ASHRAE Journal - May 2020 - 77

COLUMN BUILDING SCIENCES

FIGURE 1 Stem wall with horizontal insulation. Extruded polystyrene (XPS) or
expanded polystyrene (EPS).

Cavity Insulation
Cladding
Continuous Exterior Insulation
Protective Membrane Strip Also
Acts as Capillary Break and Acts
as a Termite and Insect Barrier
Rodent Protection for
Continuous Rigid Insulation
For Insect Protection Provide 3 ft of
Mulch and Then Drought Resistent Plants
Ground Slopes Away From
Wall at 5% (6 in. per 10 ft)

Concrete
Foundation Wall
Concrete Footing Below
Frost Depth

FIGURE 2 Stem wall with horizontal insulation. Semirigid board insulation (mineral
wool/stone wool or fiberglass). Note the requirements for damp proofing the stem
wall when using these types of board insulation.

Gypsum Board
Sealant, Adhesive or Gasket

Cladding
Continuous Exterior Insulation

Cavity Insulation

Sill Gasket

Water Control Layer/Sheathing
Protective Membrane Strip Also
Acts as Capillary Break and Acts
as a Termite and Insect Barrier

Sill Gasket

Rigid Insulation as
Bond Break Material
Polyethylene Vapor Barrier

Concrete Slab
Rigid Board Insulation
Extruded Polystyrene (XPS) or
Expanded Polystyrene (EPS)
Granular Capillary
Break and Drainage
Pad (No Fines)

Rodent Protection for Continuous
Exterior Rigid Insulation
For Insect Protection Provide 3 ft of
Mulch and Then Drought Resistent Plants
Ground Slopes Away From
Wall at 5% (6 in. per 10 ft)

Concrete Foundation Wall
Damp Prooﬁng
Concrete Footing Below
Frost Depth

Gypsum Board
Sealant, Adhesive or Gasket
Rigid Insulation as Bond Break Material
Polyethylene Vapor Barrier

Concrete Slab
Semirigid Board Insulation
Mineral Wool/Stone Wool or
Fiberglass
Granular
Capillary
Break and
Drainage Pad
(No Fines)

Capillary Break

All the insulations need to be installed over a granular
capillary break. All the insulations are capillary active-
even extruded polystyrenes (XPS).‡ This capillary break
serves a second function. It provides for control of radon
and other soil gas ingress when it is passively vented up
through the roof to the atmosphere with a radon or soil
gas vent. It gets better. This capillary break also acts as
a "drainage pad" where groundwater can be drained
through the foundation wall to the exterior. You get a
three-for-one.
And you need to seal the top of the slab-across the
rigid insulation bond break-to the top of the stem wall
using a masticed stainless steel mesh membrane strip.
This is-and I use the next word with emphasis-absolutely necessary to provide a termite barrier and insect
barrier and to provide air control layer continuity

between the perimeter wall and the slab itself, which is
acting as the air control layer of the foundation.§
Figure 1 and Figure 2 show rigid board insulations-foam
plastics such as extruded polystyrene (XPS) or expanded
polystyrene (EPS) and semirigid board insulation such
as mineral wool/stone wool or ﬁberglass, respectively.
Both require a polyethylene vapor barrier in direct contact with concrete. Don't even think of placing a sand
layer between the polyethylene vapor barrier and concrete (see "Concrete Floor Problems," ASHRAE Journal,
January 2008).
In both Figure 1 and Figure 2, the insulation layers
extend horizontally inward-typically 4 ft (1.2 m) with
a thermal resistance of R-10. This level of thermal

‡But,

but, but there is test that shows we can submerge XPS insulation in a tub of water for a year, and then take it out and weigh it.
See, it does not absorb any water. Ah, yes, but now put one side of the XPS in contact with the water and create a temperature difference across the insulation-different result, eh? What are the odds that you will have thermal gradient across thermal insulation?
Yes, the submerged water test is another stupid test method that should be ignored, right up there with putting a 20 inch water column
(5000 Pascals) on a building wrap or house wrap that does not have nail holes or screw holes. What are the odds that you are going to
have nail holes or screw holes when you attach cladding with-wait for it-nails or screws?

§In some jurisdictions an independent inspection agency may be required to inspect and certify such termite control prior to commencing
framing.

ASHRAE Journal - May 2020

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