ASHRAE Journal - December 2023 - 19

COLUMN BUILDING SCIENCES
FIGURE 7 IECC Climate Zone 7 and 8. Vented cathedral ceiling, 2 × 6 wall, interior insulated basement.
Roofi ng
Roofi ng Paper
Roofi ng Sheathing
2 in. Vent Space
16 in. I-Joist Rafter
Insulation Baffl e Prevents Wind Blowing Through
Insulation and Maintains 2 in. Clearance Under
Roof Sheathing
Water Protection Membrane
(Ice-Dam Protection
Where Required)
R-50 Insulation: Cellulose,
Fiberglass or Mineral Wool
Continuous Soffi t Vent
Sealant
Vinyl or Aluminum Siding
R-15 (IECC Zone 7) or R-20
(IECC Zone 8) Rigid Insulation
with Joints Taped: Extruded
Polystyrene or Foil-Faced
Polyisocyanurate
Perforated Plastic Bracket for
Insect/Rodent Protection of
Rigid Insulation
Ground Slopes Away At 5%
(6 in. per 10 ft)
Impermeable Backfi ll
Granular Backfi ll
Concrete Foundation Wall
Dampproofi ng
Sealant Bead Sealing
Rigid Insulation to
Foundation Wall
Stone Drainage Pad
Filter Fabric
Perforated Drain Pipe
Capillary Break Over
Footing (Dampproofi ng
Or Membrane)
Concrete Footing
R-10 Rigid Insulation with
Joints Taped: Extruded
Polystyrene or Foil-Faced
Polyisocyanurate
Wood Furring 24 in. o.c.
Wall Gypsum Must be Installed
and Caulked to Rigid Insulation
with Sealant Before Ceiling
Gypsum Board is Installed
1/2 in. Gypsum Board with a Class
III Vapor Retarder (Latex Paint)
2 × 6, 24 in. o.c. Advanced Framing
R-20 Insulation: Cellulose, Fiberglass,
Mineral Wool or Open-Cell Spray Foam
Sealant
R-20 Insulation: Cellulose, Fiberglass,
Mineral Wool or Open-Cell Spray Foam
Sealant
Foam Filter
Sill Gasket
1/2 in. Gypsum Board with a Class
III Vapor Retarder (Latex Paint)
2 × 4 Wood Frame Wall
R-13 Insulation: Cellulose, Fiberglass,
Mineral Wool or Open-Cell Spray Foam
R-5 Rigid Insulation with Joints Taped: Extruded
Polystyrene or Foil-Faced Polyisocyanurate
Joint in Rigid Insulation to Facilitate
Application of Sealant
Treated Wood Bottom Plate
Sealant Bead Sealing Rigid Insulation to Slab
Sill Gasket Capillay Break
4 in. Concrete Slab
Polyethylene Vapor Barrier
R-10 Rigid Insulation Extruded
Polystyrene
Sub-Slab Stone Layer (No Fines)
Rigid Insulation as Bond Break
Drain Pipe Through Footing
control condensation within the framing cavities. There
is no interior vapor barrier on the interior surface of the
wall; there is a vapor " retarder " (Class III vapor retarder-
semipermeable latex paint). The reason there is no
interior vapor barrier is to permit drying to the interior.
For Climate Zone 8, this assembly does not require
any cavity insulation per the 2021 IECC due to the R-20
continuous rigid foam. However, fi lling the cavity may
be needed or preferred for fi re protection, acoustics and
energy effi ciency.
Attic Ventilation Through
Continuous Ridge Vent
An alternative to this wall assembly
that meets both thermal and vapor
control code requirements is to use
an R-13 2 × 4 wall with R-10 (Climate
Zone 7) or R-12.5 (Climate Zone 8) rigid
foam on the outside.
The rigid foam on the wall's exterior
allows construction without using
structural sheathing; alternative wall
bracing methods are used instead,
and the rigid foam provides a backstop
for the cavity insulation and some
structural support to the siding. Many
contractors have found this more
economical. The R-5 rigid insulation on
the interior of the concrete foundation
wall is required to control condensation
within the interior frame wall as there is
no interior vapor barrier on the interior
of the frame wall. There is a Class III
vapor " retarder " (semipermeable
latex paint). The reason there is no
interior vapor barrier on the interior
of the frame wall is to permit drying
to the interior. Additionally, the R-5
rigid insulation on the interior of the
concrete foundation wall reduces the
rate at which moisture in newly placed
concrete moves to the interior.
The plastic L-bracket protecting
the bottom edge of the exterior rigid
insulation should be perforated to
facilitate drainage.
The vertical " short " strip of rigid
insulation where the basement
concrete slab intersects the exterior
concrete basement wall has two lines
of continuous sealant. The fi rst seals the concrete slab
edge to the interior surface of the vertical strip. The
second seals the top back corner of the vertical strip to
the concrete foundation wall. The two seals provide air
control layer continuity between the concrete slab and
the concrete wall to control radon and other soil gases.
Horizontal insulation on the underside of the
concrete fl oor slab is for comfort and to control dust
mites in carpets by reducing the relative humidity in
the carpets.
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