Document ybV51bNEzad38d6emGEV0Kb3V

132 CHAPTER 6 1949 Guide Coefficients for frame construction are corrected for the effect of framing where such correction would increase the coefficients, but not where the correction would decrease the coefficients7. It should be noted that the effects of poor workmanship in construction and installation have an increasingly greater percentage effect on heat transmission as the coefficient becomes numerically smaller. Failure to meet design estimates may be caused by lack of proper attention to exact compliance with specifications, and a factor of safety may be employed as a precaution when it is judged desirable. Roof Coefficients Computations for wood shingle roofs applied over wood stripping are based on 1 by 4 in. wood strips, spaced 2 in. apart. Values for roofs con taining Spanish and French clay roofing tile are assumed the same as for slate roofs. Values for pitched roofs in Table 16 apply where the roof is over a heated attic or top floor so that the heat passes directly through the roof structure, including any interior finish material. Combined Ceiling and Roof Coefficients - If the attic, space between the ceiling and roof is unheated, the combined coefficient from room air below the ceiling to exterior air can be calculated from the following formula: Rt = and (4) (5) where U = combined coefficient to be used with ceiling area Rr = total resistance of ceiling and roof Uc coefficient of transmission of ceiling UT = coefficient of transmission of roof n = ratio of roof area to ceiling area It should be noted that the over-all coefficient U should be multiplied by the ceiling area to determine heat loss and not by the roof area. Values of U, and {/,, should be calculated using a value of 2.2 (the reciprocal of one-half the air space resistance) rather than 1.65 for the conductances of surfaces facing the attic, since the attic is 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 t/x. Where attic wall areas are large it is preferable to estimate the attic temperature as illustrated in Chapter 14 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. 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 Heat Transmission Coefficients of Bunding Materials 133 c Limestone or sandstone. .............. ' 6 These figures may be used with sufficient aocuracy for concrete walla with stucco exterior finish. * Expanded slag, burned clay or pumioe. s ? Thir.knofts of plaster assumed f in. 9 Thickness of plaster assumed 1 in. - * Based on 2 in. furring strips; one air spsoe. x