ASHRAE Journal - December 2023 - 15

COLUMN BUILDING SCIENCES
grade portions of the stem wall should be coated with
dampproofi ng to control capillary uptake.
IECC Climate Zone 5C : Unvented Roof, 2 × 6 Wall, Vented
Crawl Space (Figure 5)
This unvented roof assembly should not be used
where the ground snow load is greater than 50 lb/ft2
(293 kg/m2), as this assembly may not adequately
prevent ice damming, and vented roofs should be
used instead.
The rigid foam insulation at the roof deck is required
to control condensation and to meet the energy
effi ciency code (2021 IECC). There is no interior vapor
barrier; 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.
The roof deck rigid foam R-value required for vapor
control depends on the R-value of the cavity insulation.
In Climate Zone 5C, the R-value of the rigid foam should
be at least R-20 per Table 806.5 of the 2021 International
Residential Code (IRC). However, if a higher amount
of cavity insulation were to be used, a higher R-value
rigid foam would need to be used to control vapor.
DOE-funded research has shown that the R-value of the
rigid foam should be at least 40% of the total R-value
in Climate Zone 5C, which is more conservative than
code. Figure 5 shows R-25 rigid insulation and R-35
cavity insulation. This meets the 40% vapor control
recommendation: 25 / (35 + 25) = 42%. Regardless of
which type of cavity insulation is used, there must not be
a gap between the cavity insulation and the sheathing/
rigid foam above it, per Section 806.5 of the 2021 IRC.
The combined R-value of the rigid foam and the cavity
insulation must be at least R-60.
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. This must be balanced
with the difference in insulation costs for the two
framing approaches.
The R-7.5 rigid insulation on the exterior of the wall
framing is required per IRC Table R702.7(3) to control
condensation within the framing cavities as there is
no interior vapor barrier. There is a Class III vapor
FIGURE 4 IECC Climate Zone 4C. Vented attic, 2 × 6 wall, conditioned crawl
space.
Roofi ng
Roofi ng Paper
Roofi ng Sheathing
Insulation Baffl e Prevents Wind Blowing
Through Insulation and Maintains 2 in.
Clearance Under Roof Sheathing
Raised-Heel Roof Truss Provides Increased
Depth of Roof Insulation at Perimeter
Continuous
Soffi t Vent
R-60 Insulation:
Cellulose/Fiberglass or
Mineral Wool
Polymer-Modifi ed or Standard
Portland Cement Stucco
R-10 Rigid Insulation:
2 in. Expanded Polystyrene
Water Control Layer
(Building Paper, Housewrap
Or Building Wrap)
Sealant
Weep Screed
R-5 Rigid Insulation with
Joints Taped; Extruded
Polystyrene
Top Courses,
Filled Solid
Ground Slopes Away
From Wall at 5%
(6 in. per 10 ft)
Dampproofi ng
If Exterior Grade is Lower
Than Interior Crawlspace
Grade, No Perimeter Drain
Is Necessary
Ceiling Gypsum Board Installed
Before Wall Gypsum Board with
Room Perimeters and Caulked
with Sealant to Top Plates
Gypsum Board with (Latex) Paint
2×6, 24 in. o.c. Advanced
Framing
R-20 Insulation: Cellulose/
Fiberglass or Mineral Wool
Sealant at Corner of Bottom
Plate and Subfl oor or Gasket
Under Bottom Plate
R-20 Insulation: Cellulose/
Fiberglass or Mineral Wool
Protective Membrane Also
Acts as Capillary Break
R-13 " Blanket " Insulation:
4 in. Polyethylene-Faced
Fiberglass
Continuous Polyethylene
Vapor Barrier/Air Barrier
(All Joints Taped) Taped to
Perimeter Rigid Insulation
Masonry Foundation Wall
Concrete Footing
Below Frost Depth
Capillary Break Over
Footing
" retarder " (semipermeable latex paint). The reason
there is no interior vapor barrier is to permit drying to
the interior.
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 plastic L-bracket protecting the bottom edge of
the rigid exterior wall insulation should be perforated
to facilitate drainage.
The R-10 rigid insulation on the underside of the fl oor
framing is also a vapor barrier that prevents moisture
from the vented crawlspace from damaging the fl oor
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