ASHRAE Journal - July 2022 - 38

BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
Throttling Flow, But Not
Necessarily Stopping It
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
Stucco over concrete masonry units (CMUs) aka " block walls " is a Florida thing-that
sometimes migrates north. We know a lot about how they work, but sometimes we
forget. And then subtle things are no longer subtle, especially when we change materials.
Why do we change materials? Ah, we sometimes run out of the stuff we have
always used, or we don't like some of the stuff we have always used for reasons that
might make sense or for reasons that might need therapy-or we want to kick up the
performance a notch or two or three.
I am not going to limit myself to Florida. I am going
to focus on " interior " insulation options for stucco
over CMUs* constructed in International Energy
Conservation Code (IECC) Climate Zones 1A, 2A and
3A. Note the " A " means " Moist. " " B " means " Dry " -you
can pretty much do anything you want in places it is
dry. In Climate Zones 4 and higher you need a different
approach. We were here before ( " Avoiding Mass
Failures-How to Insulate Mass Buildings on the Interior
and Not Get Into Trouble, " ASHRAE Journal, May 2018). In
*In this wall assembly the " water control layer " is the exterior
painted stucco. The " air control layer " is also the exterior painted
stucco. One " vapor control layer " is the exterior paint on the
stucco. We shall soon see that we will need an additional vapor
control layer on the inside of the CMU block wall. " The " thermal
control layer " will be the interior insulation.
†If we insulated the CMU block walls on the outside, we wouldn't
be having this discussion. The issues go away when you use the
" Perfect Wall " or an exterior insulated finish system (EIFS). The
vapor control layer goes on the outside of the CMU block walls
when using such systems. So why not always just insulate on the
outside? Cost, tradition, hurricane performance and skilled trades
factor into that discussion.
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places that have something called " winter, " you do not
have inward vapor drive all the time-you also have outward
vapor drive. I am also going to focus on stucco, not
metal claddings since they can't store moisture and are
typically " drained. " I am going to avoid " non-reservoir "
claddings that are drained because they are pretty easy
to deal with-you don't have to do much with non-reservoir
drained claddings.
And once again, for the record, I like all insulations-
the more the better. However, they do not all work the
same way on the interior of a stucco-clad CMU block
wall. They can all be made to work, but they do not all
work the same way. I think I mentioned that.
The subtle thing that is no longer subtle when we
change insulations is vapor flow-water vapor flow. Let's
take a simple, straightforward block wall and install
traditional stucco over the top of it. We have been doing
this forever. We also paint the stucco. We also insulate
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 - July 2022

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