ASHRAE Journal - July 2020 - 58

FIGURE 12 Wood frame assembly with exterior insulation and interior cavity insulation with brick or stone veneer and no interior vapor control layer.

Applicability
OK All hygrothermal regions, all rain exposure zones and interior climate Classes I and II.
NO Interior climate Class III buildings such as hospitals, museums and swimming pool enclosures.
Description This wall has a combined air/vapor/water control layer at the exterior sheathing, inboard
of continuous exterior insulation. The sheathing can be non-paper-faced exterior gypsum, plywood,
fiberboard or oriented strand board. This wall assembly will dry from the vapor control layer inward and
will dry from the vapor control layer outward.
Drainage Cavity Hydrostatic pressure is controlled by providing a drainage space behind the cladding.
The ideal solution is to decouple the exterior brick veneer (a "reservoir" cladding) from the wall assembly
with a ventilated and drained cavity. The cavity behind the brick veneer should be at least 1 in. (25 mm)
wide and free from mortar droppings. It must also have air inlets ("weep holes") at its base and air
outlets ("weep holes") at its top to provide back ventilation of the brick veneer. Alternately, in milder or
lower-rainfall climates, a ¼ in. (6.4 mm) drainage mat or drainable insulation can be substituted for the
1 in. (25 mm) air cavity. This option should not be used in IECC Climate Zone 5 and higher, with rainfall
over 20 in. (508 mm) per year (due to freeze-thaw risks).
Exterior Insulation Vapor impermeable (foil-faced polyisocyanurate) or vapor semi-impermeable
(extruded polystyrene) acceptable.
Water Control Layer Combined air/vapor/water control layer at the exterior sheathing. Can be fully
adhered sheet membrane, trowel-on or spray-applied coating, mechanically attached sheet or integral
water control layer within the sheathing.
Air Control Layer Combined air/vapor/water control layer at the exterior sheathing.
Vapor Control Layer (Exterior) Combined air/vapor/water control layer at the exterior sheathing;
vapor-impermeable layer at exterior of wall.
Vapor Control Layer (Interior) and Condensation Control In cold climates, condensation is limited
on the interior side of the water/air control layer and vapor control layer by installing sufficient exterior
thermal insulation on the exterior side of these control layers and limiting the interior climate class to
Class I and II. In hot-humid climates any moisture that condenses on the exterior side of the water
control layer will be drained to the exterior.
This assembly must allow inward drying; therefore, vapor-impermeable and vapor semi-impermeable

interior finishes such as vinyl wall coverings, epoxy paints and alkyd paints should be avoided.
Interior finish is Class III latex paint on interior gypsum board. The thermal resistance of the
exterior insulation to avoid condensation is specified in Table 3. This wall assembly is also
limited to interior climate Class I and II buildings.
Thermal Control Layer Stud bay cavity insulation and exterior rigid insulation.

FIGURE 13 Wood frame assembly with exterior insulation and interior cavity insulation and siding.

Applicability
OK Hot-dry, hot-humid, mixed-dry, mixed-humid, marine and cold regions.
NO Very cold and subarctic/arctic regions.
NO Interior climate Class III buildings such as hospitals, museums and swimming pool enclosures.
Description The cavity insulation in this wall assembly is spray polyurethane foam and is a variation of
Figure 8. The spray foam insulation can be low density open cell or high density closed cell spray foam.
Both foam types work in most climates.
Drainage Cavity The exterior siding should be uncoupled from the wall assembly with a ventilated
and drained cavity. The air gap behind the cladding can be provided by using a textured house wrap, a
drainage mat or furring strips at least 3/16 in. (4.8 mm) thick. If a house wrap is used to create this air
gap, it should be semi vapor permeable (i.e., not be a vapor barrier).
Exterior Insulation Vapor impermeable (foil-face polyisocyanurate) or vapor semi-impermeable
(extruded polystyrene) acceptable.
Water Control Layer Taped joints of the exterior rigid insulation (typical).
Air Control Layer Spray polyurethane foam in stud bay cavities.
Vapor Control Layer If high density closed cell spray foam, vapor control layer is spray polyurethane foam.
If low density open cell spray foam, vapor control is provided by controlling the temperature of the
condensing surface, as a result of installing exterior thermal insulation of sufficient thermal resistance,
as well as by limiting the interior climate class to Class I and II. With low density open cell spray foam,
the thickness (thermal resistance) of the rigid foam exterior sheathing is based on climate zone and
thickness of the wall framing (Table 3).
Interior vapor barrier coatings on the gypsum board such as vinyl wall coverings, oil or alkyd paints
should be avoided.
Thermal Control Layer Stud bay cavity insulation and exterior rigid insulation.
Note: High density closed cell foam can be used in all IECC Climate Zones. Where low density open cell foam can be used, the thickness or thermal resistance of the rigid foam exterior sheathing is specified by the
International Residential Code (IRC), based on Climate Zone and the thickness of the wall framing.
58

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ASHRAE Journal - July 2020

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