ASHRAE Journal - June 2022 - 54

COLUMN BUILDING SCIENCES
PHOTO 6 Exterior Aluminum Foil. What I like about
the aluminum foil on the exterior face of the exterior
continuous rigid insulation is that it is a great
water control layer, air control layer and vapor
control layer. It also gives a big boost to the structural
integrity of the rigid continuous insulation. So
yeah to all that, but to say it gives a big boost to
wall assembly R-value-slim to none.
PHOTO 7 Aluminum Foil on Cavity Insulation. This
is also know as " reflective insulation " on the inside
of batt insulation. Nope, not a radiant barrier-a
vapor barrier.
PHOTO 8 Aluminum Foil on the Inside of a Block
Wall in Florida. The " reflective insulation " was
installed over ¾ in. (19 mm) wood furring with the
foil facing the air space. The aluminum foil turned
out to be a vapor barrier on the wrong side of the
wall. Mold city.
much longer, yeah, but what about
that energy efficiency thing. Not so
much with the high levels of thermal
insulation. So, yes, I like cool roofs-
because things last longer-but, I
also like lots of insulation.
What about walls? I like white
paint and light-colored paints on
claddings because the coatings last
much longer. Unfortunately, they
reduce drying potentials because
they don't get as hot, and that can
be an issue sometimes. But on the
other hand they reduce inward
thermally driven moisture out of
reservoir claddings. Yeah, so, what
should I do on a wall? Insulate the
heck out of it on the outside, provide
water and air control layers,
drain and back-ventilate the cladding,
and color and radiation do
not matter.
How about aluminum foil on the
exterior face of that rigid continuous
insulation (Photo 6)? It is on the high
temperature side of the assembly. So
that is good for the radiation thing?
Yes. And it faces an air gap-that
cleverly located drainage gap behind
the cladding-so that works. Yes.
54
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But walls don't get as hot as roofs
and walls are really well insulated
now-R-20 to R-30 or more. So the
radiation thing is kind of a bust. It
gets worse.
For the air gap to work to increase
thermal resistance it needs to be
" airtight " and be small enough to not
have convective loops. Recall how
windows work. Once you get more
than a ½ in. (13 mm) air space, window
R-value is affected. And, isn't
the point of the air gap behind the
cladding to provide " back ventilation "
to promote drying? That makes
the " airtight " thing go out the window-pun
intended. But what I like
about the aluminum foil is that it is a
great water control layer, air control
layer and vapor control layer. It also
gives a big boost to the structural
integrity of the rigid continuous
insulation. So yeah to all that, but to
say it gives big boost to wall assembly
R-value-slim to none, and Slim just
left town.
What about that aluminum foil
(aka " reflective insulation " ) on the
inside of batt insulation (Photo 7)?
Nope, not a radiant barrier-a vapor
J U N E 2022
barrier-big difference. And the aluminum
foil layer touches the interior
wall lining, no air space-unless
there is really, really bad workmanship.
How about aluminum foil of
the back side of the gypsum board?
Nope, again, not a radiant barrier-a
vapor barrier.
Let's look at what was probably
one of the worst attempted applications
for reflective insulation-adding
aluminum foil to the inside of a
block wall in Florida (Photo 8). It was
installed over ¾ in. (19 mm) wood
furring with the foil facing the air
space. There was no real significant
temperature difference between the
interior gypsum board and the interior
surface of the block wall. The
claim here was that it was " equivalent
to R-4. " No harm, no foul. No
big deal. Nope. Big harm, big foul.
The aluminum foil turned out to be
a vapor barrier on the wrong side of
the wall. Mold city. Perforating the
aluminum didn't help much-it was
a Fick's Law thing. So not only was
the thermal performance bad, but
buildings became trashed.
Let's look at the thermal mass
thing. We learned a bunch of things
in the 1970s and 1980s with thermal
mass. Wait for it. Thermal mass is
not insulation-it just stores and
releases energy. For it to be effective
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ASHRAE Journal - June 2022

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