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216 CHAPTER 10 1954 Guide and vapor control measures are not essential. Normally the( cooled air is little, if any, colder than the dew-point temperature of the outside at mosphere and there are no areas of condensation. However, the interior vapor pressureis often below that outside and therefore, vapor, which diffuses inward, adds to the cooling load.' Vapor barriers in walls installed for .winter needs, are a help in reducing this element of the cooling load: REFERENCES , 1 Research in Rome Humidity Control, by S. C. Hite and J. L. Dray (Purdue University, Engineering Experiment Station, Research.series No.108, November 1948). '-Permeance Measurement Improved by Special Cell, by F. A. Joy and E. R. Queer (A.S-H-V.E- Trans actions, Vol. 65, 1949, p.,377). ' 1 Water Vapor Transfer Through'Building Materials, by F. A. Jay, E. R. Queer and R. E. Schreiner (Penn sylvania. Stats CoUcge,Engine&ring:Experiment.Station Bulletin No. 61, December -1948). * Remedial Measures for Building .Construction, by L. V. Teesdale (Report R1710 of U. S. Forest Products Laboratory 1947). & Condensation Control in Dwelling Constructions ( U. S. Housing and Home Finance Agency, 1949). * The Diffusion of Water Vapor Through Various Building Materials, by J. D. -Babbitt (Canadian Jour nal of Research, Vol.17; February, 1939, p. 15). -r-Methods of Moisture:ControI and Their Application to Building-Construction, by F. B. RotvJey, A. B. Algren and C. E. Lund ( University of Minnesota, Engineering Experiment Station-Bulletin No. 17). 8 The Relation of Wall- Construction to Moisture Accumulation in FiU-Type Insulation by .Henry J. Barre :(lowa State Collegeof' Agriculture and Mechanic Arts Agricultural Experiment Station Bulletin No. 271. 1940.) Moisture Migration: A Survey of Theory and Existing. Knowledge, by P. F. McDermott (Refrigerat ing Engineering, August 1941, p. 103). . - - to Permeability of Paint Films to Moisture by R. I. Wray and A. R. Van Vorst (Industrial and Engineer ing- Chemistiif.Vol. 35, p. 842,JS53);-n Water Vapor. Transmission of Building Materials Using Four Different Testing Methods, by R. R-Britton and R. C. Reichel, (17.S.Housing and Homs Finance Agency Technical Bulletin No. 12, January 1950). : 11 Water-Vapor Permeability of Building Papers and Other Sheet Materials, by E. R. Beil,' M. G. SeidJ, and N. T. Krueger. (A.S.H.V;E. Tbansactionb, Vol. 57, 1951, p. 287). 18 Value from unpublished tests of Pennsylvania State College Engineering Experiment Station. BIBLIOGRAPHY - '< Moisture Condensation in Building Walls, by H; W.Wooley (National Bureau of5tondarda, Repbrt BMSB3, K December. 14, 1940),. . ( , ; Condensation of Moisture and Its Relation to Building Construction and Operation, by. F. B. Rowley, :AV B. Algren and C. E. Lund (A.S!H.V:E.:TEANaAcnoN8, VoL 45, 1939, p. 231); ' , A Theory Covering the Transfer of Vapor Through Materials, by- F. B. Rowley (A.S.H.VJ5. Transao . tions, Vol. 45. 1939,.p.545). ... ,, Simultaneous Heat and Vapor Transfer Characteristics of an Insulating Material, by F. G. Hechler, E. R. -1' McLaughlin and E. R. Queer-(A.8.H.V.E. TransactioM, Vol. 48, 1942, p. 505). ^ .Comparative Resistance toiVapor Transmission of Various Building Materials, by L. V. Tees- dale (A.S.H.V.E. Transactions, Vol. 49, 1943, p. 124). Permissible Relative Humidities in Humidified Buildings, by Paul D. Close (A.S.H.V.E; Journal/ Section, Heating, Piping and.Air Conditioning, December, 1939, p. 768). . -` V Condensation Within Walls, by F. B. Rowley, A. B. Aigren-and C. E. Lund (A.S.H.V.B. Transactions, Vol. 44, 1935, p. 85); 1 \-J it' :;:4 CHAPTER 11 INFILTRATION AND VENTILATION Causes of Infiltration, Infiltration Due to Wind Pressure, Infiltration Due to Temperature Difference, Sealing of Vertical Openings, Infiltration Meas urement/Natural Ventilation, Wind Forces, Temperature Difference Forces, Heat Removal, Effect of Unequal Openings, Combined Wind and Temperature Forces, Types of Openings, General Ventilation Rules, Ventilation of Animal Shelters, ' " Garage Ventilation Infiltration is the air leakage through cracks and interstices, around windows and doors, and through floors and walls. Its magnitude depends on the structural design, workmanship, and condition of.the building. The rate of infiltration cannot be controlled by the inhabitants of the building to any considerable extent. Natural ventilation is the controlled displace ment of air through openings; such as windows, doors and. ventilators as well as through combustion heating devices. CAUSES OF INFILTRATION The air leakage which takes place through various apertures in buildings must be estimated in heating-and cooling calculations and enough heating or cooling capacity provided to offset the: heat lost or gained by the air leakage. The rate of air flow into and out of a building depends oh the magnitude of the pressure difference between the inside and outside of the structure and on the resistances presented to this pressure difference. The pressure difference exerted on the building walls by the air may be caused either by wind or by a difference in density of the air inside and outside the building. The effect of the wind depends on the interrelation' of the speed and direction of the wind and the exposure of the building. The effect of the difference in the density of the air depends on the magnitude of the inside-outside temperature difference, the height of the rooms,..the shape of the openings, and their elevation in the room or building. ? The effect of the difference in density is often referred to as the chimney or stack effect. The pattern of air flow through any part of the structure de- pends on both the pressure difference and the openings. In general, when the pressure difference is the result of wind pressure, air .will enter .the building through openings in the windward walls and leave through`open ings in the leeward walls or through ventilating ducts in the roof. When the pressure difference is caused by the inside-outside temperature differ ence the flow will be along the path of least resistance from inlets at lower levels to outlets at higher levels in a heated building or in the opposite direction for an air-conditioned building. < . An exact estimate of the amount of infiltration under design conditions !s difficult to make. The complicating factors include (1) variations m building construction, particularly as to width of crack or size of openings through which air -leakage takes place; (2) the variations,-in Wind velocity and direction; (3) the exposure of. the building-with respect wi air leakage openings, and with respect to adjoining buildings; (4) the 217