/Rt (5) where U = combined coefficient to be used with ceiling area. Rt = total resistance of ceiling and roof. ' U,,, = coefficient of transmission of ceiling. Ur = coefficient of transmission of roof. n = ratio of roof area to ceiling area. - It should be noted that the overall coefficient U should be multiplied by the ceiling area to determine heat loss, and not by the roof area. Values of Ur and t/ce should be calculated using a value of 2.5 (the reciprocal of one-half the air space resistance, 0.80) rather than the conductances of surfaces facing the attic, since the attic is assumed to be equivalent to an air space. If the attic contains windows, dormers, and vertical wall spaces, and if their area is small compared to that of the roof, they may be considered part of the roof area. For accuracy, the sum of the coefficients of each individual section, multiplied by its percentage of the total area, should be used as UT. Where attic wall areas are large or where louvers or vents are used, it is preferable .to estimate the attic temperature as illustrated in Chapter 12, and calculate the heat loss through the ceiling by multiplying the value of U,, for the ceiling by the difference in temperature above and below the ceiling. Table 17. Coefficients of Tbansmission (C7) of Concrete Basement Floors on Ground with Various Types of Finish Flooring U -- 0.10* Btu per (hr) (sq ft) (Fahrenheit degree temperature difference between the ground and tne air over the floor). Since authentic data are not available, this coefficient is sometimes used for concrete floors on ground For more recent procedures1* refer to National Bureau of Standardt Report BMS-203. Basement Floor, Basement Wall and Concrete Slab Floor Coefficients The heat transfer through basement walls and floors to the ground is dependent on the temperature difference between the air within and that of the ground, on the material constituting the wall or floor, and on the conductivity of the surrounding earth. The conductivity of the earth will vary with local conditions, and is usually unknown. Tests11 at the A.S.H.A.E. Research Laboratory indicate a heat flow of approximately 2.0 Btu per (hr)' (sq ft) through an uninsulated concrete basement floor, ; with a temperature difference of 20 deg between ground temperature and the air temperature 6 in. above the floor (see Table 17). For basement walls below grade only, the temperature difference for \ winter design conditions will be greater than for the floor. The test re1 / suits indicate a unit area heat loss, at mid-height of the basement wall - portion below grade approximately twice that of the same floor area. ; i For concrete slab floors laid in contact with the ground at grade level, recent tests12 indicate that for small floor areas (equal to that of a house 25 feet square) the heat loss may be calculated as proportional to the.', length of exposed edge rather than total area. This amounts to 0.81 Btu per (hr) (linear foot of exposed edge) (Fahrenheit degree difference between./the inside air temperature and the average outside air temperature). B should be noted that this may be appreciably reduced by insulating under;; the ground slab, and also along the edges between the floor and the abukdi, Heat Transmission Coefficients of Building Materials 211 ting walls. See also sections on Basement Temperatures and Heat Loss, and on Floor Heat Loss in Basementless Houses, in Chapter 12. In most calculations if the perimeter loss is calculated accurately, no other floor loss need be considered. . Glass and Door Coefficients The U values for glass sheets and hollow glass block, given in Sections A, B and C of Table 18, have been computed by methods and data given in an A.S.H.V.E. Research Paper.8 It is assumed that the surface conduct ance for convection loss to the air is 4.0 Btu per (hr) (sq ft) (F deg). It is also assumed that the glass loses heat by radiation to the ground and to the clear sky, which together have an effective radiating temperature below the air temperature. It is therefore necessary to determine, by trial and eiror, the temperature of the outdoor glass surface such that the sum of the radiation and convection losses equals the heat conducted through the glass section, and equals the heat delivered to the glass from the heated space. This heat flow, divided by the air-to-air temperature difference, results in a U value which is used in the usual manner. The equivalent surface conductance for radiation and convection combined, based on air. io-surface temperature difference, therefore varies from about 5.5 for sin gle glass to about 6.6 for double glass for exactly the same environmental design conditions. Curtains, draperies, Venetian blinds, etc., will result in lower glass surface temperatures than when the windows are not covered. It is assumed that the room air temperature equals the average tern, perature of the room surfaces seen by the glass. Special consideration should be given to those cases where the glass sees interior surfaces at temperatures differing greatly from the room air temperature, i.e., such cases as in sun rooms, greenhouses, and some panel heated rooms, or where there is an unusual amount of air motion in the vicinity of the glass. Although based on zero outdoor air, the values change only slightly with different design temperatures, being about 5 percent greater for a 30 F outdoor design temperature. In computing the Table 18 values, consideration of the dependence of the indoor surface conductances upon temperature and direction of heat flow leads to surface conductances averaging about 1.50 for block and vertical glass, and about 1.80 for horizontal glass, as compared to the value of 1.46 used in computing U values given in other tables in this chapter. These values should therefore be used in estimating the tem perature at which condensation on glass surfaces will occur. The application factors given in Section D of Table 18 are based upon hot box tests summarized in a research bulletin,15 and are approximate only. In practice, some variation in heat flow through windows having the same ratio of glass to sash area, may be expected because of difference *n construction details and in air space edge effects. The high conductance . aluminum and steel sash must be taken into consideration where exces?ve amounts of metal sash and frames are involved. This is particularly nnportant when they are in close proximity to radiation heat sources and are consequently subjected to high differential temperatures. Heat transmission coefficients for wood doors, with and without glass w>rm doors are .given in Table 19. ^ind Velocity Effect on U Values Tables 5 through 8, 13-A, 14-A, 15, parts of Table 18, and Table 19 show lues of U for winter calculations, for an outside wind velocity of 15 mph.