Document r62dJ9Eq8R9X5NJ9BqyX5ZaOG

CHAPTER 5 Air Leakage_________ ;________ ill oCeakiaye Causes of Infiltration; Infiltration Due to. Wind Pressure, Infiltration Through Walls, Window and Door Leakage, Crack Method, Air Change Method, Infiltration Due to Temperature Difference, Sealing of Vertical Openings THE air leakage which takes place through various apertures in s. buildings must be considered in heating arid cooling calculations, and properly evaluated. This infiltration as it is sometimes designated takes place through cracks around doors and windows, through solid walls and through fireplaces and chimneys. Although the latter sources of leakage may be considerable, they are often neglected on the assump S tion that dampers would be closed during periods of extreme cold weather or else that the fireplace will be in use at such times and will therefore contribute to the heat supplied and therefore lessen the heating load. Infiltration Through Walls - Data oninfiltration 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 to 25 per cent of that through windows and doors in a 10-story office building, 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 CAUSES OF INFILTRATION Table 1. Infiltration Through Walls3 Expressed in cubic feet per square foot per hour The displacement of heated air in buildings by unheated outside air is due to two causes, namely, (1) the pressure exerted by the wind and (2) the difference in density of outside and inside air because of differences in Ttp* or Wall Wiftn Vblocitt, Miles per Hour 5 10 IS 20 25 30 . temperature. The former is generally referred to as infiltration and the latter as stack or chimney effect. In either case an exact estimate of the amount of infiltration under .t design conditions is difficult to make. The complicating factors include (1) variations in building construction particularly as to width of crack 8H in. Brick Wall {pJasteredi: 2 0.02 4 0.04 8 12 19 23 0.07 0.11 0.16 0.24 or. size of openings through which air leakage takes place, (2). the varia tions in wind velocity and direction, (3) the exposure of the building with 13 m. Bnck Wan--(plaste7ed;;:: 1 0.01 4 0.01 7 12 16 21 0.03 0.04 0.07 0.10 respect to air. leakage openings and with respect to adjoining buildings, (4) the variations in outside temperatures as influencing the chimney effect, (5) the relative area and resistance of openings on the windward -a Frame Wall, with lath and plaster** 0.03 0.07 0.13 0.18 0.23 0.26 and leeward sides and on the lower floors and on the upper floors, and (6) the influence of a planned air supply and the related outlet vents. Tight 'construction is essential for preventing large heat loss due to infiltration. . 8The values given in this table are 20 per cent less than test values to allow for building up of pressure in rooms and are based on test data reported in the papers listed in chapter footnotes. bWall construction: Bevel siding painted or cedar shingles, sheathing, building paper, wood lath and 3 coats gypsum plaster. INFILTRATION DUE TO WIND PRESSURE 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. "i. 4 't 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. , A-S.H.V.E. Research Reports No. 786--Infiltration Through Plastered and Unplastered Brick Walls, 5? , Sr 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 ir* Braatz (A.S.H.V.E. Transactions, Vol. 35, 1929, p. 183). No. 851--Air Infiltration Through Various lypes of Wood Frame Construction, by G. L. Larson, D. W. Nelson and C. Braatz (A.S.H.V.E. Trans actions, Vol. 36, 1930, p. 99).