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106 CHAPTER 9 1960 Guide Table 2------Variation. in Surface Conductance Coefficient for Vertical Surfaces with Different Temperatures of Surrounding Surface Surroonding Swfstt Tomporatvro 75 f 70 F 69 F 60 F 50 F Convection--Btu per (hr) (sq ft). 6.6 6.6 6.6 6.6 6.6 Radiation--Btu per (hr) (sq ft).. 4.4 8.6 9.6 17.0 24.9 Total--Btu per (hr) (sq ft) ___ 11.0 15.2 16.2 23.6 31.5 tion, is illustrated in Table 2, which applies to a vertical sur face at 80 P, with ambient air at 70 F and with radiation exchange corresponding to an effective emissivity of 0.83.4 In many cases, because the heat resistance of the internal parts of the wall is high compared with the surface resistance, the surface factors are of minor importance. In other cases, e-g., single glass windows, the surface resistances constitute almost the entire resistance and are therefore very important. An analysis of various factors affecting surface conductance and the difference between surface and air temperatures will be found in Reference 5. (See also Chapter 30.) The convection part of the surface conductance is affected markedly by air movement. This is illustrated by Fig. 4, which shows the results of tests' made on 12-in. square samples of different materials at a mean temperature of 20 F, and for wind velocities up to 40 mph. These conductances include the radiation portion of the coefficient which, for the conditions of the teste, was about 0.7 Btu per (hr) (sq ft) (F deg). More recent tests7 on smooth surfaces show that surface length also affects significantly the convection part of conductance; the average value decreases as the surface length increases. More over, observations8 of the magnitude of low temperature radi ant energy received from outdoor surroundings show that only under certain conditions may the out-of-doors be treated as a black body radiating at air temperature. Because of these factors, the selection of surface conduc tance coefficients for a practical building becomes a matter of judgment. Surface conductances are shown in Tables 3 and 4. In calculating the overall heat transmission coefficients for the vails, etc., of Tablet B through 16, the appropriate indoor and outdoor surface coefficients given in Table 4 for Air Surfaces have been used. Both values combine the effects of convection and radiation, and are applicable to ordinary building ma terials. They should not be used for low emissivity surfaces such as bright metal. For exposed reflective surfaces refer to Table 3, Section A, and to footnotes under Table 16. In special cases, where surface conductances become im portant factors in the overall rates of heat transfer, more se lective coefficients may be required. Principles and data given in Chapter 5, Heat Transfer, may be applied in such cases. Air Space Conductance The transfer of heat across an air space involves the bound ary surfaces as well as the intervening air, and depends mark edly on the orientation of the air space and the direction of heat flow. The coefficients given for air space conductance represent the total conductance from one surface bounding the ah space to the other. The total conductance is the sum of a component due to radiation and a component due to con vection and conduction combined. These components may vary independently of each other. The radiation portion of the coefficient is affected by the temperature of the two boundary surfaces, and by their re spective surface emissjvities t, the combined effect of which is expressed by means of the effective emissivity E of the air space. The radiation component is not affected by the thickness of the space or by its orientation or direction of heat flow. The heat transfer by convection and conduction combined, how ever, is markedly affected by the orientation of the air space and the direction of heat flow, is significantly affected by the temperature difference across the space and in some cases by the thickness of the space, and is affected to only a small extent by the mean temperature of its surfaces. For air spaces usually employed in budding construction, the radiation and convection-conduction components may vary independently of each other. Table 3, Section C, gives the thermal conductances and resistances of air spaces of uniform thickness and moderately Fig. A.... Curves Showing Relation Between Surface Conductance for Different Surfaces at 20 F Mean Temperature smooth surfaces, based on experimental measurements con ducted at the National Bureau of Standards.* Although the conductances of air spaces vary to some extent with thick ness in the range over Y in., average values are tabulated for the range from H. in. to 4 in., for all except horizontal spaces with beat flow downward. The error involved by averaging is less than 10 percent in the extreme case and less than 5 per cent in most. For more exact values Reference 9 may be con sulted. For narrow air spaces, which may be defined as those for which tiie product of the cube of the thickness of the space in inches times the temperature difference (Fahrenheit de grees) across the space is less than 3 for heat flow horizontally or downward, or less than 1 for heat flow upward, the con ductance is the sum of the radiative heat transfer coefficient and that for conduction alone through air, since convection is practically suppressed. The radiation component can be computed by means of Equation 4 and Table 4 of Chapter 5; tiie conduction component can be computed using the con- Heat Transmission Coefficients of Building Materials 107 ductivity of air at the appropriate mean temperature (see Table 1, Chapter 5). The effects of different mean temperatures, temperature differences, wM effective emissivities are indicated in Table 3 of this chapter, Section C. As