Handbook of air conditioning and refrigeration / Shan K

20.2 BUILDING LEAKAGE AREA AND BUILDING TIGHTNESS
Effective Leakage Area
Building Air Leakage
AIR SYSTEMS: BASICS AND CONSTANT-VOLUME SYSTEMS 20.5
Currently, the building tightness or the building air leakage can be indicated by the following
indices: (1) an effective leakage area, (2) air leakage per unit area of building shell, and (3) air
change per hour (ACH) at a specific indoor-outdoor pressure differential.
The effective leakage area of a building Ae,l, in ft2 (m2 ), is the amount of open wall area that allows
the same volume flow rate of air flowing through it as flowing through the actual building when the
pressure difference across the open wall area and the shell of the actual building is the same. The
Ae,l value is usually determined by a fan pressurization method, in which a blower installed at the
door of a building is used to maintain a pressure difference across the building shell. The airflow
V˙
rate , in cfm (m 3 ), can then be measured , and the A e,l value is calculated as
2 �p
V˙ � Cd Ae,1√ � � C d A e,l 4005√�p
where C d � discharge coefficient; its value is 1 for effective leakage area
� � air density, lb/ft 3 (kg/m 3 )
�p � pressure differential between outdoor and indoor air at same level, in.
WC (Pa)
(20.1)
According to ASHRAE Handbook 1997, Fundamentals, effective air leakage areas for representative
residential building components at an outdoor-indoor pressure difference of 0.016 in. WC
(4 Pa) are as follows:
When air flows through a crack or gap, the flow never becomes fully developed. The relationship
between overall air leakage rate V˙ leak, in cfm (L/s), and the indoor-outdoor pressure difference �p,
in in. WC (Pa), can often be represented by a widely used air leakage equation as
V˙ leak � C flow A w (�p) n
Best estimate, in. 2 /ft 2
External walls
Cast-in-place concrete 0.007
Clay brick cavity wall, finished 0.0098
Lightweight concrete block, painted 0.016
Window
Wood frame
Uncaulked 0.025
Caulked 0.004
Double-hung, not weatherstripped 0.12
Double-hung, weatherstripped 0.031
Door, average 0.015 in. 2 /lftc (linear ft. crack)
Ceiling 0.026
(20.2)