Environmental Engineering Reference
In-Depth Information
associated with temperature differences (between indoor and outdoor envi-
ronments) and the speed of the wind.
Infiltration occurs as a result of the inflow of air through cracks and a
variety of unintentional openings (leakage areas) in the building envelope.
Infiltration only occurs through leakage areas where internal pressures are
negative relative to those outdoors. In residential structures, infiltration typ-
ically occurs at the base and during windy conditions on the lee (downwind)
side of buildings.
When infiltration occurs, it replaces and displaces air in the building
interior. As a consequence, air must also flow outward through cracks and
other leakage areas. In residential structures, such air outflows (exfiltration)
typically occur through ceiling areas and upper wall locations where internal
pressures are positive.
A. Stack effect
On calm days or during calm periods during the day, infiltration and exfil-
tration occur as a consequence of pressure differences associated with dif-
ferences between the inside and outside temperature (
T). During the heat-
ing season in seasonally cool to cold climates, warm air rises and creates a
positive pressure on ceilings and upper walls of small residential (and non-
residential) buildings. This upward flow of warm air produces negative
pressures, which are at a maximum at the base of the structure. These
negative pressures cause an inflow (infiltration) of cool or cold air, with
maximum inflows where negative pressures are (in absolute terms) the high-
est. Infiltration causes air to be drawn in from both the outdoor environment
and from the ground (soil gas).
An idealized characterization of pressure conditions in a single-family
dwelling on a cool day is diagramed in
Figure 11.2
.
As can be seen, an area
of neutral pressure exists between negative and positive pressure environ-
ments. This is the neutral pressure level or neutral pressure plane (NPP). At
∆
Figure 11.2
Generalized pressure conditions in a small house on a cool, calm day
under closure conditions. (From Lstiburek, J. and Carmody, J.,
Moisture Control Hand-
book
, Van Nostrand Reinhold [John Wiley & Sons], New York. With permission.)
