Document mqqa8gyrJ5099bV3RyjGd697J

116 CHAPTER 6 properly described as one of the following: -- = over-all or air-to-air resistance 1949 3uide - = resistance per unit thickness (resistivity) --c = resistance of a material (surface-to-surface) -- = film or surface resistance air space resistance Heat Transmission Coefficients of Building Materials 117 and ^ fix .. 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 over-all coefficient of heat transmission for a given construction is comparatively simple, but accurate values of conductivities and conductances must be used to obtain satisfactory re sults. In addition there are sometimes parallel heat flow paths of different Fig. 1. Typical Variation of Therhal Condoctivitt with Density--for Fibrous Material CALCULATING OVER-ALL 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 -1 fli *4~ R H* R -4- 4" Rn v (1) where, Ri + Rt, etc. are the individual resistances of the wall components.; Rt = total resistance: \ , .. For a wall of a single homogeneous material of conductivity k and thick ness a;, with surface coefficients/i and/0, , L . Then by definition; Rt = -1+ *7 +1/i * u . :/ (2) . For a wall with air space construction arid consisting of two homogeneous materials of conductivities fci-and fci, thicknesses ii and ^.respectively, and separated by an air space of conductance o, - Fig. 2. Typical Variation of Thermal Conductivity with Mean Temperature resistances in the same wall, and these may necessitate modification of the formula. In such cases calculated results should be checked by test meas urements. The determination of the fundamental conductivities and conductances requires considerable skill and experience to obtain accurate results. It is recommended that thermal conductivities of homogeneous materials be determined by means of the Guarded Hot Plate1. 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, because coefficients for the same material 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 coraidered homogeneous.... Most, insulating materials,. except air spaces and reflective types, are of a porous nature and consist of combinations of solid matter, with small air cells. The thermal' conductivity of these X