ASHRAE Journal - December 2023 - 13

COLUMN BUILDING SCIENCES
Closed-cell spray polyurethane foam signifi cantly
improves the wind uplift resistance of the roof assembly
under hurricane conditions.
A drainage mat is provided behind the stucco layer
installed over the frame wall to control hydrostatic
pressure. This assembly is drained to the exterior at a
weep screed located at the bottom of the wood framing.
It should not be drained into the concrete masonry unit
(CMU) wall.
The wood frame 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 fl uctuations in labor and materials.)
Although the minimum R-value of the insulation in
this wall is R-13 per the IECC, fi re code may require
the entire cavity to be fi lled, likely resulting in a higher
R-value for a 2 × 6 wall.
Avoid vinyl wallpaper and oil-based paint or coatings
in Climate Zone 2A. These wall coverings are vapor
impermeable and increase the risk of condensation
within the wall.
The CMU wall's interior is insulated with rigid
insulation that also controls inward vapor drive out of
the CMU mass wall. The rigid insulation's thickness is
typically selected to allow electrical boxes to be installed
on CMU mass wall's interior surface without having to
chisel the masonry blocks to seat electrical boxes. Only
R-6 rigid foam insulation is needed on CMU wall's interior
to meet energy effi ciency code, but thicker insulation may
be needed to accommodate electrical outlets.
The polyethylene vapor barrier under the concrete
slab extends over the top of the CMU stem wall to act as a
capillary break.
Portions of the CMU wall may need to be fi lled solid
to meet structural requirements or to accommodate
anchor bolts for a sill plate.
IECC Climate Zone 3B: Unvented Attic, 2 × 6 Wall,
Monolithic Slab on Grade (Figure 3)
The insulation in this unvented attic is located on the
FIGURE 2 IECC Climate Zone 2A. Unvented attic with spray foam at roof deck,
2 × 6 wall, concrete masonry unit (CMU) wall, elevated slab.
Tile Roofi ng
Purlins
Roofi ng Paper
Roof Sheathing
R-49 Open-Cell or Closed-Cell
Polyurethane Foam
Unvented Roof Assembly
Baffl e Backstop
For Spray Foam
Insulation
Polymer-Modifi ed or
Standard Portland
Cement Stucco
Paper-Backed Lath
Drainage Mat
Water Control Layer
(Building Paper, Housewrap,
Or Building Wrap)
Plywood or OSB
Sheathing
Membrane Strip Covering
Wood Frame-Masonry
Intersection Under Weep
Screed Flashing
Polymer-Modifi ed (PM)
Or Standard Portland
Cement Stucco)
Control Joint
Fill Concrete
Block Stem
Wall with Concrete
Stucco Damp-Proofed
With Latex Paint to
Reduce Water Absorption
Masonry Wall
Ground Slopes
Away From Wall at
5% (6 in. per 10 ft
Concrete
Footing
Below Frost Depth
underside of the roof sheathing. With tile roofi ng and
vapor-permeable roofi ng paper, no venting or vapor
diffusion ports are required for this roof assembly. If the
roofi ng material were shingles, vapor diffusion ports
would be needed. Unvented attics are more resistant to
entry of embers from wildfi res.
The purpose of the rigid foam insulation on the
exterior of the wall is to resist inward vapor drive
from the absorptive stucco cladding. This wall has no
structural sheathing. Instead, it has metal bracing (not
shown) for racking resistance.
D E C EM B E R 2 0 2 3 ashrae.org ASHRAE JOURNAL
13
Horizontal Fire Stop
Vertical Wood Furring
R-6 Minimum Rigid Foam Insulation:
Extruded Polystyrene
Or Polyisocyanurate
" Seat " in Concrete to Receive
Masonry Block
Concrete Slab
Polyethylene Vapor Barrier
Sub-Slab Stone Layer
(No Fines)
Latex Paint (Class III Vapor Retarder)
2×6, 24 in. o.c. Advanced Framing
1/2 in. Gypsum Board with Latex Paint
(Class III Vapor Retarder) Caulked, Glued or
Gasketed to Bottom Plate
R-13 Minimum Cavity Insulation: Cellulose,
Fiberglass, Mineral Wool or Open-Cell Spray
Foam; 2 × 6 Cavity Shown Completely Filled
For Fire Protection
R-13 Minimum Insulation: Cellulose,
Fiberglass, Mineral Wool or Open-Cell
Spray Foam
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ASHRAE Journal - December 2023

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