ASHRAE Journal - July 2023 - 42

COLUMN BUILDING SCIENCES
FIGURE 3 Reducing thermal shock.
Shingles
Roof Sheathing
Roofing Underlayment
Rigid Insulation
Closed-Cell 2 lb/ft3
(32 kg/m3) Spray
Polyurethane Foam
reaction followed by cooling ( " cure shrink " )
results in a huge stress that eventually results
in the crack. Sometimes this is true. Sometimes
it is not. Sometimes it is the moisture stress. If
the side crack occurs, the bond at the underside
of the roof sheathing typically does not fail.
Sometimes both occur.
Let's also put this into perspective. This does
not happen often, but it does happen. We don't
have lots of totally completely impermeable
fully adhered roofing membranes-coupled with
black shingles.
So what do we do to deal with this? There are
lots of ways. Here are a bunch:
* Install the closed-cell spray foam correctly.
FIGURE 4 Increase drying and reducing thermal shock.
Shingles
Roof Sheathing
Roofing Underlayment
Vent Space (2 × 4s
Installed " Proud " )
Vapor Permeable Fully
Adhered Membrane
Roof Sheathing
Closed-Cell 2 lb/ft3
(32 kg/m3) Spray
Polyurethane Foam
FIGURE 5 More thermal shock reduction.
Shingles
Roof Sheathing
Roofing Underlayment
Rigid Insulation
Fully Adhered
Membrane
Roof Sheathing
Closed-Cell 2 lb/ft3
(32 kg/m3) Spray
Polyurethane Foam
Not too thickly on surfaces that are not too hot or
too wet.
* Install foam products that have a high
enough density to resist thermal shock. Don't
install ultralow density closed-cell spray foam in
cold climates.
* Install a layer of rigid insulation against the
underside of the roof sheathing prior to installing
the closed-cell spray foam. This reduces the
thermal shock (Figure 3).
* Install a vapor permeable fully adhered
membrane on the top side of the roof sheathing
coupled with a " vented " " over-roof " (Figure 4).
And by the way, the only way to really control icedamming
is to have a " vented " " over-roof " over
this membrane. And yes we were here before. See
" Dam Ice Dam, " ASHRAE Journal, June 2010.
* Install a layer of rigid insulation over the
top surface of the roof sheathing and another
layer of roof sheathing, roofing underlayment
and shingles. This reduces the thermal shock
(Figure 5).
And then we can get back to enjoying Rice
Krispies and that beloved gnome " Pop. "
Acknowledgments
A big thank you to Henri Fennell from North
Thetford, Vt, a legend in the spray foam industry,
and Chris Schumacher from Waterloo, Ontario,
Canada, a legend in the building science
community, for keeping me out of too much
trouble on this piece.
42
ASHRAE JOURNAL ashrae.o rg
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ASHRAE Journal - July 2023

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

Contents
ASHRAE Journal - July 2023 - Intro
ASHRAE Journal - July 2023 - BB1
ASHRAE Journal - July 2023 - BB2
ASHRAE Journal - July 2023 - Cover1
ASHRAE Journal - July 2023 - Cover2
ASHRAE Journal - July 2023 - 1
ASHRAE Journal - July 2023 - Contents
ASHRAE Journal - July 2023 - 3
ASHRAE Journal - July 2023 - 4
ASHRAE Journal - July 2023 - 5
ASHRAE Journal - July 2023 - 6
ASHRAE Journal - July 2023 - 7
ASHRAE Journal - July 2023 - 8
ASHRAE Journal - July 2023 - 9
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ASHRAE Journal - July 2023 - 11
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ASHRAE Journal - July 2023 - Cover3
ASHRAE Journal - July 2023 - Cover4
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