ASHRAE Journal - July 2022 - 44

COLUMN BUILDING SCIENCES
FIGURE 6 Rigid Mineral Wool/Rigid Fiberglass Insulation. You can use vapor-open insulation such as mineral
wool and fiberglass board insulation on the interior of the wall in conjunction with an interior coating acting as a
vapor throttle. Note the continuous rigid insulation approach. No thermal bridge. The framing is on the inside of
the continuous rigid insulation.
FIGURE 7 Cellulose, Fiberglass and Mineral Wool Cavity Insulation. You can use vapor-open insulation such as
cellulose, fiberglass and mineral wool cavity insulation on the interior of the wall in conjunction with an interior
coating acting as a vapor throttle.
those interested). How about using
expanded polystyrene (EPS)? That
works. And closed-cell spray polyurethane
foam (ccSPF)? Yup. What
do we have to be careful about? No
interior vinyl wall coverings. None.
Ever. Don't go there. Can I use fabric
wall coverings? Yeah, ok.
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ASHRAE JOURNAL ashrae.o rg
What if I don't want to use foil-faced
isocyanurate rigid insulation or XPS
or EPS or ccSPF? What if I wanted to
use rigid mineral wool or fiberglass
batts or cellulose insulation? Yes,
but you have to " throttle " the inward
vapor flow out of the CMU block wall.
You can install an interior coating
J U LY 2022
on the interior surface of the CMU
block wall (Figure 5). Here comes
another " Joe Rule. " This interior coating
should be less than 5 perms " wet
cup. " Now you can use vapor-open
insulation such as cellulose, fiberglass
and mineral wool on the interior
of the wall (Figures 6 and 7).
Anything else we have to be careful
about? Yes of course. A couple of
things. Don't forget to put a " seat " in
the concrete slab at the perimeter to
handle the rainwater draining down
the CMU block cores (Photo 8). The
seat acts as flashing, directing the
rainwater to the exterior of the wall.
What else? Windows. You absolutely
have to drain the window
opening to the exterior (Figure 8,
Figure 9 and Figure 10). You absolutely
must use a fluid-applied coating
around the entire perimeter of the
rough opening (Photo 9). The head
of the window opening is also a big
deal. The fluid-applied coating over
the " wood buck " at the underside
of the CMU block at top of the window
rough opening coupled with a
concrete lintel acts as a " dam " and
a " gutter " to prevent water from
draining into the window from the
wall at top of the window opening.
When we control both the inward
and outward vapor flow in painted
stucco-clad CMU block walls that are
internally insulated, we pretty much
can use any interior insulation
system we want-XPS, EPS, SPF, cellulose,
fiberglass and mineral wool.
Neat, eh?
So what happens if we replace the
CMU with cast-in-place concrete
or precast and no stucco? Well, the
concrete itself is an awesome vapor
throttle. We typically don't need
to add anything on the interior if
the interior insulation and interior

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ASHRAE Journal - July 2022

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