indicated, use may be made of interpolation and moderate extrapolation of conductance values in the table to obtain conductances for conditions moderately different from those given. Interpolation of re sistance values is not recommended, especially in relation to emissivity values. Table 3, Section B, gives values for the surface reflectivities fund emissivities of materials used as boundaries of air spaces in building construction, for total radiation at ordinary build ing temperatures. Effective emissivities for various combina tions of these materials, for use in conjunction with Section C of Table 3, are given in the last two columns of Section B. When considering heat transfer across air spaces in building construction, the emisivities of the boundary surfaces must be known. The possibility of change in emissivity of highly reflective surfaces due to exposure to conditions promoting chemical action, deposition of dust, soiling of the surface, or the application of coatings, even though transparent to the eye, must be considered in selecting a material for use.10 Surface emissivity values should be obtained by tests. OVERALL COEFFICIENTS AND THEIR PRACTICAL USE The values in Tables 3 and 4 for component elements and materials were selected by the ASHAE Technical Advisory Committee on Insulation as representative for dry materials at 75 F mean temperature. They are based on available pub lished data obtained by the guarded hotplate method {ASTM 077-45) or by the guarded hot box method {ASTM C23654T). Because there are variations in commercially available materials of the same type, not all of these selected represen tative values will be in exact agreement with data for indi vidual products. The exact value for a certain manufacturer's material can be secured from unbiased testa or from guaran teed data of the manufacturer. The most exact method of determining the heat transmis sion coefficient for a given combination of building materials assembled as a building section is to test a representative section in a guarded hot box. However, it is not practicable to test all the combinations which may be of interest in building construction. Experience has indicated that V values for many constructions, when calculated by the methods given in this chapter, using accurate values for the component materials, are in good agreement with values determined by guarded hot box measurements. Caution Although the validity of calculating V values for all of the types of constructions in Tables 5 through 16 has not been fully demonstrated, calculated values are given because measured values are not available. It is emphasized that the calculated values in Tables 5 through 16 are given for the convenience of the user. In calculating U values, exemplary conditions of compo nents and installations are assumed, i.e., that inflating rnaterials are uniformly of the nominal thickness and conduc tivity, that air spaces are of uniform thickness and surface temperatures, that effects due to moisture are not involved, and that installation details are in accordance with design Some evidence of departures of measured values from cal culated values for certain insulated constructions is given in Building Materials and Structures Report BMS 151, National Bureau of Standards- In order to provide a reasonable factor of safety to account for departures of constructions from exemplary conditions, in part due to field construction re quirements ami practices, some may wish, before making corrections for framing (as indicated in Fig. 6), to increase moderately the calculated V values of the insulated walls, floors and ceiling sections obtained from Table 16. Where reflective air spaces are involved, increases of U values up'to 10 percent for applications where heat Sow is horizontal or upward, and up to 20 percent where heat flow is downward, appear reasonable on the basis of present information. Heaf Row Through Panels Containing Metal The transmittance of a panel which includes metal or other highly conductive material extending wholly or partly through insulation should, if posable, be, determined by test in the guarded hot box. When a calculation is required, a good ap- QXVATtOM For enlarged taction of Zone A, too Fig. 6 Fig. 5 .... Gypsum Roof Deck on Bulb Tees proximation can be made by a Zone Method. This involves two separate computations--one for a chosen limited portion, Zone A, containing the highly conductive element, and the other for the remaining portion of ampler construction, called Zone B. The two computations are then combined, and the average transmittance per unit of overall area is calculated. The basic laws of heat transfer are applied, i.e., adding area conductances C-A of elements in parallel, and adding area re sistances 1/C-A of elements in series. The surface shape of Zone A is determined by the metal element. For a metal beam (Fig. 5) the Zone A surface is a strip of width W, centered on the beam. For a rod perpendicu lar to panel surfaces it is a circle of diameter W. The value of W is calculated from Equation 4, which is empirical. W - m + 2d (4) where m -- width or diameter of the metal heat path terminal, inches. d -- distance from panel surface to metal, inches. The valued of d should not bo taken less than 0J> in. (for still air). In general, the value of W should be calculated by Equa tion 4 for each end of the metal heat path, and the larger value, within the limits of the basic area, used as illustrated in Example 1. Example /* Calculate the transmittance of the roof deck shown in Figs. 5 and 6. Tee-bars on 24 in. centers support glass fiber form boards, gypsum concrete and built-up roofing. The conductivities of components are: steel 312; gypsum concrete 1.66; glass fiber 0.25. The conductance of built-up roofing is 3.0. (GestuuwS n p. US)