Document XzY6g9VjxgneZVEE8J6oDrqNd
American Society of Heating and Ventilating Engineers Guide, 1934
As in the case of the simple wall, /i and /0 are always the inside and outside surface coefficients for the two materials in contact with air. If the air is still (no wind), then for the same material f\ and/0 are the same, and/i = f0\ but, if the outside air is in motion, then /0 is always greater than fi and will increase as the wind velocity increases. Values for /i in still and moving air have been determined for various building materials at the University of Minnesota under a cooperative research agreement with the Society1. The range of values for ordinary building materials is comparatively small and for practical purposes may be assumed constant for either still air or any given wind velocity, particularly in view of the fact that the surface resistances usually comprise only a small part of the total resistance of the construction, except in the case of thin, highly conductive walls.
Table 1. Conductances of Air Spaces a at Various Mean Temperatures
Mun Temp
Deo Fa.hu
0.128
Conductances op Air Spaces fob Various Widths in Inches
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
1.670
1.065
40
2.470
1.480
1.288
1.193
1.125
1.112
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
li440
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
Thermal Resistance of Air Spaces, by F. B. Rowley and A. B. Algren (A.S.H.V.E. Transactions. Vol. 35. 1929).
The conductances of air spaces at various mean temperatures and widths, for ordinary building materials, are given in Table 1. These results were likewise obtained at the University of Minnesota under a co operative research agreement with the Society.
Values for k and C, the conductivity and conductance of building materials and insulations, are given in Tables 2, 3, 4, 5 and 6, and are taken from the published values of various investigators. It should be noted that values of k and C.as well as U are dependent on the temperature range, and it is therefore desirable that the investigator determine heattransmission values under conditions approximating those existing under actual conditions. Recommended values for calculating the coefficients of transmission of various types of construction are given in Table 7.
Surface Conductances as Affected by Air Velocity. Temperature and Character of Surface, by F. B. Rowley. A. B. Algren and J. L. Blackshaw (A.S.H.V.E. Transactions, Vol. 38, 1930). See also references at end of chapter.
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Chapter 5--Heat Transmission
While most building materials have surfaces which show similar characteristics as far as the transmission of heat is concerned, it is a wellknown fact that certain surfaces such as aluminum bronze, gold bronze, aluminum foil, or in fact any metallic, highly polished surface presents a greater resistance to heat transmission than the surface of the average
building material.
.
., .
.
The greater heat resistance of such metallic surfaces is due primarily to
their higher reflectivity and consequent lower emissivity of radiant heat.
The use of multiple layers of metallic surfaces, combined with air spaces
of low resistance, provides a definite insulating effect. Factors for single
air spaces of various thicknesses bounded by aluminum foil are given in
A^nmnanvino1 tabulation:
Conductances of. Air Spaces Bounded by.^Aluminum Foil Under Various Conditions and. for:Various Widths
CbaBACTER
or
Surfaces
0.25
0.28
0.33
--Width^op'Aib Space Inches 0.35 0.375 0.4 0.5 0.62 0.675 0.7-
0.75
1.00 1.50
3H
Authobitt
0.69& 0.64* 0.65d
0.59*
0.62a 0.57& 0.54* O.S4d
0.43*
0.554
0.53 0.556 0.46c 0.46d
0.480 0.566 0.41*
0.42*
0.42*
E. R. Queer,* Pennsylvania State College
i |
poo
!
000
MNUi
Plain
Foil To Plain Foil
0.82* 0.74*
0.88 0.81* 0 94* 0.83
0.88* 1.00**
0.75//
0.55*
0.60m 0.63**
,0.61*
0.502/
0.48" 0.5290 0.52"
0.4766 0.4909
02
40
0.48** 0.48<W 0.49// 0.46/7
^N0
Ralph B. Mason |
J. L. Gregg**
0.64** 0.72*
Prof. G. B1 Wilkes, Mass. Inst, of Tech.
Crumbled Foil Surfaces
1.40
1.11
0.74
0.80 0.88
0.41 J.L. Gregg*?
Prof. G. B. Wilkes, Mass. Inst, of Teeh.
a--Mean Temp. 95 F; 9 in. horizontal air space.
6-- * c--
95 F; 9 in. vertical air space.
65 F;
"" ""
d-- 65 F; 9 in. horizontal air space.
e-- 65 F; 11% in. vertical air space.
/- "
97.2 F: high side, 123.5 F low side, 70.9 F.
9-- A--
* 129.7 F " ", 174.2 F
. 70.7 F.
" 152 F; " ",232.1 F " , 72.0 F.
i-- 94.7 F " ", 122.3 F " ` ,67.1 F.
t 129.7 F " ", 191.8 F " 67.6 F.
J-- m--
" 190.4 F 92.5 F
127.1 F
" " "
", 258.8 F ", 121.7 F ", 190.8 F
" M 4.
" H >
68.4 F. 63.3 F.
,63.5 F.
n-- " 162.9 F " ", 262.3 F " " 63.6 F.
P-- 0-- " r--
t-- ** ,
106.0 F 145.0 F
182.6 F * 105.6 F
" ", 136.4 F " " , 75.5 F.
" ", 214.2 F 44 M , 75.7 F.
" "
", 290.0 F " . 135.4 F
44 44
, 75.3 F. , 75.8 F.
NOTES t-- u--
9--. --' X-- If-- 2--
a6a6----
ec-- ddee-- //-
j=
ti--
"
" " " ** " " *' "
"
** 141.5 F;
**. 207.0 F
" .76.0F.
177.0 F;
". 278.3 F
" , 75.8 F.
" 99.0 F; " 137.6 F;
", 129.7 F ". 205.8 F
" , 68.2 F. " .69.4F.
174.7 F;
", 279.6 F
" , 69.8 F.
105 F; wooden separators occupying 6% ofspace
107 F; " " . .............
*' 106 F:
100 F
'99 F
92 F
93 F
92 F
" 98 F '* 97 F fiber insulation separators occupiyng 8%
of space.
115 F;
From data in paper entitled. Importance of Radiation in Heat Transfer Through Air Spaces {Heating, Piping and Air Conditioning,
November, 1931).
^
IComputed from data in article entitled. Thermal Insulation with Aluminum Foil (Industrial and Engineering Chemistry, March, 1933).
Computed from data in paper entitled, Properties of Metal Foil as an Insulation (Refrigerating Engineering, May,*1932).
Coefficients of transmission of various types .of,, wall, ceiling, floor and roof construction with aluminum insulation can be_ readily calculated. The present installation practice indicates that air spaces of l/i in. to 1^2 in. are preferred but manufacturers' recommendations should be closely followed in the application of aluminum foil insulation.
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