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184 CHAPTER 9 1955 Guide: through the ceiling by multiplying the value of t/<* for the ceiling by theS 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 4' dependent on. the temperature difference between the air within and that 2 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.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 deg between ground temperature andthe 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- r mended for calculation where it is desirable to allow for the small base-' ment floor heat loss, e.g., for heated basements. 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. ;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 the1length 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). It should be noted that this may be appreciably reduced by insulating underthe ground slab, and also along the edges between the floor and the abut1 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.13 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 error, 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 I surface conductance for radiation and convection combined, based on air- | to-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. "I It is assumed that the room air temperature equals the average tefflj I perature of the room surfaces seen by the glass. Special consideration. I wvuivicuVd UJ AJuuuAug materials 185 Table 7- Coefficients of Transmission (U) of Frame Walls* These coefficients are expressed in Btu per (hour) (square foot) (Fahrenheit degree difference in temperature between the atr on the two sides), and are based on an outside wind velocity of 16 mph. No Insulation Between Studs* (See Table 6) Wall N u k b m l 5. Note a. * fete assuummeedd ^4ciginu.vetthhuii;ccckkb."1 0f studdin* neglected. ? urri^ipjft1 ^n.^nomia&i thickness) betwo__---- - ,*** aoa Small air space and mortar Nominal thickness, 1 in. between building paper aMndd 'bbrriciakfvv"eneeer'nneegMlec^tHed0.gS ""*>* WOOd-