Document 822N8qY45jjoJEQa0K3v93oN5

American Society of Heating and Ventilating Engineers Guide, 1934 yellow pine floor (actual thickness, 25/32 in.) placed directly on 6 in. concrete on the ground, is determined as follows: 1_______ U= 1 0.781 6.0 = 0.48 Btu per hour per square foot per degree difference 1.65 + 0.80 + 12.0 in temperature between the ground and the air immediately above the floor. The thicknesses upon which the coefficients in Tables 8 to 18, inclusive, are based are as follows: ' Brick veneer........ ...................................................................................... 4 in. Plaster and metal lath........................................................................... M in. Plaster (on wood lath, plasterboard, rigid insulation, board form, or corkboard)........ ...................... ...................................... K in. Slate (roofing)........................................................................................ 14 in. Stucco on wire mesh reinforcing...... .............................................. 1 in. Tar and gravel or slag-surfaced built-up roofing ........................ 1- in. Lumber (S-2-S)...1........................................ ,............................. lj^-in. Lumber (S-2-S)............. ................ ........................................... ljf6 in. 2- in. Lumber (S-2-S)......... ........................................................ ;...... 1% in. 2y$-in. Lumber (S-2-S).......................................................................... 2ys in. 3- in. Lumber (S-2-S)................................................................. :....... 2% in. 4- in. Lumber (S-2-S)......................................................................... 3% in. Finish flooring (maple or oak).__........ .............................................. 1Hs in. Solid brick walls are based on 4-in. face brick and the remainder common brick. Stucco is assumed to be 1 in. thick on masonry walls. Where metal lath and plaster is specified, the metal lath is neglected. Rigid insulation refers to the so-called board form which may be used structurally, such as for sheathing. Flexible insulation refers to the blankets, quilts or semi-rigid types of insulation. Actual thicknesses of lumber are used in the computations rather than nominal thicknesses. The computations 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 were taken 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 17 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. Combined Coefficients of Transmission If the attic is unheated, the roof structure and ceiling of the top flodr 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: ' v = ,U*XUc* n X Ut -{- C/ce (6) 80 Chapter 5--Heat Transmission where 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. Example 1. Calculate the coefficient of transmission (U) of an 8-in. brick wall with V in of plaster applied directly to the interior surface, based on an outside wind exposure f 15 mph. It is assumed that the outside course is of face brick having a conductivity f 9 20 and that the inside course is of common brick having a conductivity of 5.0, the thicknesses each being 4 in. The conductivity of the plaster is assumed to be 3.3, and the inside and outside surface coefficients are assumed to average 1.65 and 6.00, respectively, for still air and a 15 mph wind velocity. Solution, k (face brick) = 9.20; x = 4.0 in.; k (common brick) = 5.0; x = 4.0 in.; k (plaster) = 3.3; * = \4'm.\fi = 1.65;/0 = 6.0. Therefore, 1 U= 1 4.0 4.0 0.5 6.0 + 9.20 + 5.0 + 3.3 1 1.65 1 - 0.167 + 0.435 + 0.80 + 0.152 + 0.606 = 0.46 Btu per hour per square foot per degree Fahrenheit difference in tempera ture between the air on the two sides. Computed Transmission Coefficients Computed heat transmission coefficients of many common types of building construction are given in Tables 8 to 18, inclusive, each con struction being identified by a serial number. For example, the coefficient of transmission ( U) of an 8-in. brick wall and in. of plaster is 0.46, and the number assigned to a wall of this construction is 1-B, Table 8. 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, the average coefficient of a surface in still air is 1.65 Btu per hour per square foot per degree Fahrenheit, whereas the average coefficient of an air space in an outside wall is about 1.10 Btu per hour per square foot per degree Fahren heit difference between the two surfaces, at a mean temperature of 40 F. 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 over-all 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 t/Ce 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 Tieat 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, 81