Document bO9z8Mv8qnv9neY5VmwNR6wm0

American Society of Heating and Ventilating Engineers Guide, 1937 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: Ur X Uce (6) where U = combined coefficient to be used with ceiling area. UT = coefficient of transmission of the roof. Z7ce -- coefficient of transmission of the ceiling. n -- the ratio of the area of the roof to the area of the ceiling. * Stating the formula in terms of the total heat resistance of the ceiling and roof, 1 ,, 1, 1 U R Uce + Ur X n . (7) In selecting'the values to be used for UT and t/ce 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 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 Z7r. In this case an approximate value of Ur 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. 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 128 Chapter 5;--Heat Transmission Coefficients and Tables { 32 F. Additional information on the inside and outside temperatures to be used in heat loss calculations is given in Chapter 7. REFERENCES effects of Air Velocities on Surface Coefficients, by F. B. Rowley, A. B. Algren and [ L. Blackshaw (A.S.H.V.E. Transactions, Vol. 36, 1930). Wind Velocity Gradients Near a Surface and Their Effect on Film Conductance, by F C Houghten and Paul McDermott (A.S.H.V.E. Transactions, Vol. 37, 1931). Conductivity of Concrete, by F. C. Houghten and Carl Gutberlet (A.S.H.V.E. Transactions, Vol. 37, 1931). Surface Coefficients as Affected by Direction of Wind, by F. B. Rowley and W. A. Eckley (A.S.H.V.E. Transactions, Vol. 37, 1931). Insulating Effect of Successive Air Spaces Bounded by Bright Metallic Surfaces, by L W. Schad (A.S.H.V.E. Transactions, Vol. 37, 1931). Importance of Radiation in Heat Transfer Through Air Spaces, by E. R. Queer (A.S.H.V.E. Transactions, Vol. 38, 1932). Properties of Metal Foil as an Insulating Material, by J. L. Gregg (Refrigerating Engineering, May, 1932). Thermal Insulation with Aluminum Foil, by R. B. Mason (Industrial and Engineering Chemistry, March, 1933). The Heat Conductivity of Wood at Climatic Temperature Differences, by F B. Rowley (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, June, 1933). Thermal Insulation of Buildings, Technical Paper No. 11 (American Architect, May, 1934). Insulating Value of Bright Metallic Surfaces, by F. B. Rowley (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, June, 1934). Thermal Properties of Concrete Construction, by F. B. Rowley, A. B. Algren and Clifford Carlson (A.S.H.V.E. Journal, Heating, Piping and Air Conditioning, January, 1936). Thermal Properties of Concrete Construction, by F. B. Rowley, A. B. Algren and Robert Lander (A.S.H.V.E. Journal Section, Heating, Piping and Air Conditioning, November, 1936). Heat Insulation as Applied to Buildings and Structures, by E. A. Allcut, University of Toronto, 1934. House Insulation, Its Economies and Application, by Russell E. Backstrom (Report of the National Committee on Wood Utilization, United States Government Printing Office, 1931). Heat Transmission Through Building Materials, by F. B. R.owley and A. B. Algren, University of Minnesota Engineering Experiment Station Bulletin No, 8. Calculation of Heat Transmission, by Margaret Fishenden and Owen A. Saunders. Heating, Ventilating and Air Conditioning, by Harding and Willard, Revised Edition, 1932. Heat Transmission, by W. H. McAdams. Industrial Heat Transfer; by Shack. PROBLEMS IN PRACTICE 11 What is the coefficient U and how is it applied? The coefficient U is the heat loss through walls, ceilings, and floors and the value depends upon the construction and material, expressed in Btu per'hour per square foot per degree difference in temperature between the inside and. outside. To determine the total heat loss, multiply U for each material by the square feet of surface and the temperature difference. 129 , i