Document 8R3oaXkag2R2Jp0Zqy0DVzX65

% l210 CHAPTER 9 1958 Guide: # U = 1/Rt (5)= ` 2 where U. = combined coefficient to be used with ceiling area. Rr = ;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. . :.i ' IIt 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" I of Ur and (/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- airspace. 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 Ur. 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 {/M for the ceiling by the difference in temperature above and. below the ceiling. Table 17. Coefficients of Transmission. (U) 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 the air over the floor). Since authentic data are not available, this coefficient is sometimes used for concrete floors on ground^/ For more recent procedures12 refer to National Bureau of Standards Report BMS-103. ' Basement Floor, Basement Wall and Concrete Slab Floor Coefficients;; The heat transfer through basement walls and floors to the ground in 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 floojp: with a temperature difference of 20 deg between ground temperature and{ the air temperature 6 in. above the floor (see Table 17). , /jgo For basement walls below grade only, the temperature difference winter design conditions will be greater than for the floor. The test-rev,: suits indicate a unit area heat loss, at mid-height of the basement portion below grade approximately twice that of the same floor area. lgv; For concrete slab floors laid in contact with the ground at grade levpl,;; recent tests12 indicate that for small floor areas (equal to that of a hous?;; 25 feet square) the heat loss may be calculated as proportional to .wAi length of exposed edge rather than total area. This amounts to 0.81 B per (hr) (linear foot of exposed edge) (Fahrenheit degree difference betweeP;; the inside air temperature and the average outside air temperature). should be noted that this may be appreciably reduced by insulating ub<fe. the ground slab, and also along the edges between the floor and the abirtgs 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 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 surface conductance for radiation and convection combined, based on airto-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 tem 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 now 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. h Thtf aPP^catl0n factors given in Section D of Table 18 are based upon onl r tCS*'S summarized in a research bulletin,13 and are approximate Practice, some variation in heat flow through windows having in San?e rati of glass to sash area, may be expected because of difference of a?Hi8 .c*''on details and in air space edge effects. The high conductance sive ^minum and steel sash must be taken into consideration where exces- import metal sash and frames are involved. This is particularly are nr> ant w"e? they are in close proximity to radiation heat sources and nsequently subjected to high differential temperatures.. trar|smission coefficients for wood doors, with and without glass storm doors are given in Table 19. Wmd Velocity Effect on U Values valuesfof 13-A, 14-A, 15, parts of Table 18, and Table 19 show tor winter calculations, for an outside wind velocity of 15 mph.