Document jmna1rkbK0JXMZ2dka2OMYm25

136 CHAPTER 6 1948 Guide " Table 18. Coefficients of. Transmission (7) .of Doors, Windows, Skylights . ; and Glass Block Waias ; Coefficients are expressed in Bin Per (hour) (square foot) (Fahrenheit degree differencein Iketemperature between the air inside and outside 'of the door, tvindoa, skylight or wall) and are.based:on an outside wind velocity of 16 mph. - ' Section A.. Windows and Skylights Single V 1.13 Double 0.45ae Triple 0.281* Section B. .Solid Wood- Doorsbc - Nominal Thickness - Inches 1 1H m m 2 m 3 Actual Thickness Inches1 , *56 m m m m: 22HH V....... .Exposed Doob 0.69 0.69 0.62 0.51 0.46 0138 0.33 V` With Plass Storm Door : 0.42! 0.38 0.35 0.35 0.32 0128 : 0.25 Section C. Description u Still Am Both Sides u Still Am Inside 15 mph Outside Hollow Glass Smooth surface glass blocks Block Walls: 7J x 3% in. thick__ Ribbed surface glass blocks;-. 7Jx7% x 3% in;,thick_____| 0.40 0.38 -I 0.49 0.46 "See Heating, Ventilating and Air Conditioning, by Harding and Willard, revised edition, 1932,......... ^Computed using C 1.16 for wood;/! = 1.65 and fo -- 6-0. It;Is sufficiently accurate to use the same coefficient of transmission for doors containing thin wood panels as that of single panes of glass, namely, 1.13 Btu per (hpur) (square foot) (degree difference between inside and ontside air temperatures). `These values may also be u^ed with sufficient accuracy for wood storm doors. Neglect storm-doors if loose and use values for exposed doors. > Air spaces assumed tobe$ in. or more-in width. .................. Combined Ceiling and Roof Coefficients If the attic space between ceiling aiid roof is Unheated, the combined coefficient from room air below the ceiling to exterior air can be calculated from the following,formula. where U Ut X UT + ................n. . . 17 = combined coefficient to be used with ceiling area. Uf = coefficient of transmission of roof. Utx = coefficient of transmission of ceiling. ' = ratio of roof area to "ceiling area: " ...... (5) 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-hailf 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 Heat Transmission (Coefficients of Building Materials - 137 individual section multiplied'by its percentage of the total area should - be uSed as UT. 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 / 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; . Tests 8 at the A.S.H.V.E. Research Laboratory indicate a heat flow of approximately 2.0 Btu per (hour) (square foot) through an uninsulated concrete base ment floor with a temperature difference of 20 F between ground tem perature and the air temperature 6 in. above the floor. Based on this result, a coefficient of 0.10 Btu per .(hour) (square foot) (Fahrenheit degree difference) is recommended for calculation where it is desirable to allow for the small basement 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 winter 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, approximately twice that of the same floor area. For concrete slab floors laid in contact with the ground at grade level,' recent tests 9 indicate that for small floor areas (equal to that of a house 25 ft 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 (hour) (lineal foot of exposed edge) (Fahrenheit degree difference between the inside air temperature and the average outside air tempera ture). It should be' noted that this may be appreciably reduced by insu lating the edges of the floor from the abutting wall. CONDENSATION IN BUILDINGS Water vapor in the air within a building condenses if it comes in contact with surfaces at or below its dew-point temperature. It also will be transmitted into or through a wall, floor or ceiling, if a vapor pressure difference exists between the opposite sides, at a rate determined by the permeability of the materials encountered 10 (see Table 17 Chapter-15). Building practice must take account of these facts in avoiding (1) surface condensation on interior building surfaces (walls, ceilings, roofs or glass) and (2) interstitial condensation or accumulation of condensation in the voids within the structure. The conditions under which surface con densation will take place are directly dependent on surface temperature and upon the relative humidity of the air in contact. Limiting maximum relative humidities for walls, roofs or glass having transmission coefficients up to 1.2 Btu for outside temperatures from --30 F to 40 F and for 70 F inside temperature may be obtained from Fig. 4 u. Surface condensation may be controlled by air conditioning, ventila tion for the purpose of removing water vapor, particularly from laundries and from kitchens during lengthy periods of cooking, elimination, of sources of vapor such as unvented gas stoves, or by local application of . heat or insulation to raise surface temperatures.