ASHRAE Journal - January 2015 - 43

COLUMN BUILDING SCIENCES

is that in cold climates the vapor
transmission of the air impermeable insulation installed on the
underside of the roof deck matters.
The most common air impermeable
insulation used in these applications is spray polyurethane foam
(SPF). There are two types: 0.5 lf/ft3
(5 m/m3) density foam and 2.0 lf/
ft3 (22 m/m3) density foam-often
referred to as "open cell" foam and
"closed cell" foam due to their differences in vapor transmission.
Open cell foam has a vapor transmission of around 30 perms for a
1-in. (25 mm) thickness and closed
cell foam has a vapor transmission
of around 2 perms for a 1-in. (25
mm) thickness.
You can use closed cell in all climate zones. You cannot use open
cell in IECC Climate Zones 5 or
higher unless the foam is used with
a Class II (or Class I) vapor retarder.
The reason should be fairly obvious. With open cell foam (Photo 2) a
significant amount of moisture via
vapor diffusion will pass upwards
through it during cool periods and
during the winter in cold climates-
too much to be driven back down
during sunny weather and during
the summer. Additionally, with open
cell foam the attic space it encloses
must be conditioned to remove
moisture (see "Cool Hand Luke
Meets Attics," ASHRAE Journal, April
2014).
The key to the "air impermeable"
approach is that interior air needs
to be prevented from accessing
the underside of the roof deck. If
interior air gets to the roof deck
and if the roof deck is cold enough,
condensation can occur. If enough
moisture condenses and accumulates for long enough under the

"right" conditions, bad stuff can
happen like mold, rot and corrosion.
With closed cell foams you have
a couple of interesting issues that
you have to address. The application
is an exothermic reaction. If you
spray the foam on too thickly, you
will burn the place down. No joke.
You can't spray more than a 2-in. (51
mm) layer at a time so that the heat
dissipates.
Also, the closed cell foams experience huge shrinkage stresses
during the application process. If
these stresses are not controlled
or restrained correctly, things can
move in weird ways. That is why
"picture-framing" is necessary when
spraying rafter cavities or cavity
spaces between the top chords of
trusses. With "picture-framing," the
perimeter of the space is "foamed"
first. The stresses, if not controlled,
can be great enough to cause the
foam itself to split. Or, more dramatically, pop off the roof sheathing or
other surface loud enough to wake
occupants months later. The forces
are large enough to deform roof
sheathing-roofs can appear "wavy"
if picture framing is not done or not
done correctly.
There has been much discussion
about open cell being better under
a roof deck than closed cell because
if there is a leak the leak either
dries faster or you can see it sooner.
Not true. There is no compelling
evidence one way or the other.
None. I started all this SPF stuff
under roof decks in the early 1980s.
I have been looking at this roof leak
issue since the very beginning. I
have played with mock-ups and
wetted them and watched them. I
have taken apart existing leaking
roofs all over the place, open cell,

PHOTO 2 Open Cell Spray Polyurethane Foam (SPF).
You cannot use open cell in IECC Climate Zones 5
or higher unless the foam is used with a Class II (or
Class I) vapor retarder. With open cell foam a significant amount of moisture via vapor diffusion will pass
upwards through it during cool periods and during the
winter in cold climates-too much to be driven back
down during sunny weather and during the summer.
Additionally, with open cell foam the attic space it
encloses must be conditioned to remove moisture.

closed cell, SIPs, compact, etc. With
the mock-ups and the field experience, I have place, open cell, closed

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