A.Mike Rogers of Green
Homes America, a home-performance contracting
company with locations across the country,
responds: The short answer is yes, it does
matter. You will often get different cfm50 results
depending on whether you are depressurizing
(pulling air out) or pressurizing (pushing air in).
Because depressurization is the standard approach
in the U.S. and Canada, you will generally want to
depressurize so that you can better compare apples
to apples, either across buildings or as "before"
and "after" measures of one particular building.
(Europeans prefer to take both pressurized and
depressurized readings, and average the two.)
Why is depressurization used? The most cited
reason is that it more accurately mimics natural
leakage where mechanical ventilation backdraft
dampers are present. Depressurizing tends to close
the dampers, whereas pressurizing tends to open
them. In cold climates, some people have also
expressed concern about blasting concentrated cold
air into one area (at the blower door) when
pressurizing, potentially killing indoor plants.
When a structure is depressurized, cold air leaks
in from many diffuse points.
In terms of finding leaks, sucking and blowing
have different advantages. Depressurizing makes it
easier to feel the presence of leaks with your
hands. Pressurizing makes it easier to find leaks
with some sort of artificial smoke.
The one time pressurizing definitely trumps
depressurizing is when there is concern about
drawing pollutants into a home - fiberglass fibers
from the attic, rat poop, mold, or mildew from a
crawlspace, or ashes from the fireplace.
Incidentally, as barroom tales illustrate, it is
absolutely not a good idea to use a blower
door in either direction with a fireplace or