Environmental Engineering Reference
In-Depth Information
Figure 11.5
Infiltration rates measured in 312 North American houses in the early
1980s. (From Grimsrud, D.T. et al., LBL-9416, Lawrence Berkeley Laboratory, Berke-
ley, CA, 1983.)
more than 300 U.S. houses was measured on a one-time basis. As can be
seen in
Figure 11.5
,
approximately 80 to 85% of houses tested had a daily
average air exchange rate of <1 air change per hour (ACH). Infiltration/exfil-
tration rates in low-income housing were on average significantly higher;
approximately 40% had infiltration values >1 ACH.
On a population basis, these one-time measurements of infiltration/exfil-
tration-induced air exchange were likely to have demonstrated a reasonable
estimate of ventilation conditions in housing stock existing at the time (early
1980s). Since then, construction practices have changed (tighter building
envelopes are now the norm), and significant weatherization measures have
been implemented to reduce energy losses in low-income housing. Weath-
erization measures using retrofit tightening of building envelopes in low-
income housing have, however, only been moderately effective (on average,
≤
25% reduction in building leakage and infiltration-associated air exchange).
It is highly probable that construction practices in the past several decades
have significantly increased the stock of housing units in North America,
northern Europe, and other developed regions and countries which have
lower air exchange (and thus ventilation) rates than older houses. Decreasing
natural ventilation rates have been a cause for concern among policy makers
in various governmental agencies, utilities which have supported weather-
ization measures, public health groups, and research scientists. It was and is
widely believed among environmental and public health professionals that
decreasing natural ventilation rates associated with infiltration/exfiltration-
reducing measures are likely to cause an increase in indoor contaminant levels
and health risks associated with increased exposures.
D. Leakage characteristics
Air exchange rates in buildings associated with thermal and wind-induced
pressure differences are affected to a significant degree by building leakage
characteristics. Typical leakage areas are indicated in
Figure 11.6
for a single-
