Document R2JNK24xnBzLJ8pnvoGnkq44n

American Society of Heating and Ventilating Engineers Guide, 1930 Table 10. Conductivities (k) and Conductances (C) of Building Materials and Insulations Based on Tests Conducted at the University of Minnesota, By F. B. Rowley P Material Description Density (Lb. per Cu. Ft.) Mean Temp. (Deg. Fahr.) CoNDUC-a TIVITY (fe) OR Conduc tance (C) Concrete.... Stone 1-2-4 mix... Dry Zero... Pliable slab form of insulation made from ceiba fibres_____ Fir sheathing and building paper........... ........................ Firsheathing, building paper and pine lap siding..........- Firsheathing, building paper and stucco. Gypsum Tile... Solid.. Gypsum Tile_ Solid_______ Gypsum Fibre Concrete.^..... 87M% gypsum and 12)4% wood chips Lath and ^ in. Plaster.____ Masonite......... ....................... Total thickness in........................ .... Rigid insulation made from exploded wood fibre. Pine lap Aiding and building paper....... ............................ Lap siding 4 in. wide.. Plaster. Thickness M in................. ..................... Sheet Rock IVrofill Roofing, Plaster board, gypsum fibre concrete 2)4 in .thick....................... Sprayo-Flake_____ ______ and 3-ply roof covering-----------------Shredded paper with silica binder-------- 143.0 51.8 75.6 51.2 17.9 52.4 5.9 68.8 30.0 20.0 20.0 69.9 75.9 74.4 70.0 77.6 15.5 73.0 76.0 61.1 9.46 0.23 0.71b 0.50b 0.82b 1.66 2.96 1.66 2.50b 0.32 0.85b 8.8b 0.58b 0.28 aln addition to the conductivity values for the authorities listed, considerable work of importance per taining to the heat transmission of various types of construction and materials has been done by the late CPorollfe.gJeo. hn R. Allen and Pro/. A. J. Wood of the Engineering Experiment Station of Pennsylvania State bFor thickness stated or used in construction, not per 1 in. thickness. pSee Chapter LX, by Chas. H. Herter of the Report of the Insulation Committee. A. S. R. Annual Meeting, 19Z2. Revised to 1924, entitled. Heat Transmission of Insulating Materials for a more compre hensive collection of heat transmission data relating to building and insulating materials. Note that actual thicknesses of lumber are used in the computations, rather than nominal thicknesses. On account of the fact that the internal resistances of metal and single thicknesses of building paper (used because of its value as a wind stop' only) and roofing felt are very small, these resistances were neglected in the calculations, in accordance with standard practice. The computation^ for wood shingle roofs applied over wood stripping are based on 1 by 4 in. wood strips, spaced 2 in. apart. Since no reliable figures are available concerning the conductivity of Spanish and French clay roofing tile, of, which there are many varieties, the figures for such types of roofs weretaken the same as for slate roofs, as it is probable that the values of U . for these,two types of roofs will compare favorably. The coefficients of transmission of the pitched roofs in Table 34 apply where the roof is over a heated attic or top floor, such that the Heat passes directly through the roof structure including whatever finish, if any, is applied to the underside of the roof rafters. By a heated attic is meant an attic to which heat is supplied directly from the furnace or boiler by means of radiators, warm-air registers, or other means. If the attic is unheated, the roof structure and ceiling of the top floor must both be taken into consideration, and the combined coefficient,of transmission determined. The formula for calculating the combined coefficient of transmission of a top-floor ceiling, unheated attic space and pitched roof, per square foot of roof area, is as follows: 26 Chapter 2--Heat Losses from Buildings where U = U, X n X Ur Uce Ucc Ur = coefficient of transmission of the roof. Uce -- coefficient of transmission of the ceiling, n = the ratio of the area of the roof to the area of the ceiling. (10) In using this formula, a, correction-factor must be applied. As the amount of heat transferred through an air space is proportional to the difference of the fourth powers of the absolute temperatures of the surfaces enclosing the air space, a greater amount of heat is absorbed or emitted by radiation by the surfaces enclosing an unheated attic than by the surfaces of a wall or ceiling in a room under still-air conditions, where the surrounding objects are only slightly higher in temperature than the interior surfaces of the walls and ceiling; For example: According to the most reliable information at present available, the average coefficient of a surface in still air is 1.34 B.t.u. per hour per square foot per degree fahrenheit, whereas the average coefficient of an air space in an outside wall is about 1.10 B.t.u. per hour per square foot per degree fahrenheit difference between the two surfaces, at a mean temperature of 40 deg. fahr. An air space coefficient of 1.10 is equivalent to a surface coefficient of 2.20 for each of the two surfaces enclosing the air space, where the overall transmission is computed by using the coefficients of the two surfaces enclosing the air space instead of the coefficient of the air space itself. Hence, in determining the values of UT and Uce to be used in the formula, the coefficients for the surfaces of the roof and ceiling enclosing the attic should be increased to allow for the additional amount of heat transferred by radiation, and a coefficient of 2.20 may be used with sufficient accuracy for each of these surfaces, although in very precise work a correction should be made to allow for the fact that the area of a pitched roof over an unheated attic is greater than the area of the ceiling, and hence, the amount of heat absorbed by radiation by each square foot of roof surface is less than is given off by radiation by each square foot of ceiling surface. The following examplelwill illustrate the use of this formula: Determine the combined coefficient of transmission of a roof constructed of asbestos shingles applied over wood sheathing on rafters, an unheated attic, and a wood lath and plaster ceiling, based on a roof having a Ys pitch, for which the value of n is 1.2. Ur = 1 4.02 + 1 2.20 1 6.00 + 0.781 1.00 = 0.605 Uce -- 1.34 T 2.20 + 1 2.00 > 0.588 Substituting these values in the preceding formula: U = 0.605 X 0.588 1.2 X 0.605 + 0.588 = 0.271 B.t.u. per hour,, per square foot of roof area per 1 deg. fahr. difference in temperature between the air near the underside of the ceiling and the -iif* f' ' 27