ASHRAE Journal - July 2022 - 42

COLUMN BUILDING SCIENCES
FIGURE 4 Extruded Polystyrene (XPS). It is fairly common to use XPS rigid insulation
on the interior of stucco-clad CMU block walls. The inward vapor flow rate
through the rigid insulation out of the CMU block wall needs to be " slower " than
the vapor flow rate through the interior lining-painted gypsum board.
PHOTO 7 Rigid Insulation Vapor Throttle. It is fairly common to use extruded polystyrene
(XPS) rigid insulation on the interior of stucco-clad CMU block walls.
FIGURE 5 Interior Coating Vapor Throttle. You can install an interior coating on
the interior surface of the CMU block wall. This interior coating should be less
than 5 perms " wet cup, " but more than 1 perm " wet cup. "
keep it from absorbing water. But when we paint the
stucco or add " magic " stuff, we shouldn't reduce its ability
to dry to the exterior. With respect to paint, we want
a paint that is " liquid water closed " but " vapor open " -
hydrophobic and vapor permeable. The hydrophobic
part is easy. The vapor permeable should be easy but is
not. Especially when folks try to have the paint also be
able to span cracks in the stucco. Guess what? Stucco
has cracks. All stucco has cracks. Always has. Always will.
Yup, I will get arguments about this. Trust me-be conservative.
Assume the stucco will have cracks even as you
try to get it to have no cracks.
Back to the paint thing. When we formulate paint to
be able to span cracks, the paint typically becomes less
vapor permeable. It is very, very difficult to get " crack
spanning paint " that is also vapor permeable. It is possible,
but not cheap. If the paint on the stucco is too
impermeable, it bubbles, blisters and peels (Photo 6).
Here comes a " Joe Rule. " The paint on the stucco needs
to be greater than 10 perms " wet cup " or it will bubble,
blister and peel.
Back to staying out of trouble. The second thing we have
to do is control the rate of inward vapor flow so it does
not damage the interior finishes or things like cabinets
on the interior of exterior walls.‡ Inward vapor flow can
‡ We have talked about this before-back-ventilate cabinets.
42
ASHRAE JOURNAL ashrae.o rg
J U LY 2022
be " throttled " -slowed down-by the permeance of the
interior insulation (also Figure 3). It doesn't need to be
stopped-but it could be if the interior insulation controls
condensation at the interface of the interior side of the
CMU and the interior insulation. Here comes another
" Joe Rule. " If you totally stop the inward vapor flow with
a " for real " vapor barrier such as foil-faced isocyanurate
rigid insulation, and the thermal resistance of the foilfaced
rigid insulation should be R-5 or greater. With R-5
or greater you are controlling the temperature of the
" condensing surface " by " warming " it up.
It's fairly common to use extruded polystyrene (XPS)
rigid insulation on the interior of stucco-clad CMU block
walls (Figure 4 and Photo 7). The inward vapor flow rate
through the rigid insulation out of the CMU block wall
needs to be " slower " than the vapor flow rate through
the interior lining-painted gypsum board. Painted gypsum
board is typically greater than 10 perms " wet cup. "
The extruded polystyrene is about 1 perm " wet cup "
when it is 1 in. (25 mm) thick (approximately R-5 for
http://ashrae.org
ASHRAE Journal - July 2022

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