Document bBqb3yp4LxzX7v9wjpr5Re5p0

HEATING VENTILATINC. AIR CONDITIONING GUIDE 1944 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. Table 1. Infiltration Through Walls Expressed in citbicfeet per squarefoot per hour Ttpe of Wall Wind Vrlocitt, Mn.TM feb Hour 5 10 , 15 20 25 30 / 8^ in. Brick Wall {plastered.." 1.75 4.20 7.85 12.2 18.6 22.9 0.017 0.037 0.066 0.107 0.161 0.236 13 in. Brick Wall {plastered!" 1.44 3.92 7.48 11.6 16.3 21.2 0.005 0.013 0.025 0.043 0.067 0.097 Frame Wall, with lath and plaster1* 0.03 0.07 0.13 0.18 0.23 0.26 The values given in this table are 20 per cent less than test values to allow for building up of pressure m rooms and are based on test data reported in the papers listed at the end of this chapter." bWall construction: Bevel siding painted or cedar shingles, sheathing, building paper, wood lath and v 3'coats gypsum plaster. 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. Infiltration'Through Walls Data on infiltration through brick and frame walls are given in Table l1. The brick walls listed in this table are walls which show poor workman ship and which are constructed of porous brick and lime mortar. For good workmanship, the leakage through hard brick walls with cementlime mortar does not exceed one-third the values given. These tests indicate that plastering reduces the leakage by about 96 per cent; a heavy coat of cold water paint, 50 per cent; and 3 coats of oil paint carefully, applied, 28 per cent. The infiltration through walls ranges from 6 tc 25 per cent of that through windows and doors in a 10-story office building, A.S.H.V.E. Research Reports No. 786--Infiltration Through Plastered and Uriplastered Brick Walla by F, C. Houghten and Margaret Ingels (A.S.H.V.E. Transactions, Vol. 33, 1927, p. 377) No 826--^ Air Infiltration Through Various Types of Brick Wall Construction, by G. L. Larson. D W. Nelson and C. Braatz (A.S.H.V.E. Transactions, Vol. 35, 1929, p. 183). No. 851--Air Infiltration Through Various Types of Wood Frame .Construction, by G. L. Larson, D. W. Nelson and C. Braatz (A S H V E Trans actions, Vol 30, 1930, p. 99). CHAPTER 5. AIR LEAKAGE with imperfect sealing of plaster at the baseboards of the rooms. With perfect sealing the range is from 0.5 to 2.7 per cent or a practically negligible quantity, which indicates the importance of good workmanship in proper sealing at the baseboard. It will be noted from Table 1, that the infiltration through properly plastered walls can be neglected. The value of building paper when applied between sheathing and shingles is indicated by Fig. 1, which represents the effect on outside construction only, without lath and plaster. The effectiveness of plaster properly applied is no justification for the use of low grade building paper or of the poor construction of the wall containing it. Not only is it difficult to secure and maintain the full effectiveness of the plaster but also it is highly desirable to have two points of high resistance to air flow with an air space between them. The infiltration indicated in Fig. 1 is that determined in the laboratory and should be multiplied by the factor 0.80 to give proper working values. Window and Door Leakage There are two methods of estimating air leakage through window and door cracks, namely, (1) the crack method and (2) the air change method. The crack method is generally regarded as being more accurate than the ..purely arbitrary air change method, provided the variables entering into <the crack method, such as crack width and clearance, can be properly evaluated. ' Crack Method The crack method is based on known air leakage factors for various types of windows and widths of crack and clearance. The wind velocity and length of crack are also considered when the crack method is employed. 119 '