Document Yj2BXLemQk66y3mzgG6D8N8zk

168 CHAPTER 9 1952 Guide resistance value of 1/0.25 = 4.0. Therefore, 4 hr would be required for the flow of one Btu for each square foot of area and each degree of temperature differential. CALCULATING OVERALL COEFFICIENTS From Chapter, 5, Equation 7, the total resistance to heat flow through a wall is equal numerically to the sum of the resistances in series. Rt -- Ri +. Ri + Rt + R + ... d" Rd (1) where,- Ri + Rt, etc., are the individual resistances of the wall components. iJT = total resistance. For a wall of a single homogeneous material of conductivity k and thickness x, with surface coefficients/i and/o, w 1r 1 +h / <!) Heat Transmission Coefficients of Building Materials - 169 . is recommended that thermal conductivities of homogeneous materials be determined by means of .the Guarded Hot Plate.1 For determination of conductances, a Guarded Hot Box method* is generally used. Tables 1 and 2 give. conductivities and conductances which are quite generally used in calculation, and which have been selected from various sources. Wherever possible, the properties of the material and test con ditions are given. In selecting and applying heat transmission values to any construction, caution is necessary, since coefficients for the same mate rial may differ because of variations which occur in test methods, in the materials themselves, or in the temperature of the material when tested. Conductivity of Homogeneous Materials Thermal conductivity is a property of a homogeneous material and of types of building materials such as lumber, brick, and stone, which may be considered .homogeneous. Most insulating materials, except reflective Fig. 1. Typical Variation op Thermal Conductivity with Density--for Fibrous'Material. Then by definition, 17 = l/R-t For a wall with air space construction and consisting.of two homogeneous materials of conductivities: h and fcj, thicknesses Xi and **, respectively, arid separated by an air space of conductance a, = r/ri + tk, d--a .+ tkt + /--,, (3) and U = 1/Rt In the case of types of building materials having non-uniform or irregu lar sections such as hollow clay tile or concrete blocks, it is necessary to use the conductance C of the section unit as manufactured instead of a conductivity k. The resistance of the section 1/C.is therefore substituted for. x/k in Equations 2;and 3. , ... . '. ;. . CONDUCTIVITIES AND CONDUCTANCES The method of calculating the overall coefficient of heat transmission for a given construction is comparatively simple, but accurate values of con ductivities and conductances must be used to obtain satisfactory results. In addition, there are sometimes parallel heat flow paths of different re sistances in the same wall, and these may necessitate modification of the formula; In such cases calculated results should be checked by test. The determination of the fundamental conductivities and conductances requires considerable skill and experience to obtain accurate results. It with Mean Temperature types, are of a porous nature and consist of combinations of solid matter with small air cells. The thermal conductivity of these materials will vary with density, mean temperature, size of fibers or particle, degree and extent of bond, between particles, moisture present, and the arrange ment of fibers or particles within the material. The effect of density upon conductivity (at constant mean temperature) is illustrated for two fibrous materials in Fig. 1. It will be noted that for each there is an optimum density for lowest conductivity.. Typical varia tion of conductivity with mean temperature is shown in Fig. 2. Thermal Conductivity of Soil The following statements are based largely on results of a study8 made in the Engineering Experiment Station, University of Minnesota, and pub lished in Bulletin No. 28. Tests were made on nineteen different soils which'represented a wide textural variety, including gravel, sand, sandy loam, silt loam and clay, as well as some crushed rocks and a fibrous peat. Moisture contents in tots varied from air-dried values to those greater than the optimum moisture content; densities varied from a loosely-poured condition to the maximum density obtainable by heavy ramming. The general findings of the investigation are as follows: Effect of Temperature. Soils were tested at several mean .temperatures. The degree of influence of temperature depends upon whether it is above or below frees-