UN ITED STATES . I NSTALL ATI ON INFOR MATION . R ESI DEN TIA L
I N S T A L L AT I O N P R O C E D U R E S F O R
T H E R M A X ™ O R S T Y R O F O A M ™
B R A N D R I G I D I N S U L AT I O N
Closed Crawl Space
The closed crawl space is gaining popularity
for its effectiveness in reducing energy loss
and minimizing the potential for moisture
problems. Dow offers the following guidelines
for constructing a closed crawl space with an
insulation system that includes THERMAX™ or
STYROFOAM™ brand rigid insulation on interior
crawl space walls, a continuous poly vapor barrier
to prevent evaporation of ground moisture into
the crawl space, and a means to condition the
crawl space to keep the relative humidity below
70 percent to mitigate mold and mildew growth.
(See Figure 1.)
A b o u t T H E R M A X ™
R i g i d I n s u l a t i o n
THERMAX™ rigid insulation boards have a glassfiber-reinforced polyisocyanurate foam core,
with special laminated aluminum facers that
enhance moisture and damage resistance. This
reinforcement is chemically modified to provide
improved fire performance. THERMAX rigid
insulation boards can be left exposed in crawl
spaces per National Evaluation Services report
NER-681. THERMAX rigid insulation boards are
also Factory Mutual approved as a “Wall-Ceiling
Construction, FM Approvals Standard FM 4880,
Metal-Faced – Class 1 Fire Rated to a max, 30
ft. high.” Consult the local building codes in your
area before starting construction.
A b o u t S T Y R O F O A M ™ B r a n d
R i g i d I n s u l a t i o n
STYROFOAM™ brand rigid insulation boards are
made from extruded polystyrene foam insulation
and have exceptional water resistance, longterm thermal performance and high compressive
strength. ASTM C578 Type X and ASTM
C578 Type IV STYROFOAM Brand Extruded
Polystyrene Foam Insulation boards, with a
maximum nominal thickness of 2 inches (51mm),
may be installed on surfaces (wall, ceiling) of a
crawl space with no coverings (no thermal or
ignition barrier) applied to the crawl space side of
the foam plastic insulation per International Code
Council Evaluation Service (ICC-ES) Evaluation
Report ESR-2142.
Figure 1
Please note that closed crawl spaces may include
components not shown, such as other ventilation
options, floor drains, radon mitigation, etc.
Before You Begin
Before installing THERMAX™ or STYROFOAM™ brand rigid insulation boards on the interior crawl space wall, remove anything
that will interfere with installation. Best results are achieved when waterproofing or damp-proofing is applied from footer to grade
on the exterior of the crawl space wall. Install foundation drainage per code requirement.
The crawl space may contain utilities including, but not limited to, mechanical equipment, electrical wiring, fans, plumbing, gas
or electric hot water heaters, and gas or electric furnaces. When THERMAX or STYROFOAM brand insulation boards are used as
stated above in the crawl space, the following conditions must be met:
1. Under-floor (crawl space) ventilation is provided in accordance with IBC Section 1203.3 or IRC Section R408.1, as applicable; or
unvented crawl spaces are permitted under the conditions prescribed in IRC Section 408.3.
2. Combustion air is provided in accordance with IMC (International Mechanical Code) Section 701 and 703,
or IRC Section M1701 and M1703.
Installation on the interior of the crawl space wall should comply with 2009 IRC Section R408, 2009 IECC Section 402.2.9, Dow’s
ESR 2142 report and all other applicable code requirements. Additional useful information on closed crawl space systems can be
found at www.crawlspaces.org. Consult the local building codes in your area before beginning construction.
Materials Checklist
To insulate closed crawl spaces,
you will need:
• THERMAX™ or STYROFOAM™
brand rigid insulation boards –
select from any of the following:
- STYROFOAM Brand
Square Edge (25psi)
Insulation
- STYROFOAM Brand
Scoreboard (25psi) Insulation
- STYROFOAM Brand
Tongue and Groove
(25psi) Insulation
- THERMAX Sheathing
(with foil facing)
- THERMAX White Finish
Insulation
• Utility knife or small handsaw
• Straight edge
• Measuring tape
• Pencil
• Safety glasses
• Water-based adhesive mastic
• 6-mil poly liner
• Duct tape
• WEATHERMATE™
Construction Tape (optional)
• Spray urethane foam such as
GREAT STUFF PRO™ Gaps
& Cracks Insulating Foam
Sealant (optional)
• FROTH-PAK™ Foam Insulation
kit (optional)
• 6" galvanized nails
• 1" nailing tins
Cutting The Boards
THERMAX™ or STYROFOAM™ brand
rigid insulation boards may be cut
with a straight edge and utility knife or
with a small handsaw. (See Figures
2a and 2b.)
Figure 2a
Figure 2b
Installation
1. Attach a Type 1 Vapor Retarder such
as a 6-mil poly liner to the interior
stem wall with water-based adhesive
mastic. The edges of the vapor
retarder should extend at least 6
inches up the stem wall. Extend the
wall poly liner onto the ground about
1 to 2 feet from the wall. (See
Figure 3.)
Figure 3
2. Fasten the THERMAX™ or
STYROFOAM™ brand insulation
boards to the interior of the
crawl space wall using powerdriven masonry nails with 1-inch
washers or caps. The boards
shall be permanently fastened to
the interior of the stem wall and
extend downward from the top
of the stem wall to the exterior
finished grade level and then
vertically and/or horizontally
for at least 24 inches. In some
situations, an inspection strip may
be appropriate. An inspection strip
is a 2-inch to 3-inch strip around
the top of the stem wall without
insulation, which allows inspection
for termites. (Check with your local
code officials and pest control
operators regarding this option.)
Space fasteners approximately
2 feet on center vertically and
4 feet on center horizontally.
(See Figure 4a.)
Note: If the distance between
the exterior grade level and the
crawl space floor is greater than
24 inches, the rigid insulation
board on the wall should extend at
least 24 inches below the exterior
grade. (See Figure 4b). If the
2
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