indoor-outdoor air leakage of apartments and commercial buildings

7.1. Current Knowledge
Buildings are often divided into two categories: places where people live, which we call
"residential" buildings, and places where people work, which we will call" commercial
buildings" although this is not technically the correct term (since government buildings, schools
and other non-commercial buildings are also workplaces).
Residential buildings can be divided into (1) single-family houses and (2) multi-family
residences. Commercial buildings (as we have defined them above) can be divided into many
sub-categories: office buildings, small or large retail buildings, schools, etc.
Of all of the many categories and sub-categories of buildings, the only category for
which air exchange rates and leakage parameters are well known is single-family detached
houses. Vast amounts of data are available for single-family homes, mostly as a result of
"energy audit" programs that seek to quantify house leakiness or identify leaky homes in order
to implement energy efficiency programs. The available data are subject to selection bias and
other problems, but the overall picture is characterized well enough that most practical
questions that rely on knowledge of the statistical distribution of house leakage parameters can
be answered (Chan et al., 2005).
In contrast, the overall situation for commercial buildings and for apartment buildings
is: data are sparse, and there are complications in both measuring and conceptualizing building
leakage because some commercial buildings are compartmentalized into discrete stores, offices,
etc. in such a way that air exchange between compartments can interact with air exchange
between the building and the outdoors. One implication of the interaction between indoor flow
and indoor-outdoor air exchange is that it is difficult to predict the air exchange rate as a
function of wind, indoor-outdoor temperature, and building leakage parameters. In contrast, in
single-family homes, which are small in absolute size and which have large surface-to-volume
ratio, very simple formulae relate the environmental conditions and leakage parameter to the
air change rate. This is not true for more complex buildings.
Persily (1999) has shown that, contrary to expectation of some experts, air infiltration is
significant in commercial buildings. VanBronkhorst et al. (1995) estimate that infiltration
accounts for 10% to 20% of the heating load in all office buildings nationwide, although they
estimate it to have little effect on cooling loads, in part because of lower winds and lower
indoor-outdoor absolute temperature difference in summer compared to winter.
Although air infiltration in commercial buildings is significant, the air exchange rate due
to HV AC operation is almost always larger than the air infiltration rate. Therefore, removal of
indoor pollutants, delivery of outdoor pollutants, and energy costs are largely determined by
the details of HV AC design and operation. Moreover, since HV AC systems often mix air from
different parts of the building, and deliver outdoor air approximately equally to different parts
of the building, predicting indoor exposures to outdoor pollutants can be done fairly accurately
using knowledge of HV AC operation alone. For these reasons it is somewhat understandable
that little effort has gone into modeling air infiltration rates in commercial buildings, or into
experiments to determine the relationship between building leakiness and air exchange for
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