ASHRAE Journal - March 2022 - 68

COLUMN BUILDING SCIENCES
the exterior plywood and OSB sheets or by
selecting plywood or OSB sheets that have
an integral water control layer and sealing
the joints between the plywood and OSB
sheets. Note that when the thermal resistance
of the wall assembly exceeds R-60
in Climate Zones 5 and higher it is recommended
that the exterior sheathing be
plywood or gypsum sheathing rather than
OSB due to the reduced vapor transmission
of the OSB.
Single Top Plate
1/2 in. Plywood Connecting Interior and Exterior Walls
2 × 4 Structural Stud
Wall at 24 in. o.c.
Latex Paint on 1/2 in.
Gypsum Board
3 1/2 in. Cellulose Insulation
1/2 in. Plywood or OSB
With Joints Taped
Figure 6: Offset Frame Wall with
Exterior Spray Foam
Fiberglass batt, blown cellulose, sprayed
cellulose and sprayed fi berglass are typically
used to insulate the stud space.
Fiberglass, blown and sprayed cellulose
are air permeable materials used in
the stud space of the wall, allowing possible
air paths between the interior and
exterior as well as convective looping in
the insulation. Dense pack cellulose has
less air permeance, but does not control
air leakage. Plywood and OSB sheets are
air impermeable. When joints between
the plywood and OSB sheets are properly
sealed with the exterior spray foam, an
effective air control layer (air barrier system) can be
created.
4.5 in. Space Between
Sheathing and Offset Framing
Cellulose
Insulation at Rim Joist
Cellulose Cavity
Insulation
Figure 7 Double Stud Wall.
* 2 × 6 advanced wood frame wall
* Plywood or OSB sheathing with joints taped/sealed
* Plywood or OSB or gypsum sheathing
* Drainage space creating a minimum 1/4 in. (6.4 mm)
ventilation gap/drainage gap behind cladding
* Fiberglass, cellulose or spray foam cavity insulation
* 2 × 3 or 2 × 4 exterior wood frame wall with cellulose or
fiberglass cavity insulation
* Housewrap or liquid- or trowel-applied or integral water
control layer
Cladding
2 × 3 Framing Wall
1/2 in. Fiberboard,
Plywood, OSB or
Gypsum Sheathing
Building Paper,
Housewrap or
Building Wrap
Furring Strips
Rain control is typically addressed by using the high
density spray foam as the water control layer.
Fiberglass or cellulose in the stud cavity are vapor
permeable, while plywood and OSB are moderately permeable,
and high density spray foam is impermeable.
The high density spray foam reduces the risk of winter
condensation by increasing the temperature of the
condensation plane. The level of vapor control is determined
in the IRC ( " Hybrid Attics and Hybrid Walls-
Simple Physics, Complicated Language, " ASHRAE Journal,
October 2017).
Figure 7: Double Stud Wall
Fiberglass batt, blown cellulose, sprayed cellulose and
sprayed fi berglass are typically used to insulate the stud
space. Cellulose or fi berglass insulation is installed in
the exterior truss cavity.
68
ASHRAE JOURNAL ashrae.o rg M A R C H 2 0 2 2
Plywood and OSB sheets are air impermeable. When
joints between the plywood and OSB sheets are properly
sealed with tape, mastic, caulk, etc., an effective air control
layer (air barrier system) can be created.
Rain control is typically addressed by either installing
a water control layer over the exterior plywood and OSB
sheets or by selecting plywood or OSB sheets that have
an integral water control layer and sealing the joints
between the plywood and OSB sheets. Note that when
the thermal resistance of the wall assembly exceeds R-60
in Climate Zones 5 and higher it is recommended that
the exterior sheathing be plywood or gypsum sheathing
rather than OSB due to the reduced vapor transmission
of the OSB.
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ASHRAE Journal - March 2022

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