ASHRAE Journal - July 2013 - 42

BUILDING SCIENCES
hour (ach), there is no correlation between ventilation rate and concentration
(Figure 1). As air change is increased
and formaldehyde concentrations are
reduced by dilution, emission rates are
simultaneously increased. The faster you
dig in a sand pile, the more sand comes
pouring in.
There is no way to win with formaldehyde unless you don’t put it in the building in the ﬁrst place or seal it up so it
does not get out of the stuff you put in the
building or build the building from. Why
not just double or triple the ventilation
rates? Sure. Spoken like a true believer
or Kool-Aid drinker. Rates like that don’t
work because we can’t afford them and
folks won’t accept them. Good luck trying to sell 1.5 ach or higher as a standard
or a practice or a code. The best way is
don’t put formaldehyde-emitting stuff
in your building or let formaldehydeemitting stuff emit. Dilution will not
help you at the rates that are practical
to dilute.
So how to do source control in a real
building? Guess what? The “Indoor airPLUS” speciﬁcations section§ brought to you courtesy of the
U.S. Environmental Protection Agency is a great place that
tells you how to get it done. It does not deal with occupant
stuff—the stuff occupants bring into their houses or the stupid
stuff occupants do in their houses—but it does deal with the
stuff you build the building out of. Nice job, EPA folks.
I have already ranted about ﬁlling your place to keep stuff
with stupid stuff and doing stupid stuff inside. Don’t do it.
Can’t say much more than that. I am also not going to rant
about the need for sealed combustion appliances and the need
to make basements dry. Been there and done that already. #
Just do it.
What do I do when I have this beautiful enclosure constructed
out of good stuff that is tight, and I want to ventilate it right and
have a clothes dryer, a kitchen range hood, a ﬁreplace and an
attached garage? A lot of you are not going to like what I am
going to say. Sorry in advance. Gulp.
I am going to start with garages. The absolute best way to
deal with a garage is not to attach it to the house. OK, I said

Unconditioned
Makeup Air
60% to 70%
of Hood
Exhaust

Figure 2: Capture the Effluent. The
first thing we make sure is that the exhaust hood actually works to capture
the effluent. The hood must overhang
the cooking surface big-time. The
absolute best approach is to use a
backshelf hood with side panels and
large overhangs on both sides and the
front. Backshelf hoods can typically
use 30% less exhaust to capture bad
stuff compared to other hoods. Side
panels can get you another 30% improvement. Note the direct makeup air
using a modified backwall approach
such that this makeup air is introduced
at floor level. This direct makeup air
introduced at floor level should never
provide more than 60% to 70% of
the hood exhaust. Why? Ah, we need
a zone of negative pressure around
the cooking surface. You don’t want to
push the bad stuff; you want to pull
the bad stuff.

it. I know that is not a practical answer, but it is what it is. So
what about attached garages? Now, it gets ugly. If you build to
code, you will get consistently below 3 ach at 50 Pa; and if you
install an exhaust-only ventilation system at the rates you are
required to ventilate at, you will pull stuff out of the attached
garage and into the house.
Good luck with trying to isolate the garage by constructing
the demising wall/ﬂoor/whatever “tight enough.” Not going
to happen. And, trying to do pressure diagnostics to verify
a “tight enough” demising wall using some arbitrary and
capricious ratio/value/test procedure ranks right up there
with most hygrothermal simulations. The takeaway is don’t
suck on the house, and you will not end up sucking on the
garage.**
Attached garages mean you had better install balanced ventilation systems or supply ventilation systems in houses and
call it a day.†† It is possible to make the garage more negative
than the house even if it has an exhaust-only ventilation system.
This is done by sucking on the garage using a continuously

www.epa.gov/iaplus01/pdfs/construction_specifications.pdf
“Understanding Basements” ASHRAE Journal, July 2006, and “Learning from Mistakes” ASHRAE Journal, February 2010.
**Or the crawlspace, or the slab, or the basement. You only suck on a building if you think radon is valuable and you want to mine it.
††Also, make sure that you do not create negative pressures because you have done dumb things with your duct systems such as installing leaky ductwork in vented
attics, vented crawlspaces or using building cavities as returns.
‡‡I really have a strong opinion on the machine shop garage ventilation thing. To this day, I never willingly fly in a helicopter.
§§I do more than my share of high-end custom home stuff in extreme climates such as Aspen, Colo., and Naples, Fla. Usually, my clients have homes in both places
plus a home somewhere else out of the country for tax purposes and an apartment in NYC that they fly to in their private jets, worrying about their carbon footprints.
#

ASHRAE Journal

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July 2013

ASHRAE Journal - July 2013

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