ASHRAE Journal - April 2021 - 52

COLUMN BUILDING SCIENCES

PHOTO 8 Open-Cell Spray Polyurethane Foam (ocSPF). When you spray ocSPF
into wall cavities from the interior, the ocSPF can act as the air control layer and
thermal control layer. It can't act as the vapor control layer. It is too vapor open.
This is an issue when you spray ocSPF on the underside of roof/attic assemblies, and you can end up with problems ( " Ping Pong Water and the Chemical
Engineer, " ASHRAE Journal, October 2016).

FIGURE 1 " Ping Pong Water. " Water vapor from the interior passes up through
the ocSPF and is stored in the wood-based roof sheathing-and then driven
out by solar radiation. In and then out, in and then out-a " ping " followed by a
" pong, " leading to an increase in moisture accumulating at the ridge, requiring
a means of moisture removal-either air supply and return from the house or a
dehumidifier.

Shingles
Roofing Paper
Wood Based
Roof Sheathing
Open-Cell,
Low-Density Spray
Foam

(ocSPF), if applied on the exterior of wall assemblies, can't
act as the water control layer. A fluid-applied water control layer needs to be applied over its exterior surface.
When you spray ccSPF into wall cavities from the
interior, the ccSPF can act as the air control layer, vapor
control layer and thermal control layer ( " Interior Spray
Foam, " ASHRAE Journal, February 2020). When you spray
ocSPF into wall cavities from the interior, the ocSPF can
act as the air control layer and thermal control layer. It
can't act as the vapor control layer. It is too vapor open.
This is an issue when you spray ocSPF on the underside
of roof/attic assemblies (Photo 8), and you can end up
with problems ( " Ping Pong Water and the Chemical
Engineer, " ASHRAE Journal, October 2016). Water vapor
from the interior passes up through the ocSPF and is
stored in the wood-based roof sheathing-and then
driven out by solar radiation. In and then out, in and
then out-a " ping " followed by a " pong, " leading to an
increase in moisture accumulating at the ridge (Figure 1),
requiring a means of moisture removal-either air supply and return from the house or a dehumidifier.
Note that this also happens with fiberglass, mineral
wool and cellulose (Photo 9). All of these insulations also
require either air supply and return from the house
coupled with a vapor diffusion port or a dehumidifier
( " Venting Vapor, " ASHRAE Journal, August 2015). Further
note that none of this is an issue with ccSPF. Let me
repeat. None of this is an issue with ccSPF because it is
not vapor " open. "
On to fiberglass and mineral wool batts installed in
wall cavities, roof cavities, on the ceilings of attics or on
52

ASHRAE JOURNAL

ashrae.org

APRI L 2021

Each " Ping " and " Pong "
Bounces the Water Molecules
Up the Slope

PHOTO 9 Fiberglass and Mineral Wool and Cellulose Underside Roof Deck
Insulation. Water vapor from the interior passes up through the vapor open
insulation and is stored in the wood-based roof sheathing-and then driven out by
solar radiation. In and then out, in and then out-a " ping " followed by a " pong, "
leading to an increase in moisture accumulating at the ridge, requiring a means
of moisture removal-either air supply and return from the house coupled with a
vapor diffusion port or a dehumidifier.

the underside of roof/attic sheathing. They obviously
can't act as the water control layer, the air control layer
or the vapor control layer. They can act as the thermal
control layer if airflow through them is limited or controlled (Photo 10 and Photo 11). We learned this with our
" Thermal Metric Project. " 1 In fact, the Thermal Metric
Project showed that all cavity insulations functioned if
they were combined with air control layers and if convection was controlled (Figure 2).
Let me be specific and obvious. If batts are installed
and fitted " tightly " without voids, they work ( " WUFI:
Barking Up the Wrong Tree?, " ASHRAE Journal, October
2015). Additionally, in roof/attic applications windwashing must be controlled ( " Bobby Darin and Thermal
Performance, " ASHRAE Journal, October 2012).
How about cellulose-the netted and damp spray kind?
They can only act as the thermal control layer (Photo 12).
They can't act as the air control layer-despite what folks
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ASHRAE Journal - April 2021

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