ASHRAE Journal - March 2022 - 66

COLUMN BUILDING SCIENCES
Single Top Plate
permeable. The EPS insulating sheathing
reduces the risk of winter condensation
by increasing the temperature of the condensing
surface (condensation plane),
and reduces the risk of summer inward
vapor drives by slowing the vapor movement
into the enclosure from the stucco.
The level of vapor control in insulating
sheathing walls is determined in the IRC
( " Hybrid Attics and Hybrid Walls-Simple
Physics, Complicated Language, " ASHRAE
Journal, October 2017).
Figure 4: Advanced Framing with
Multiple Layers of Insulating Sheathing
Fiberglass batt, blown cellulose, sprayed
cellulose and sprayed fi berglass are typically
used to insulate the stud space. Two
or more layers of mineral fi ber insulation
boards are installed over plywood or OSB
sheathing.
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 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.
Fiberglass or cellulose in the stud cavity
are vapor permeable, while plywood
and OSB are moderately permeable.
Insulating sheathing reduces the risk
of winter condensation by increasing
the temperature of the condensing
66
ASHRAE JOURNAL ashrae.o rg M A R C H 2 0 2 2
Cellulose
Insulation at
Rim Joist
EPS Insulation
Capillary Break
FIGURE 4 Advanced Framing with Multiple Layers of Insulating Sheathing
* 2 × 6 advanced wood-frame wall
* Plywood or OSB sheathing
* Multiple layers of mineral wool insulating sheathing
Single Top Plate
Taped and Painted
1/2 in. Gypsum Wall
Board as Interior Finish
2 × 6 Interior Framing
Member at 24 in. o.c.
Plywood/OSB
Sheathing; Joints
Taped/Sealed
Cellulose Insulation in
Wall Cavity
Plywood/OSB
Sheathing
Housewrap
Cladding
Furring Strips
Cellulose
Insulation at
Rim Joist
Capillary Break
Ledger Board
Figure 5 Truss Wall.
* 2 × 6 advanced wood-frame wall
* Plywood or OSB sheathing with joints taped/sealed
* Plywood or OSB or gypsum sheathing
* Fiberglass, cellulose or spray foam cavity insulation
* 2 × 3 or 2 × 4 exterior truss with cellulose or fiberglass cavity
insulation
* Housewrap or liquid- or trowel-applied or integral water control layer
* Drainage space creating a minimum 1/4 in. (6.4 mm) ventilation gap/drainage gap behind cladding
* Fiberglass, cellulose or spray foam cavity insulation
* Liquid- or trowel-applied or integral water control layer
* Drainage space creating a 3/4 in. (19 mm) ventilation
gap/drainage gap behind cladding
Plywood Cavity CLosure
at Top of Assembly
2 × 3 Exterior Truss
2 × 6 Stud Wall at 24 in. o.c.
Taped and Painted 1/2 in. Gypsum
Wall Board as Interior Finish
Cellulose Insulation in
Wall Cavity
Insulation Boards
OSB Sheathing; Joints
Taped/Sealed
1 × 4 Furring Strips
Mineral Fiber
Wood or Fiber Cement
Siding

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ASHRAE Journal - March 2022

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