ASHRAE Journal - July 2023 - 39

COLUMN BUILDING SCIENCES
PHOTO 1 Closed-cell spray foam is often installed on the underside of roof
sheathing.
sheathing are 5 in. (127 mm) or more thick.‡
At 5 in.
Always consider them as " requirements. " Several
passes or lifts are typically required to meet the thermal
resistance requirements of the assembly. The installed
foam in each pass or lift needs to " cool " before the next
pass or lift is applied.
Figure 1 illustrates good installation practice that
maximizes bond surface. It is referred to as " classic
picture-frame " installation- " doubled up. " The
perimeter of the " framing bays " are sprayed fi rst with
a " fi llet " of foam prior to the application of the " main "
buildup followed by a second layer of " picture-framing. "
Are we done? Nope. There is more. Closed-cell 2 lb/ft3
(32 kg/m3) " high density " spray polyurethane foam
also has a low vapor transmission-less than 2 perms
(120 ng/Pa·s·m2) per inch of thickness. When you get
to 2 in. (51 mm) thick, the typical vapor transmission
is less than 1 perm (60 ng/Pa·s·m2), and that classifi es
the material as a Class II vapor retarder. Most closedcell
spray foam applications on the underside of roof
the vapor transmission is approximately 0.25 perm
(15 ng/Pa·s·m2), give or take.§
Well, that sounds awesome-you don't need a vapor
barrier in a cold climate. Yup, but nothing is ever
without complications. In cold climates we are worried
about this thing called " snow " and ice-damming. Many
folks install a fully adhered roofi ng membrane under
typical shingles as a guard against ice dam-related
roof leaks. The most common fully adhered roofi ng
membrane is less than 0.05 perms-a Class I vapor
retarder. A for-real " vapor barrier " is defi ned as less
than 0.1 perms.
Now it gets interesting. The top side of the roof
sheathing has a layer that is 0.05 perms (3 ng/Pa·s·m2),
and the underside of the roof sheathing has a layer that
is 0.25 perms (15 ng/Pa·s·m2). The vapor permeance
is " fi ve timesÓ greater at the bottom than the top. Ah,
come on, who cares, both are so small it does not matter.
I remind you of " mischievous " and that " Pop " gnome
character. No upward drying is possible through the
fully adhered " totally vapor impermeable " roofi ng
membrane. None. Let me repeat the " none. "
But there is some upward vapor fl ow-not a lot but
some-and we have seasons, one of which is called
winter. And in the winter it is not always sunny all the
time, and we have real people inside doing what they
do to be called real people, and sometimes during the
winter the humidity inside goes above 30%.#
FIGURE 1 Good installation practice.
Classic Picture-Frame Fillet
Main Buildup
Best-Practice Final-Pass Fillet
‡One of the most common roof rafter thicknesses is a 2 × 6, and closed-cell spray foam is applied slightly " thinner " than the thickness
of a 2 × 6 so that it does not have to be " shaved " when folks install interior fi nishes-hence the 5 in. (127 mm). At 5 in. its thermal resistance
is approximately R-30 (RSI 5.3).
§Those are " weasel words, " as the range can be quite large based on installation skill, mix control, temperature, etc. One of the best
studies of spray foam in real assemblies was done by Straube, Smith and Finch: https://tinyurl.com/straubesmithfi nch
#And much higher if you install a humidifi er and listen to the CDC and the Harvard School of Public Health. Yes, we were here before:
see " Covert COVID Thoughts, " ASHRAE Journal, February 2022.
J U LY 2 0 2 3 ashrae.o rg ASHRAE JOURNAL
39

https://www.tinyurl.com/straubesmithfinch http://ashrae.org

ASHRAE Journal - July 2023

Table of Contents for the Digital Edition of ASHRAE Journal - July 2023