ASHRAE Journal - November 2022 - 26

BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
Letting Airflow Out
Of and Into Closets
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
Airflow needs to come out of closets, and yes, airflow needs to come into closets-
because closets need air change. But imagine a world in which the building code does
not allow returns to be installed in closets to control mold and other contaminants.
That would be insane right? Welcome to the real world.
Let me make the case for closet air change-in cold
climates, hot-humid climates and mixed-humid climates.
Then let's look at the arguments against codeallowed
returns. And finally let's look at a sleazy way to
get around the code if the " authority having jurisdiction " -the
" local big dog chief building official " -does
not agree with the argument. Note that the local chief
building official has the authority to make exceptions
to the code language. A good code official* who knows
stuff is more important to know than a good surgeon.
Lots of good surgeons-well, there used to be back in
the old days.
A classic example of mold occurring in a cold climate
closet is that of an exposed closet on an exterior wall
(Figure 1). Its heat loss is high, and the closet will be
colder than the bedroom attached to it. If the vapor
*Way back when-before I was an engineer-I was a young
builder. The local building departments where I was building
(Ontario, Canada) adopted me and taught me things-like how to
build houses that actually worked. The inspectors would come out
and say things like, " Ah, young Joseph, I see you are about to install
windows. Let me tell you a little bit about flashing " and " When
you insulate the attic, you need to know about ice dams-it snows
a lot around here. " Building officials were friends and allies, not
enemies. They still are today-you just need to talk to them more.
The code-making process is broken, not the building departments,
but that is another story for another day.
26
ASHRAE JOURNAL ashrae.o rg N O V E M B E R 2022
pressure in the bedroom is the same as the vapor pressure
in the closet, and the closet is colder, then the relative
humidity in the closet will be higher than the relative
humidity in the bedroom. If the closet experiences
mold, it is apparent that the relative humidity in the
closet is greater than 80%. However, the key question
is this: is the relative humidity above 80% because the
closet is too cold, or is it because there is too much moisture
present (high vapor pressure) in the closet? Is this
a surface temperature mold problem, or is this a vapor
pressure mold problem-or a combination of the two?
It is usually a combination of the two. Controlled ventilation
(air change) in the house will reduce the vapor
pressure (and therefore relative humidity) in the house.
Yeah! But, folks are now wanting higher interior relative
humidities in houses for health reasons (yes, we were
here before-ASHRAE Journal, February 2022, " Covert
COVID Thoughts " ). The temperature of the closet can be
raised by increasing the heat flow to the closet or decreasing
the heat flow out of the closet to the exterior (more
insulation, especially continuous exterior insulation).
But what if you can't do the insulation thing because
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.
http://www.buildingscience.com https://www.ashrae.org/
ASHRAE Journal - November 2022

Table of Contents for the Digital Edition of ASHRAE Journal - November 2022
