ASHRAE Journal - March 2021 - 52

COLUMN BUILDING SCIENCES

FIGURE 2 Concrete basement foundation with interior rigid insulation.

Gypsum Board with Semipermeable (Latex) Paint

Vinyl Siding

PHOTO 2 Floor Slab Insulation. Continuous rigid
insulation is located under the basement floor slab.
This insulation can be any rigid insulation product.
All work if installed over a granular layer.

Sealant, Adhesive or Gasket

Sealant at Corner of Bottom Plate and Subfloor or
Gasket Under Bottom Plate

Adhesive
Sealant, Adhesive or Gasket

Cavity Insulation
Sealant
Sealant, Adhesive or Gasket

Plastic L-Bracket for Insect/
Rodent Protection of Rigid
Insulation

Sill Gasket
Concrete Foundation Wall
Unfaced Extruded or Expanded Polystyrene Rigid Insulation
(Vapor Semipermeable with Taped or Sealed Joints)

For Insect Protection Provide 3 ft of Mulch
And Then Drought-Resistance Plants
Ground Slopes Away From
Wall at 5% (6 in. per 10 ft)

Gypsum Board Thermal Barrier Necessary When Rigid
Insulation is Not Rated for Exposed Application

Exterior rigid insulation is located
at the rim joist floor framing to conSealant
trol summer condensation. When
Free-Draining Backfill
Gypsum Board Held Up From Slab
insulating sheathing is not used,
Dampproofing
Concrete Slab
rigid insulation should be installed
Extruded Polystyrene, Expanded Polystyrene Or
High Density Mineral Wool
to the interior of the rim joist or an
Filter Fabric
air impermeable insulation should
Stone Drainage Bed
be applied at the rim joist assembly.
Note the capillary break at the top of
Perforated
Drain Pipe
the footing. Also note the continuous
Sub-Slab Stone Layer
Concrete
Footing
(No Fines)
Capillary Break Over Footing
rigid insulation under the basement
Drain Pipe Through Footing
(Dampproofing or Membrane)
floor slab (Photo 2). Further note the
air sealing of the rigid insulation at
is common and often leads to odor, mold, decay and cor- the top perimeter edge of the concrete floor slab-a sealrosion problems.
ant is used to seal the top of the concrete slab to the rigid
The most common examples of configurations of coninsulation, and an additional sealant is used to seal the
trol layers and control approaches follow for residential
rigid insulation to the interior of the concrete perimeter
basement foundations.
foundation wall. These two seals are necessary to control
soil gas ingress.
Impermeable Backfill

Gypsum Board Over Furring Strips

Concrete Basement Foundation With
Interior Rigid Insulation

Concrete Basement Foundation With
Interior Spray Polyurethane Foam Insulation

Applicability-All Hygro-Thermal Regions
Figure 2 illustrates a concrete basement foundation
with interior rigid insulation. The key to this assembly
is the use of non-water-sensitive rigid insulation on
the interior that still permits drying to the interior. The
recommended permeance of the interior rigid insulation layer is approximately 1 perm. This typically limits
the thermal resistance of the interior rigid insulation
layer. No interior vapor control layer is located within
the frame wall, thereby permitting inward drying. All
interior concrete surfaces are wrapped with the rigid
insulation layer-particularly at the top of the wall and at
foundation " step downs. "
52

ASHRAE JOURNAL

ashrae.org

MARCH 2021

Applicability-All Hygro-Thermal Regions
Figure 3 illustrates a concrete basement foundation
with interior spray polyurethane foam insulation.
Polyurethane spray foam insulation can be directly
applied to the interior of concrete foundation walls.
High density closed-cell spray foam should be used. The
recommended permeance of the interior spray foam
insulation layer is approximately 1 perm. This typically
limits the thermal resistance of the interior spray foam
insulation layer. An insulated frame wall assembly can
be located to the interior of the interior spray foam insulation to increase the assembly thermal resistance. No

http://ashrae.org

ASHRAE Journal - March 2021

Table of Contents for the Digital Edition of ASHRAE Journal - March 2021