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Chapter 1
Introduction
1.2.a Stack Effect:
Stack effect is the tendency for warm buoyant air to rise and leak out the top of the building and be replaced by
colder outside air entering the bottom of the building (note: when outside air is warmer than inside air, this
process is reversed). In winter, the stack effect creates a small positive pressure at the top of the building and
small negative pressures at the bottom of the building. Stack effect pressures are a function of the temperature
difference between inside and outside, the height of the building, and are strongest in the winter and very weak
in the summer. Stack induced air leakage accounts for the largest portion of infiltration in most buildings.
1.2.b Wind Pressure:
Wind blowing on a building will cause outside air to enter on the windward side of the building, and building air
to leak out on the leeward side.. At exposed sites in windy climates, wind pressure can be a major driving force
for air leakage.
1.2.c Point Source Exhaust or Supply Devices:
Chimneys for combustion appliances and exhaust fans (e.g. kitchen and bath fans) push air out of the building
when they are operating. Air leaving the building from these devices causes a negative pressure in the building
which draws outside air into holes and cracks in the building envelope. Supply fans (e.g. positive pressure
ventilation fan) deliver air into the building creating a positive pressure which pushes inside air out of the
building through holes and cracks in the building envelope. (The interaction of ventilation fans on building air
leakage and pressures is discussed in Chapter 10)
1.2.d Duct Leakage to the Outside:
Leaks in forced air duct systems (to the outside) create pressures which increase air leakage in buildings. Leaks
in supply ducts act like exhaust fans causing negative building pressures. Leaks in return ducts act like supply
fans creating positive pressures in buildings. (Duct leakage and duct leakage diagnostics are discussed in more
detail in Chapter 9).
1.2.e Door Closure Coupled with Forced Air Duct Systems:
Research has shown that in buildings with forced air duct systems, imbalances between supply and return ducts
can dramatically increase air leakage. For example, a study conducted in Florida showed that infiltration rates in
many houses were doubled whenever the HVAC system fan was operating due to pressures caused by door
closure. In the Florida houses, closing of bedroom doors created large duct imbalances by effectively cutting off
the bedroom supply registers from the central return registers located in the main part of the house. (Duct
leakage and duct leakage diagnostics are discussed in more detail in Chapter 9)
1.3 Common Air Leakage Sites
Common air leakage sites are shown in Figure 3 below. Notice how as warm air rises due to the stack effect, it
tends to escape through cracks and holes near the top of the building. This escaping air causes a slight negative
pressure at the bottom of the building which pulls in cold air through holes in the lower level. Air sealing
activities should usually begin at the top of the building because this is where the largest positive pressures exist
and where many of the largest leakage sites (and potential condensation problems) can be found.
4
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