ASHRAE Journal - July 2023 - 38

BUILDING SCIENCES
COLUMN
Joseph W. Lstiburek
That Popping Noise You Hear is Not a Breakfast Cereal...
Snap, Crackle and Pop
Goes the Spray Foam
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE
Snap, Crackle and Pop are beloved gnome characters-cartoon mascots of Rice
Krispies breakfast cereal produced by a small company you might have heard of:
Kellogg's.* Rice Krispies are one of the best things to come out of Michigan besides
the Wolverines. " Pop " is " a mischievous yet also clumsy youngster and the center of
attention. " † Yup, sometimes spray foam can be considered to be " mischievous-and
the center of attention. "
Here is the issue. Closed-cell 2 lb/ft3 (32 kg/m3) " high
density " spray polyurethane foam has tremendous
adhesive and strength properties as well as high thermal
resistance and airtightness. Closed-cell spray foam
is often installed on the underside of roof sheathing
(Photo 1). If the bond between the closed-cell spray foam
and the underside of the roof sheathing fails, it fails
quickly. Very quickly. And it lets everyone know that it
has failed quickly. It shouts out the failure. Yup. You
hear the " pop. " It wants to be the center of attention-
and lets you know.
There are a couple of reasons for this type of failure.
The most common are thermal shock in the winter, poor
installation technique or overheating of the foam during
installation on a hot day. If the spray foam is installed
*Kellogg's is a huge multinational company with worldwide corporate
office headquarters in Battle Creek, Mich.-with a history going
back to 1906, which is when I think was the last time my Toronto
Maple Leafs won the Stanley Cup.
†Thank you https://en.wikipedia.org/wiki/Snap,_Crackle_and_Pop
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too thickly ( " too-thick passes " ), the heat buildup from
the exothermic reaction in the foam can be intense.
This can be exacerbated by too-hot substrates-summer
work on the south elevation with black underlayment or
roofing. This can result in a core density that is too low
and that makes the spray foam dimensionally unstable
in the core. This makes it difficult for the spray foam to
handle thermal stress. The resulting thermal stress is
often the greatest during cold weather. So the " failure "
can happen months after the installation.
It is important to note that the " too-thickly " refers
to " each pass " of spray application, not the overall
thickness of the installation. The maximum thickness
per pass or per " lift " is determined by the manufacturer,
as the chemistry varies from manufacturer to
manufacturer. Typical " lifts " vary between 2 in. (51 mm)
and 5 in. (127 mm). This next point can't be stressed too
strongly-follow the manufacturer's recommendations.
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.
http://www.buildingscience.com https://en.wikipedia.org/wiki/Snap,_Crackle_and_Pop http://ashrae.org
ASHRAE Journal - July 2023

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