Document wKYqYQBaz9OMxbY2KJYJz54V4

104 CHAPTER 9 1960 Guide and tMcVgays of the homogeneous material, the conductance of non-homogeneous materials (such as concrete blocks), the surface conductances of both sides of the construction, and the conductances of any contained air spaces. These items are discussed in the pages that follow. Conductivities and Conductances The method of calculating the overall coefficient of heat transmission for a given construction is comparatively simple, but accurate values of conductivities and conductances must be used to obtain satisfactory results. In addition, there are sometimes parallel heat-flow paths of different resistances 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 ob tain accurate results. It is recommended that thermal con ductivities of homogeneous materials be determined by means of the Guarded Hot Plate.1 For determination of conduc tances, a Guarded Hot Box method* is generally used. Conductivity of Homogeneous Materials Thermal conductivity is a property of a homogeneous ma terial and of types of building materials such as lumber, brick, and stone, which may be considered homogeneous. Most insulating materials, except reflective types, are of a porous nature and consist of combinations of solid matter with voids. Such materials including fibrous, cellular, or granular matter are generally known as mass or bulk in sulations. The thermal conductivity of these materials will vary with density; mean temperature; size of voids, fibers, or particles; degree and extent of bond between particles; mois ture present; and the arrangement 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. Typical variation of conductivity with mean tempexatureis shown in Fig. 2. Thermal Conductivity of Soil The following statements are based largely on results of a study* made in the Engineering Experiment Station, Uni versity of Minnesota, and published 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 tests varied from air-dried values fig. 1.... Typkai Variation of Thermal Conductivity with Density--for fibrous Material fig. 2 .... Typical Variation of Thermal Conductivity with Mean Temperature to those greater than the optimum moisture content; densi ties varied from & 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 freezing. For increases of moisture content exceeding about 6 to 12 percent, the conduc tivity of frozen soil becomes progressively greater than that of the unfrozen soil. Effect of Density. Density affects the thermal conductivity of a soil in about the Mmi'i manner for all soils, at any moisture content, and for either the frozen or unfrozen condition. On the average, each one pound per cubic foot increase in dry density increases the thermal conductivity by about 3 per cent. Effect of Moisture. An increase in moisture content, up to the point of saturation, causes an increase in thermal con ductivity. The rate of increase in typical soils was as follows: average conductivities, in Btu per (square foot) (hour) (Fahrenheit degree per inch), of four sands at a density of 110 lb per-cu ft were: 6.8 at 2.5 percent moisture, 8.9 at 5 percent moisture, 11.2 at 10 percent moisture: Five soils of a fine texture at a density of 100 lb per cu ft, gave average con ductivities of 6.7 at 10 percent moisture, and 9.5 at 20 per-; cent. Thus, the doubling of moisture content within the ranges cited increases the conductivity by approximately 30 or 40 percent. At higher moisture contents tire percentage increase would be less. Effect of Soil Characteristics. The thermal conductivity of the soil, at a given density and moisture content, varies in general with the texture of a soil, being relatively high for coarse-textured soils and relatively low for fine-textured soils. The mineral composition of the soils also affects the conduc tivity. Quart* tends to give high values, whereas minerals such as ptagioclase-feldsp&r and pyroxene, which are constituents' ofjbasic rocks tend to give low values of thermal conductivity. These points are illustrated by the values in Table 1 which lists seventeen soils in approximate order of their magnitude of thermal conductivity from greatest to least for seven dif-' ferent density-moisture content conditions. Some of the values in this table have been determined by extrapolation and are consequently approximate. Blank spaces in tire table indicate that the density or moisture content, or both, are such that no tests were possible for that condition or that no tests were sufficiently close to permit a reasonable extrapolation of the data. Granular soils, particularly those with high quartz con tents, head the tabulation or have tire greatest conductivity HeatSfransmission Coefficients of Building Materials 105 Table I___ Thermal Conductivity (k) Value, of Soils in Approximate Order of Decreasing Values* Mean TMipewtan--40 F MmiJiattical AnoJ/iU % by Weipfcf Moirfwe Content--% Sofl 0ignotion Grovel Over 2> am Send sat Goy 0-5 to 0.005 te 2.00 mm 0.05 mm 0.005 rora 100 4 10 Pry Oenrfiy-A par w ft 110 120 90 no 20 90 100 Lowell Sand.................-.................. Crushed.Trap Rock........................ 0.0 15.5 0.0 27.5 0-0 80.0 25.5 16.2 27.0 Fairbanks Silt Loam........... ........ Fairbanks Silty Clay Loam......... Northway Silt Loam...................... 0.0 3.0 0.0 0.0 0.0 1.0 100.0 79.0 99.9 70.0 100.0 19.4 70.3 77.0 57.9 63.0 97.0 97.0 1.9 7.6 9.2 21.0 0.0 0.0 5.5 0 .1 2.5 0.0 0.0 0-6 4.2 6.8 21.2 10.0 10.0 27.5 3.0 0.0 20.1 80.9 18.5 0.0 0.0 78.0 11.5 63.8 64.4 27.0 13.6 12.0 16.0 11.5 16.0 10.0 14.0 8.5=fc 10-5 8.5 11.0 22.0 13.6 9.0b 13.0 6.0 7.5 9.5 5.5 7.5 10.0 6.5 9.5 5.0 6.0 7.0 4.5 4,5 4.5 4. Orb 6.5 5.5 6.0 15.0 13.5 13 10.0 8.5 7.5 5.5 9.0 8.0 10.0 5.0 9.0 7.5 5.0 9.0 7.5 9.5 4.0=fc 7.0 e.Osb 7.0 31 7* ico %n *0 2 i "TO UOISTURE CONTEXT - PERCENT Is SILT 1 CLAY SOILS FROZEN Vu N s\V ft, V N|V \ \\ i -s. )O MOISTURE CONTENT - PERCENT Fig. 3 .... Determining Thermal Conductivity of Sals from Density and Moisture Content at a given condition. Sandy loam soils are midway in the table and finft gmined soils such as clay and silt loam are last. Estimating Thermal Conductivity. The four diagrams of Fig. 3 are presented to aid in the estimate of the thermal con ductivity of any soil. Two of the charts are for sands or sandy soils, and two for sit and clay soils. One of the diagrams for each type of soils is for the frozen, and the other for the un frozen condition. It is expected that these charts will give conductivity values with a precision of 25 percent. The effect of such factors as density, moisture content, freezing, or tex ture may be easily approximated by use of these graphs. Specific Heat of Soils Tests to determine specific heat were run on twelve soils. On five of tire soils, tests were made at three or four mean temperatures ..varying from about 10 to 140 F. The specific heat values of all twelve soils varied-by oniy-a small amount (about 0.01), and averaged 0.19 at 140 F. The specific heat values of the soils decreased with a decrease in temperature. The average value at zero F would be about 0.16. Values at temperatures between zero and 140 F can be estimated by considering a straight-line relationship between (he two values given. Surface Conductance The surface conductance of a wall is the combined heat transfer to or from the wall by radiation, convection, and con duction. Each of the three portions making up the total may vary, independently of the others, thus affecting the total conductance. The beat transfer by radiation between two surfaces is controlled by the character of the surfaces (emissivity), the temperature difference between them, and the solid ftogfo through which they see each other. The heat transfer by convection and conduction is controlled by the roughness of the surface, by air movement, and temperature difference between the air and the surface. The importance of the effect of temperature of surrounding surfaces on the surface conductance, due to the effect on radia-