Document 8RxN1wJvEQj7a7oBzb756yzZ

36 Chapter 4 1945 Guide Table 1. Conductances (C) for Surfaces and Air Spaces AU conductance values expressed in Btu per hour Per square foot per degree Fahrenheit temperature difference. Section A. Surface Conductances for Still Atra - Surface Emissivity of Surface of Heat Flow e = 0.83 e = 0.05 Vertical..... .................. .... ..... Downward 1.95 1.21 1.52* 1.16 0.44 0.74 Section B. Conductance of Vertical Spaces at Various Mean Temperaturesb Mean Temp Deg Fahr Conductances of Air Spaces for Various Widths in Inches 0.128 0.250 0.364 0.493 0.713 1.00 1.500 20 2.300 1.370 1.180 1.100 . 1.040. 1.030 1.022 30 2.385 1.425 1.234 1.148 1.080 1.070 1.065 40 2.470 1.480 1388 . 1.193 1.125 1.112 1.105 50 2.560 1.535 1.340 1342 1.168 1.152 1.149 60 2.650 1.590 . 1.390 1.295 1310 . 1.195 1.188 70 2.730 1.648 1.440 1.340 1350 1.240 1.228 80 2.819 1.702 1.492 1.390 1.295 1.280 1370 90 2.908 1.757 1.547 1.433 1.340 1.320 1.310 100 * 2.990 1.813 1.600 1.486 1.380 1.362 1.350 110 3.078 1.870 1.650 1.534 1.425 1.402 1.392 120 3.167 1.928 1.700 1.580 1.467- 1.445 1.435 130 - 3350 1.980 1.750 1.630 1.510 1.485 1.475 140 3.340 2.035 1.800 1.680 1.550 1.530 1.519 150 3.425 2.090 1.852 1.728 1.592 1.569 1.559 Location and Position of Air Space Section C. Conductances and Resistances of Air Spaces Faced on One Surface with Reflective Insulatlon Direction of Heat Flow Temp* Diff Deg Fahr Winter Summer Conductance* (Q No. of Air Spaces 1 23 Resistance* (r) No. of Air Spaces 123 Rafter Space (8 in.) Horizontal Horizontal 30 deg slope . 30 deg slope 30 deg slope Down Up Down Up Down . Up Down : Up 45 45 45 45 25 25 25 25 0.10 0.27 0.09 0.24 0.15 0.25 0.13 0.23 0.07 0.17 0.06 0.16 0.10 0.17 0.09 0.14 10.00 3.70 11.11 4.17 6.67 4.00 7.69 4.35 14.29 5.88 16.67 635 10.00 5.88 11.11' 7.14 Stud Space ` (3H in.) Vertical 30 0.34 2.94 40 0.23 0.13 4.35 7.69 Vertical 15 0.32 3-13 20 0.18 0.11 - 5^56 9.09 Vertical* 30 0.46 2.17 ^Radiation and Convection from Surfaces in Various Positions, by G. B. Wilkes and C. M. F. Peterson 6\.S.H.V.E. Transactions, Vol. 44, 1938, p. 513). bA.S.H.V.E. Research Report'No. 825--Thermal Resistance of Air Spaces, by F. B. Rowley and A. B. Atgren (A.S.H.V.E.-.Transactions, Vol 25, 1929, p: 165). . cThermal Test Coefficients of Aluminum Insulation for Buildings, by G. B. Wilkes, F. G. Hechler and E. R. Queer (A.S.H.V.E. Transactions. Vol. 46, 1940). ^Temperature difference is based on total space between plaster base and sheathing, flooring or roofing. These air space conductance and resistance values are based on one reflective surface (aluminum) having an emissivity of 0.05 facing each space and are based on total space between plaster base and sheath* ing. flooring or roofing. The rafter and stud spaces are divided into equal spaces. /Stud space is lined on plaster base side with loose paper with aluminum on surface facing air space. The resistance of the small air space between the plaster base and paper was 0.43. fRadiation and Convection Across Air Spaces in Frame Construction, by G. B. Wilkes and C. M. F. Peterson (A.S.H.V.E. Transactions, Vol. 43. 1937, p. 351). The recommended surface conductance for calculating heat losses for still air for non-reflective surfaces Cs 1.65 Btu. For a 15 mph wind velocity, the recommended value'is 6:0*Btu. These coefficients were derived from Fig. 1 which was based on tests conducted at the University of Minnesota, and apply to vertical surfaces. 5 'SC- C- u .fe a. $ ft; I-?- o* Heat Transmission Coefficients 87 in the emissivity coefficient due to surface coatings or chemical action6 should be investigated as to the permanence of the reflective surface for the conditions under which this material will be used. In making instal lations of this material the partitions between air spaces should be tight, particularly at the top and bottom so that air cannot circulate between adjacent spaces. When reflective insulating materials are installed with multiple air spaces, the position (vertical, horizontal or inclined) of the material in the structure must be taken into consideration. For example, the resistance to heat flow upward is about one-third that of downward flow in a hori zontal position in the same construction, as will be apparent from Table 1, Section C. However, the difference between upward heat flow throughsingle horizontal or sloping air spaces and through single vertical air spaces is comparatively small for the same temperature difference. Con sequently the same conductance value (0.46) may be used for computing coefficients involving upward heat flow through horizontal and sloping air spaces bounded on one side by aluminum foil applied to plasterboard, as for similar vertical air spaces. As already stated, a conductance value of 1.10 was similarly used in all cases for calculating the coefficients of construction involving vertical, horizontal and sloping air spaces bounded on both sides by ordinary, building materials. PRACTICAL COEFFICIENTS For practical purposes it is necessary to have average coefficients that may be applied to various materials and types of construction without the necessity of making tests on the individual material or combination of materials. In Table 2 coefficients are given for a group of materials which have been selected from various sources. Wherever possible the proper ties of material and conditions of tests are given. However, in selecting and applying these values to any construction a reasonable amount of caution is necessary; variations will be found in the coefficients for the same materials, which may be partly due to different test methods used, but which are largely due to variations in materials. . It is recommended that future determinations of thermal conductivity-be made in accor dance with the code, Standard Method of Test for Thermal Conductivity of Materials by Means of the Guarded Hot Plate'. The coefficients which have been used for the calculation of over-all coefficients are riven in Table 3. . It should be recognized that in these tables of calculated coefficients space-limitations will not permit the inclusion of all the combinations of materials that are used in building construction and the varied applications of insulating materials to these constructions. Typical examples are given of combinations frequently used, but any special construction not given in Tables ^ to 16 can generally be computed by using the conductivity'values given in Table 3 and the fundamental heat transfer formulas. For example, the tabulation of all of the values for multiple layers of insulating materials would present extensive and detailed problems of calculations for the varied application combinations, but the engineer having -the fundamental conductivity values can quickly obtain the proper coefficients.' * b- Test Coefficients of Aluminum Insulation for Buildings, by G. B. Wilkes, F; G. Hechler and *=~ R. Queer (A.S.H.V.E. Transactions, Vol. 46. 1940, p. 109). a 'Swnsored by A.S.H.V.E., A S.'j' M , AJS.R.E. and N.R.C. and approved as a Tentative Code by A S.H.V.E. and A.S.T.M. in 1942 (AJ.T.M. Designation of Code is C177-42T).