ASHRAE Journal - December 2023 - 18

COLUMN BUILDING SCIENCES
FIGURE 6 IECC Climate Zone 6. Vented over roof, unvented cathedral ceiling,
2 × 6 wall, interior insulated basement.
Roofing
Fully Adhered Roof Underlayment
2 × 4 Installed " Upright " ; 3 1/2 in. Airspace
R-30 Rigid Insulation with Joints Staggered:
Extruded Polystyrene or Polyisocyanurate
R-30 Insulation: Cellulose, Fiberglass,
Mineral Wool or Open-Cell Spray Foam
Roof Sheathing
Fully Adhered Roof
Membrane Underlayment
Air Control Layer
Drip Edge
2 in. Vent Space
Blocking
Siding Over 1 × 4 Furring
Air Gap and Furring Strips
R-11.25 Rigid Insulation with Joints
Taped: Extruded Polystyrene or
Foil-Faced Polyisocyanurate
R-20 Insulation: Cellulose,
Fiberglass, Mineral Wool or
Open-Cell Spray Foam
Sealant
Sill Gasket
Perforated Plastic Bracket for
Insect/Rodent Protection
Of Rigid Insulation
Ground Slopes Away
From Wall at 5% (6 in. per 10 ft)
Impermeable Backfill
( " Clay Cap " )
Free-Draining Backfill
Dampproofing to Grade
Sealant
Rigid Insulation as
Bond Break
Capillary Break Over Footing
(Dampproofing or Membrane)
Filter Fabric
Perimeter Drain
Drain Pipe Through
Footing Every 10 ft
8 in. Concrete Foundation Wall
Roof Ventilation Through
Continuous Ridge Vent
2 × 12
Wood Rafters
1/2 in. Gypsum
Board with a Class
III Vapor Retarder
Coating (Latex
Paint)
Continuous Rim
Joist, Rafter Joist
Spray Foam Air Seal
2 × 6, 24 in. o.c.
Advanced Framing
1/2 in. Gypsum Board w/
Class III Vapor Retarder
Coating (Latex Paint)
Sill Gasket
Spray Foam Air Seal
R-15 Rigid Insulation w/
Joints Taped: w/ Extruded
Polystyrene, Interior Gypsum
Board Lining Required for
Fire Rating or Foil-Faced
Polyisocyanurate Rated to be
Left Exposed
Gypsum Board
Treated Wood Bottom Plate
Sill Gasket
4 in. Concrete Slab
6 mil Polyethylene
R-10 Rigid Insulation:
Extruded Polystyrene
4 in. Stone Layer
(No Fines)
Concrete Footing
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
first 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
floor slab is provided for comfort reasons and to control
18
ASHRAE JOURNAL ashrae.org D ECEMBER 2 0 2 3
IECC Climate Zone 7 and 8: Vented Cathedral Ceiling, 2 × 6
Wall, Interior Insulated Basement (Figure 7)
The R-10 rigid foam at the ceiling is not required
for vapor control purposes in Climate Zone 7 as long
as sufficient ridge- and soffit-venting is provided.
However, it will aid the control of moisture by slowing
vapor transfer from the conditioned space to the roof
assembly. The rigid foam brings the total roof R-value up
to the 2021 IECC requirement of R-60. Because the I-joist
rafter is 16 in. (406 mm) and a 2 in. (51 mm) ventilation
space is needed above the insulation, only about 14 in.
(356 mm) are available for the fibrous cavity insulation.
This will typically give an R-value of about R-50, leaving
the need for the R-10 rigid foam. Note that only 1 in.
(25 mm) of ventilation space above the cavity insulation
is required by code, but 2 in. (51 mm) is recommended
in areas where ice damming could be an issue or where
rafter spans are long.
The rigid foam at the ceiling is also used to allow the
creation of a service raceway between the rigid foam and
the gypsum board below it. The rigid foam acts as the air
control layer instead of the gypsum board. The service
raceway allows the air control layer to be undisturbed
by the electrical penetrations in the ceiling. A third
purpose of the rigid foam in this application is that it
also reduces drywall cracking. From a vapor control
standpoint, however, the rigid foam and the raceway
are not required by the 2021 IRC, and R-60 fibrous
cavity insulation could be installed directly on top of
the gypsum board if cavity height allows and if the attic
is properly ventilated. In this case the gypsum board
would become the air control layer and would need to be
carefully sealed, and a Class I or II vapor retarder paint
would be recommended at the ceiling.
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 is faster to assemble. This must be
balanced with the difference in insulation costs for the
two framing approaches.
The R-15 or R-20 rigid insulation on the exterior of the
wall framing is required by 2021 IRC Table 702.7(3) to
dust mites in carpets by reducing the relative humidity
within the carpets.
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ASHRAE Journal - December 2023

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