ASHRAE Journal - September 2021 - 58

BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
Moldy Hotels Suck
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
Thirty years ago I said the moldiest buildings in the United States were air-conditioned
hotels with vinyl wall coverings that operate under a negative air pressure.
They literally " sucked. " Not much has changed. For me to point that out is amazing.
Maybe we need more attorneys. We are building hotels wrong faster than we can litigate
their lawsuits. That is not meant as a joke. Some of the most effective " technology
transfer " to practitioners in the construction industry is through the legal profession.
Some of the physics is obvious; some not so much.
Here goes once again, but with 30 years of context.
Let's look at air-conditioning, then vinyl wallcoverings,
and then pressure.
Air Conditioning
To civilians and most normal folks, it means airconditioned
buildings are cooler inside than outside,
and if we we do things correctly, the cooler inside is also
drier than the warmer outside. Here we go with the
obvious: moisture flows from the outside to the inside in
these buildings by vapor diffusion. Moisture also flows
from the outside to the inside by airflow if air moves
from the outside to the inside. See the Second Law of
Thermodynamics.
In places like Las Vegas where it is hot and dry outside,
there is negligible movement of moisture from the outside
to the inside. Different story in Miami.
In air-conditioned buildings that are located in hothumid,
mixed-humid, marine and cold climates, we
need to concern ourselves with moisture movement
from the outside to the inside. Cold climates? Really?
*I am waiting for all the fact checkers on the internet to flag this.
Go for it. Note the key word- " air-condition. " Yup waiting to be
banned for improper information.
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ASHRAE JOURNAL ashrae.o rg
S E PTEM B E R 2021
Yes, really. It gets hot and humid in Chicago in the summer,
and we air-condition there. Same for Toronto and
Boston. Installing an interior vapor barrier on the inside
of air-conditioned buildings in these climates can be a
problem. Yes, that plastic sheet on the inside of all those
condos built in Vancouver in the 1980s and 1990s was
not very helpful-it prevented inward drying of all that
stucco during the summer. Same for the plastic sheet in
Woodbury, Minn., and most of Pennsylvania. If you aircondition
and you have humid summers, there should
be no interior vapor barrier.*
Vinyl Wall Coverings
Vinyl wall coverings are vapor barriers. Really. They
are made out of vinyl. Who knew? Using the correct
terms, they are either vapor impermeable or vapor
semi-impermeable. (Yes, we were here a long time ago:
" Understanding Vapor Barriers, " ASHRAE Journal, August
2004.)
Manufacturers of vinyl wall coverings do not publish
the perm values. What a surprise. The information
below comes from two sources. The first source is
from the 2021 ASHRAE Handbook-Fundamentals, Chapter
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 http://ashrae.org
ASHRAE Journal - September 2021

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