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186 CHAPTER 9 1954 Guide ik. Uc. -- coefficient of transmission of ceiling. U, = 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 Vt and JJX 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 1 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 Ur. Where attic wall areas are large, 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 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 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. Tests12 at the A.S.H.V.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 F between ground temperature and the air temperature 6 in. above the floor. Based on this result a coeffi cient of 0.10 Btu per (hr) (sq ft) (Fahrenheit degree difference) is recom mended for calculation where it is desirable to allow for the small base ment floor heat loss, e.g., for heated basements. 5 $1 For basement walls the same coefficient may be used, but due to closer proximity to the surface of the ground, the temperature difference for win ter design conditions will be greater than for the floor. The test results indicate a unit area heat loss, at mid-height of the basement wall approxi mately twice that of the same floor area. 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) (lineal foot of exposed edge) (Fahrenheit degree difference between the inside air temperature and the average outside air temperature). It 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 abut 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 Coefficients | The U values for glass sheets and hollow glass block, given in Sections A, B and C of Table 20, have been computed by methods and data given in | an A.S.H.V.E. Research Paper.11 It is assumed that the surface.conduct-1; ance for convection loss to the air is 4.0 Btu per (hr) (sq ft) (F deg). R] Table 7- Coefficients of Transmission (U) of Frame Walls efficients are expressed in Btu per- (hour) foot) (Fahrenheit decree difference in temp between the air on the iu>o sides), and are based on an outside xaind vtiociip of 16 mph. No Insulation Between Studs (See Table 6) TYPE exterior finish INTERIOR FINISH Gttsoh (Mo. thick) THICK Wood/ I Bldo. . Paprh Wood Srcwa (Clapboard) ,/TVfy AB Metal Lath asd Plaster*...,....... -- Gypsum Board (M in.) DecoratecL. Wood Lath and mater__,____ -- Gypeiun Lath (M in.) Ptestered*_--~----- Plywood (M In*) Plain or Decorated------- ---Instating Board (M In.) Plain or Decorated*. Insulating Board Lath (M In.) Mastered'..,-- Insulating Board Lath (1 in.) Plastered*-- 0.33 032 031 031 030 0.23 0.22 0.17 0.32 032 031 0.30 030 0.23 032 0.17 Wocp ! Metal Lath and Plaster*--,-- /HtHCLe^ ) Gypsum Board (M in.) Decorated.- Wood Lath and Plaster--. Gypsum Lath (M In.) Plastered*- lywood <K in.l - -------- famish Board Lath (I bJ pSS: Metal Lath awlPtesto*.-------------- Gypsum Board (M in.) Decorated___ -.......... Wood Lath and Piaster--........ ...................... Gypsum Lath (Hm.) Plastered*...... ....... ....... Plywood 04 in.) Plain or Decorated Insulating Board 04 in.) Plain or Decorated-. Insulating Board Lath (M in.) Plastered*--.-- Ir^njlftting Board Lath (1 in.) Plastered*-------- Venieb* Metal Lath and Pfaater*_______ __ ------. Gypsum Board (H in.) Decorated...----.----t Wood Lath and Plaster--.--------------------- 1 Gypsum Lath (Min.) Plastered*------------Piyurood (M in.) Ham or Decorated----------- Insulating Board (M in.) Plain or Decorated--I Insulating Board Lath (M in.) Plastered*.------ j Insulating Board Lath (1 in-) Plastered*-- p 1Ki- ' fe,r dfeCt f studdins neglected. . Fu '*oou.uiea ^V5 in-. thick. ^ "V nominal thickness) between wood shingles and all sheathings except wood. Normw.j t^cknewl n^rt&r between budding paper and brick veneer neglected. Wajj, N nrsu