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HEATING VENTILATING AIR CONDITIONING GUIDE 1940 CHAPTER 5. HEAT TRANSMISSION COEFFICIENTS AND TABLES Rigid insulation refers to so-called insulation board which may be used structurally, such as for sheathing. Flexible insulation refers to the. blankets, quilts or semi-rigid types of insulation. !i Actual thicknesses of lumber are used in the computations rather than nominal thicknesses. The computations for wood shingle roofs applied I! 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 V'll 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 Stating the formula in terms of the total heat resistance of the ceiling and roof, 1 U >R = U,, +1 Ut X n (7) In selecting the values to be used for Ut and Z7ce it should be noted that the under surface of the roof and the upper surface of the ceiling are more nearly equivalent to the boundary surfaces of an internal air space than they are to the external surfaces of a wall. It would be more nearly correct.to use a value of 2.2 rather than the usual value of 1.65 as coef ficients for these surfaces. In most cases this would make only a minor compare favorably. The thicknesses upon which the coefficients in Tables 3 to 13 inclusive, are based are as follows: Brick veneer............................................ ................................ ............. 4 in. Plaster and metal lath;.................................. ..................................... Vi in. Plaster (on wood lath, plasterboard, rigid insulation, board form, or corkboard)______________________ ___ ___ __________ M in. Slate (roofing)----------- -------- ----------------------------------------- -......-- Yi in. Stucco on wire mesh reinforcing-..................i................................... 1 in. Tar and gravel or slag-surfaced built-up roofing % in. 1-in. lumber (S-2-S)____________________________ ______ ______ 2?32 in. 113-in. lumber (S-2-S).........................................................................1M6 in. 2-in. lumber (S-2-S).......... ................................. ............ ...................in. 2J3-in. lumber (S-2-S)..................................................... .................... 2)4 in. 3-in. lumber (S-2-S)........................................... ............................. -- 2% in. 4-in. lumber (S-2-S)---------------------------------------------------------------- 3% in. Finish flooring (maple or oak)____ ____________ _________ _____ e in. change in U. It should be noted that the over-all coefficient should be multiplied by the ceiling and not the roof area. If the unheated attic space between the roof and ceiling has no dormers, windows or vertical wall spaces the combined coefficients may be used for determining the heat loss through the roof construction between the attic and top floor ceiling. If the unheated attic contains windows and vertical wall spaces these must be taken into consideration in calculating the roof area and also its coefficient Ur. In this case an approximate value of Ut may be obtained as the summation of the coefficient of each individual section such as the roof, vertical walls or windows times its percentage of total area. This coefficient may be used with reasonable accuracy in the above formulae. If, however, there are roof ventilators such that the attic air is substantially at outside temperature, then the roof should be neglected and only the coefficient for the top floor ceiling construction used; , Solid brick walls are based on 4 in. hard brick (high density) and the remainder common brick (low density). Stucco is assumed to be 1 in. thick on masonry walls. Where metal lath and plaster are specified, the metal lath is neglected. The coefficients of transmission of the pitched roofs in Table 12 apply where the roof is over a heated attic or top floor so the heat passes directly through the roof structure including whatever finish 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 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 ceiling area, is as follows: Ut X C/ce U=UT+? (6) where U = combined coefficient to be used with ceiling area. Basements and Unheated Rooms The heat loss through floors into basements and into unheated rooms kept closed may be computed by assuming a temperature for these rooms of 32 F. The coefficients of transmission for concrete floors on ground (Table 10) are based on the assumption that the heat-resisting value of the floor extends downward and stops at the under side of the concrete. It is probable, however, that the dirt underneath has some heat-resistance value extending to a considerable depth, which would result in substan tially lower heat transmission coefficients than given in Table 10. This subject is being investigated. Additional information on the inside and outside temperatures to be used in heat loss calculations is given in Chapter 7. REFERENCES A.S.H.V.E. Research Reports: No. 852--Effects of Air Velocities on Surface Coefficients, by F. B. Rowley, A. B. Algren and J. L. >Blackshaw (A.S.H.V.E. Transactions, Vol. 36, 1930, p. 123). No. 895--Wind Velocity Gradients Near a Surface and Their Effect on Film Con ductance, by F. C. Houghten and Paul McDermott (A.S.H.V.E. Trans actions, Vol. 37, 1931, p. 301). No. 914--Surface Coefficients as Affected by Direction of Wind, by F. B. Rowley and W. A. Eckley (A.S.H.V.E. Transactions, Vol. 38, 1932, p. 33). No. 915--Conductivity of Concrete, by F. C. Houghten and Carl Gutberlet (A.S. Ut = coefficient of transmission of the roof. bce = coefficient of transmission of the ceiling. -- the ratio of the area of the roof to the area of the ceiling. H.V.E. Transactions, Vol. 38, 1932, p. 47). No. 964--The Heat Conductivity of Wood at Climatic Temperature Differences, by F. B. Rowley (A.S.H.V.E. Transactions, Vol. 39, 1933, p. 329). 114 115