Document 0gqa5GNjEX7RYv6m57oYVbQrn

HEATINC VENTILATING AIR CONDITIONING GUIDE 1941 greater the tendency for condensation to take place on the under side of the roof boards which moisture will drop on to the ceiling. Thus where thick insulations are installed between ceiling joists, it is desirable to allow openings for outside air circulation through attic space as a precaution against condensation on the underside of the roof even though barriers are used in the ceiling below. 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. 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). No. 996--Insulating Value of Bright Metallic Surfaces, by F. B. Rowley (A.S.H.V.E. Transactions, Vol. 40, 1934, p. 413). No. 1026--Thermal Properties of Concrete Construction, by F. B. Rowley, A. B. Algren and Clifford Carlson (A.S.H.V.E. Transactions, Vol. 42, 1936, p. 33). No. 1048--Thermal Properties of Concrete Construction, by F. B. Rowley, A. B. Algren and Robert Lander (A.S.H.V.E. Transactions, Vol. 43, 1937, P-33). Insulating Effect of Successive Air Spaces Bounded by Bright Metallic Surfaces, by L. W. Schad (A.S.H.V.E. Transactions, Vol. 37, 1931, p. 285). Importance of Radiation in Heat Transfer Through Air Spaces, by E. R. Queer (A.S.H.V.E. Transactions, Vol. 38, 1932, p. 77). Condensation of Moisture and Its Relation to Building Construction and Operation, by F. B. Rowley, A. B. Algren and C. E. Lund (A.S.H.V.E. Transactions, Vol. 45, 1939, p. 231). A Theory Covering the Transfer of Vapor Through Materials, by F. B. Rowley (A.S.H.V.E. Transactions, Vol. 45, 1939, p. 545). 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). Thermal Insulation of Buildings, Technical Paper No. 11 (American Architect, May, 1934). Heat Insulation as Applied to Buildings and Structures, by E. A. Allcut, University of Toronto, 1934. House Insulation, Its Economies and Applications, 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. Rowley and A..B. Algren, University of Minnesota Engineering Experiment Station Bulletin No. 8. Aluminum Foil Insulation (National Bureau of Standards Letter Circular No. LC535, October 15, 1938). Effect of Ceiling Insulation upon Summer Comfort, by T. D. Phillips (National Bureau of Standards, Report BMS52, July 1, 1940). 100 Chapter 4 AIR LEAKAGE Nature of Air Infiltration, Infiltration Through Walls, Window Leakage, Door Leakage, Selection of Wind Velocity, Crack Length used for Computations, Multi-Story Buildings, Heat Equivalent of Air Infiltration AIR leakage losses are those resulting from the displacement of heated air in a building by unheated outside air, the interchange taking place through various apertures in the building, such as cracks around doors and windows, fireplaces and chimneys. This leakage of air must be considered in heating and cooling calculations. (See Chapters 5 and 6.) NATURE OF AIR INFILTRATION The natural movement of air through building construction is due to two causes. One is the pressure exerted by the wind; the other is the difference in density of outside and inside air because of differences in temperature. The wind causes a pressure to be exerted on one or two sides of a building. As a result, air comes into the building on the windward side through cracks or porous construction, and a similar quantity of air leaves on the leeward side through like openings. In general the resis tance to air movement is similar on the windward to that on the leeward side. This causes a building up of pressure within the building and a lesser air leakage than that experienced in single wall tests as determined in the laboratory. It is assumed that actual building leakages owing to this building up of pressure will be 80 per cent of laboratory test values. While there are cases where this is not true, tests in actual buildings substantiate the factor for the general case. Mechanical ventilating systems are frequently designed to produce positive or negative pressures in an enclosure which are greater or lower than prevalent wind pressures. In such designs, if the rate at which air is specified to be introduced to or removed from the enclosure by positive means exceeds the infiltration rate, it is common practice to use the greater value in determining -the heating capacity to warm the outside air. The air exchange owing to temperature difference, inside to outside, is not appreciable in low buildings. In tall, single story buildings withopenings near the ground level and near the ceiling, this loss must be considered. Also in multi-story buildings it is a large item unless the sealing between various floors and rooms is quite perfect. This tempera ture effect is a chimney action, causing air to enter through openings at lower levels and to leave at higher levels. 101