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Heating Ventilating Air Conditioning Guide 1939 vection are controlled largely by the width and shape of the air space and the roughness of the boundary surfaces. The thermal resistances of air spaces bounded by extended parallel surfaces perpendicular to the direction of heat flow and at different mean temperatures have been determined for average building materials at the University of Minnesota in a cooperative research program with the Society. The values given in Table 1 show the results of this study and apply to air spaces bounded by such materials as paper, wood, plaster, etc., having emissivity coefficients of from 0.9 to 0.95. The conductivity coefficients decrease with air space width until a width of about % in. has been reached, after which the width has but very little effect. In these Table 1. Conductances of Air Spaces at Various Mean Temperatures Conductances of Air Spaces fob Various Widths in Inches Obq Fahb 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 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 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 ISO 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, p. 165). coefficients radiation is a large factor, and if surfaces with low emissivity coefficients are substituted for ordinary building materials the total amount of radiant heat will be reduced. The reduction in radiant heat caused by the low emissivity surface is independent of width of air space. Air spaces properly formed in combination with metallic surfaces such as. aluminum foil, coated sheet steel, and other materials having a reflective surface, possess heat repelling characteristics. Values of air spaces lined with aluminum foil on one or both sides for widths of % in. and % in. are shown in Table 2 of conductivities. A low emissivity coefficient is dependent on the permanency of the reflective surface. If a bright clean surface is covered with a thin layer of corrosive material its reflectivity is appreciably reduced2. In comparing the conductance coefficients for air spaces with and with out bright metallic surface lining it should be noted that the reduction in heat transfer is substantially as great when one surface is lined as it is when both surfaces are lined. The reason for this is that practically 95 per cent of the total radiant heat is- intercepted by one surface lining 'Aluminum Foil Insulation {National Bureau of Standards Letter Circular No. LC465. June? 1936). 92 5.Chapter Heat Transmission Coefficients and Tables nd there is but a small amount left to be stopped by the second surface f nine The effect of any low emissivity surface in stopping the trans mission of radiant heat is the same regardless of whether it is on the high or low temperature side of the air space. For materials such as aluminum oaint or bronze paint which stop only a small percentage of radiant heat there is a greater percentage of gain by addition of a second surface lining. 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. The recommended coefficients which have been used for the calculation of over-all coefficients as given in Tables 3 to 12 are marked by an asterisk. It should be recognized in these tables of calculated coefficients that 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 3 to 12 can generally be computed by using the conductivity values given in Table 2 and the fundamental heat transfer formulae. 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. /' Attention is called to the fact that the conductivity values per inch of / thickness do not afford a true basis for comparison between insulating 7 materials as applied, although they are frequently used for that purpose, f The value of an insulating material is measured in terms of its heat resistance, which not only depends upon the thermal conductivity coef ficient per inch but also upon the thickness as installed and the manner of installation. For instance the material having a coefficient of 0.50 and 1 in. thick is equal in value to a material having a coefficient of 0.25 and `j a thickness of M in.. Certain types of blanket installations are designed i to be installed between the studs of a frame building in such manner as to give two air spaces. In order to get the full value of such materials they should be so installed that each air space is approximately 1 in. or more in thickness and the air spaces should be sealed at the top and bottom to prevent the circulation of air from one space to the other. Another . common error in installing such a material is to nail the blanket on the outside of- the studs underneath the sheathing, in which case one air space is lost and also the thickness of the insulating material is materially reduced at the studs. There are certain other types of insulation which are very porous, allowing air circulation within the material if not 93