ASHRAE Journal - December 2023 - 14

COLUMN BUILDING SCIENCES
FIGURE 3 IECC Climate Zone 3B. Unvented attic, 2 × 6 wall, monolithic slab on
grade.
Roofing (Tile)
Raised-Heel Roof Truss
Roofing Paper (Vapor Permeable)
Roof Sheathing
Provides Increased Depth of
Roof Insulation at Perimeter
R-5 Rigid Insulation
Blocking: Expanded Polystyrene
Or Extruded Polystyrene
Polymer-Modified or Standard
Portland Cement Stucco
R-5 Rigid Insulation:
Expanded Polystyrene
Water Control Layer
(Building Paper, Housewrap
Or Building Wrap)
Termite Shield Set in Mastic
Serves as Capillary Break)
Removable Strip
Of Insulation and
Protection Board for
Termite Inspection
Cellular PVC Protection Board
Ground Slopes Away
From Wall at 5%
(6 in. per 10 ft)
R-10 Rigid Insulation:
Extruded Polystyrene
Dampproofing to Grade
Sealant
Sill Gasket
Sub-Slab Stone Layer
(No Fines)
4 in. Concrete Slab
6 mil Polyethylene
The water control layer in the wall assembly is
installed behind the rigid insulation. Drainage occurs
between the water control layer and the back side of
the rigid insulation.
The wall is framed with 2 × 6, 24 in. (610 mm) o.c.
advanced framing, as it uses less board footage (volume
of wood framing) than standard 2 × 4, 16 in. (406 mm)
o.c. framing and therefore is less expensive in material
cost. It is also less expensive in labor (25% fewer
framing members) and faster to assemble. Although
the code allows 2 × 4 framing and lower R-value cavity
insulation, using advanced framing and associated
cost savings more than offset the increase in the cost
of cavity insulation. (This depends on relative price
14
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Gypsum Board with Latex
Paint (Class III Vapor
Retarder)
R-49 Insulation: Cellulose,
Fiberglass, Mineral Wool
(Batts or Netted)
Or Open-Cell Spray Foam)
Sealant
Gypsum Board with Latex Paint
(Class III Vapor Retarder)
R-13 Minimum Cavity Insulation:
Cellulose, Fiberglass, Mineral
Wool or Open-Cell Spray Foam;
2 × 6 Cavity Completely Filled
For Fire Protection and Acoustics
2 × 6, 24 in. o.c.
Advanced Framing
fluctuations in labor and materials.)
Although the minimum R-value of the insulation in
this wall is R-13 per the IECC, fire code may require
the entire cavity to be filled, likely resulting in a higher
R-value for a 2 × 6 wall.
A Class III vapor retarder (latex paint) is used on
the interior surface of the walls and ceiling instead
of a vapor barrier. This allows drying to the interior.
Avoid vinyl wallpaper and oil-based paint or coatings
in Climate Zone 3. These wall coverings are vapor
impermeable and increase the risk of condensation
within the wall.
The rigid insulation on the exterior of the monolithic
slab has a removable strip to facilitate termite inspection.
The entire monolithic concrete slab should have a
polyethylene vapor barrier wrapping the underside
of the slab and footing to control capillary uptake. The
polyethylene should extend upward from the bottom of
the footing to grade on the exterior.
IECC Climate Zone 4C: Vented Attic, 2 × 6 Wall,
Conditioned Crawlspace (Figure 4)
The ceiling gypsum board is installed to the underside
of the insulated ceiling prior to the wall gypsum board
to accommodate a " filet " bead of sealant that seals
the ceiling gypsum board to the top plates creating a
continuous air control layer ( " air barrier " ) across the
entire ceiling including interior partition walls. The
gypsum board is the air control layer.
The R-10 rigid insulation on the exterior of the wall
framing is required to control condensation within the
framing cavities as there is no interior vapor barrier;
there is a vapor " retarder " (Class II vapor retarder-
semipermeable latex paint). The reason there is no
interior vapor barrier is to permit drying to the interior.
The code requires approximately 30% of the total wall
thermal resistance to be on the exterior of the frame wall
to control condensation.
The function of the R-5 rigid insulation on the interior
of the masonry foundation stem wall is to control
condensation within the blanket insulation. The R-5
rigid insulation extends down to the footing to provide
effective thermal resistance. The protective membrane
strip that is installed under the sill plate and is sealed to
the rigid insulation also controls entry of termites.
The masonry foundation stem wall should have the
block courses filled solid with grout. The entire below
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