Document oDOkNbyq8849z1Oaj06n6Yq2w

180 CHAPTER; 9 : 1958 Guide ; f : experimental measurements, conducted at the National Bureau of Stand- rj ards? Although the conductances of air spaces vary to some extent with ^ thickness in the range over % in., average values are tabulated for the range from % in. to 4 in., for all except horizontal spaces with heat flow \ downward. The error involved by averaging is less than 10 percent in f': the extreme case and less than 5 percent in most. For more exact values -J Reference 9 may be consulted. . . '5 . For narrow air spaces, which may be defined as those for which the y; product of the cube of the thickness of the space in inches times the temperature difference (Fahrenheit degrees) across the space is less than 3 ! for heat flow horizontally or downward, or less than 1 for heat flow up- ward, the'conductance is the sum of the radiative heat transfer coefficient 1 and that for' conduction alone through air, since convection is practically t; suppressed. The radiation component can be computed by means of Equation 4 and Table 4 of Chapter 5; the conduction component can be Fig. 5. Section of Concrete Wall Having Steel Tie Rods and Insulation computed using the conductivity of air at the appropriate mean tempera-,43 ture (see Table 1, Chapter 5). %;: The effects of different mean temperatures, temperature differences, and: : ; effective emissivities are indicated in Table 3 of this chapter, Section B'. j '- 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 resistance values^&j is not recommended, especially in relation, to emissivity values: ` , Table 3, Section C gives values for the surface reflectivities and emis^.';;,.; sivities of materials used as boundaries of air spaces in building construe- tion, for total radiation at ordinary building temperatures. Effective t.; emissivities for various combinations of these materials, for use in con- junction with Section B of Table 3, are given in the last two columns of / Section C. . When considering heat transfer across air spaces in building construction,.! the emissivities of the. boundary surfaces must be known The-possibility i of change in emissivity of highly reflective surfaces due to exposure to conrf- ditions promoting chemical action, deposition of dust, soiling of the surface, or the application of coatings, even though transparent to the eye, must be< Heat Transmission Coefficients of Building'Materials 18 L considered in selecting a. material for use.10 Surface. emissivity values, should be obtained by tests. .., PRACTICAL COEFFICIENTS AND THEIR USE For practical purposes it is necessary to compute average coefficients that may be applied to various materials and types of construction. '' Table' 4, gives representative values for dry materials at 75 F mean temperature, as selected by the ASHAE Technical Advisory Committee on Insulation., Since there may be some variation in the materials and in test conditions,! these selected values may not be in exact agreement with published data of; all manufacturers. The exact value for the conductivity or conductance of a certain manufacturer's material can only be secured from unbiased tests or guaranteed by the manufacturer. Caution , The conductivity or conductance values given in Tables 3 and 4 are taken from values obtained in most cases by the guarded hot plate methodASTM Standard C-177-45 which states, "Because of the requirements prescribed in this method as to conditions under which conductivity tests shall be made, it. should be recognized,that the conductivity coefficients obtained will not necessarily be the values pertaining under all service conditions.- As an example, the method provides that the conductivity coefficient shall be obtained by test on dry specimens, while in service such a condition will seldom be realized." The user should realize that the average conductivity and conductance values given in Table 4 do not necessarily apply to all products of the same general description. In using these values, judgment should be exercised with regard to the extent to which the product (either as received or as applied) will comply with the tabulated values. Exact conductivities or conductances for specific materials should be obtained from the manufac turer. Because V values for heat flow upward, such as for ceilings in winter, rnay differ greatly from U values for heat flow downward, such as for ceil ings in summer, a serious error may be made in calculating heat losses or gains if the appropriate U value is not used. (See Chapter 13 for recom mended temperature gradients for cooling loads.) Due to the inconsistency of actual building construction, because of the human element, many designers do not use overall coefficients of transmis sion lower than 0.10 for walls and particularly roofs, except where the construction is installed under continuous supervision. It should also be noted that these coefficients do not include any factor of safety and that factor of* syf|erv`s'on ls used, some designers incorporate a 10 percent Computed Heat Transmission Coefficients the^v^6 Mna*ys`s ,f any waU construction for the purpose of calculating termir,erfk efficient of heat transmission U, it is first necessary to deor a TM u6 Pa^s of heat flow, that is, whether they are parallel or series, heat t mbfnatl?n both. This is in accordance with the basic laws of while in,nS state that in parallel flow the conductances are additive, termine ,^nfsflw the resistances are additive. Likewise, in order to de- The ' "e resistance for the wall, the conductance must be known, importance of this analysis cannot be over-emphasized. This is