Document LKE7vjo2wrpJpY2NBB154y02z
HEATING VENTILATING AIR CONDITIONING GUIDE 1942
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 Air
Position op Surface
Mnr-irnntal Horizontal Vertical. ....
.....................
Direction of Heat Flow
Upward Downward
Surface Emissivity
e = 0.83
1.95 1.21 1.52
e = 0.05
1.16 0.44 0.74
Section B. Conductance of Vertical Spaces at Various Mean Temperatures**
Mean Temp Dec Faur
Conductances op Air Spaces for Various Widths in Inches
0.128
0.250
0.364
0.493
0.713
1.00
1.500
20 30
40
2.300
2.385 2.470
1.370
1.425 1.480
1.180 1.234 1.288
1.100
1.148 1.193
1.040 1.080 1.125
1.030 1.070
1.112
1.022
1.065 1.105
50
2.560
1.535
1.340
1.242
1.168
1.152
1.149
60
2.650
1.590
1.390
1.295
1.210
1.195
1.188
70
2.730
1.648
1.440
1.340
1.250
1.240
1.228
80
2.819 - 1.702
1.492
1.390
1.295
1.280
1.270
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
3.250
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
Section C. Conductances and Resistances of Air Spaces ______________Faced with Reflective Insulation
Location and Position of
Air Space
Direction
of
. Heat Flow
Temp Diff Deg Fahr
Winter Summer
Conductance* (O
No. of Air Spaces
1 23
Resistance*
(r)
No. of Air Spaces
23
Rafter Space (8 in.)
Horizontal Horizontal
Down Up
45 45
0.10 0.27
0.07 0.17
10.00 14.29 3.70 5.88
Horizontal Horizontal
Down Up
25
0.09 0.06
11.11 16.67
25
0.24- 0.16
4.17 6.25
30 deg slope 30 deg slope
Down Up
45 45
0.15 0.25
0.10 0.17
6.67 10.00 4.00 5.88
30 deg slope 30 deg slope
Down Up
25
0.13 0.09
7.69' 11.11
25
0.23 0.14
4.35 7.14
Stud Space
<3M in.) Vertical/
Vertical
30 0.34
2.94
40
0.23 0.13
4.35 7.69
Vertical/ : 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 (A.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. Algren (A.S.H.V.E. Transactions, Vol. 35, 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. fThese 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- mg, 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.
sRadiation 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).
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CHAPTER 4. HEAT TRANSMISSION COEFFICIENTS AND TABLES
largely by the nature of the surface and the temperature difference between the boundary surfaces of the air space. Conduction and con vection are controlled largely by the width and shape of the air space and the roughness of the boundary surfaces.
The conductances of vertical air spaces bounded by such materials as paper, wood, plaster, etc., are given in Table 1, Section B, having emis sivity coefficients of 0.8 or higher, and with extended parallel surfaces perpendicular to the direction of heat flow. A conductance of 1.10 Btu per hour per square foot per degree Fahrenheit temperature difference (resistance. = 0.91) based on this table was used for calculating the overall coefficients given in Tables 3 to 12 inclusive for air spaces % in. or more in width. Air space tests2 reported by Wilkes and Peterson resulted in comparable values. For 3% in. horizontal air spaces having an effective emissivity of 0.83, the conductance for heat flow upward was 1.32 and for heat flow downward, 0.94. The conductance for a similar vertical air space was 1.17, the resistances of course being the reciprocals of these values in each case.
A large part of the heat transferred across air spaces bounded by ordi nary materials is by radiation. Therefore, if such air spaces are faced with metallic surfaces such as aluminum foil, coated sheet steel or other low-emissivity, infra-red reflective metal surfaces, the radiant.heat trans fer will be substantially reduced, thus causing the major portion of the remaining transmitted heat to be by convection. Table 1, Section C, gives conductances and resistances for air spaces bounded by one reflec tive surface having an emissivity of 0.05. It will be noted that the con ductance values given in this table are a function of the temperature differences across the space rather than mean temperature, the larger the temperature difference, the larger the conductance. The radiant heat transfer is the same regardless of whether the low emissivity surface is on the high or low temperature surface of the space, and is independent of the width of the space. To minimize the convection transfer the vertical air space should be at least % in. in width. A conductance of 0.46 was used for computing the overall coefficients in Tables 3 to 12 inclusive for air spaces bounded by aluminum foil applied to plasterboard.
When referring to reflective heat-insulating surfaces, the term brightness which deals with visible light has no specific meaning and should'be avoided3. Emissivity and reflectivity definitely define the radiating and reflecting properties and values may be determined directly for long wave length radiation corresponding to room temperature. As previously stated, the values in Table 1, Section C, are based on an emissivity of the reflective surface of 0.05. Obviously for higher emissivity values the conductances will increase accordingly. For example, non-metallic reflec tive materials are available having emissivity values approximately midway between those of metallic reflective insulations and ordinary building material surfaces. These materials will have a correspondingly higher radiant heat transfer and where such materials are under con sideration, due allowance should be made for the higher emissivity value in arriving at the proper air space conductance.
'Radiation 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).
5ome Reflection and Radiation Characteristics of Aluminum, by C. S. Taylor and J. D. Edwards (A.S.H.V.E. Transactions, Vol. 45. 1939, p. 179).
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