ASHRAE Journal - March 2021 - 51

COLUMN BUILDING SCIENCES

For example, we put coarse gravel
(no ﬁnes) and a polyethylene vapor
control layer under a concrete slab
to keep the water vapor and water
in the ground from getting into the
slab from underneath. The gravel
and polyethylene do nothing for
the water already in the slab. This
water can only dry into the building.
Installing ﬂooring, carpets or tile
over this concrete before it has dried
sufﬁciently, or if a top-side vapor
control layer is not installed, is a
common mistake that leads to mold,
buckled ﬂooring and lifted tile.
Similarly, we install dampproofing on the exterior of concrete
foundation walls and provide a
water-managed foundation system
to keep water vapor and water in the
ground from getting into the foundation from the exterior. Again, this
does nothing for the water already
in the foundation wall. When we
then install interior insulation and
ﬁnishes on the interior of a foundation wall in a manner that does not
permit drying to the interior, mold
will grow.
Foundation wall and slab assemblies must be constructed so they
resist water vapor and water from
getting in them, but they also must
be constructed so it is easy for water
vapor to get out when it gets in, or if
the assembly was built wet to begin
with (as they typically are).
Drying a foundation wall assembly or ﬂoor slab after it is insulated
and after surface ﬁnishes have been
installed should only be done using
diffusion ( " letting them breathe " ),
not airﬂow ( " ventilation " ). Allowing
interior air (that is usually full of
moisture, especially in the humid
summer months) to touch cold
foundation surfaces will cause

condensation and wetting, rather
than the desired drying. It is important that interior insulation assemblies and ﬁnishes be constructed as
airtight as possible but vapor permeable. This will prevent interior
moisture-laden air from accessing
cold surfaces during both the winter and summer and still allow the
assemblies to dry. It is extremely
important not to have a vapor barrier on the interior of internally
insulated basement assemblies.
The traditional approach to basement moisture control has been
to locate the water control on the
outside and then allow drying to
the inside. Drainage, water control
layers (water-prooﬁng), capillary
control layers (dampprooﬁng) and
vapor control layers (dampprooﬁng)
have historically been located on
the outside of basement perimeter
walls, and crushed stone layers and
plastic vapor barriers have been
located under concrete slabs. The
operative principle has been to keep
the liquid ﬂow due to groundwater
and the liquid ﬂow due to capillarity
out of the structure and locate vapor
control layers (vapor barriers) on the
outside-and allow inward drying to
the basement space where moisture
can be removed by ventilation or
dehumidiﬁcation.
Two generic basement foundation
approaches are typical: insulate the
inside or insulate the outside. The
most logical location from the physics perspective is to locate the insulation on the outside. By locating
the insulation layer outward of the
structure and outward of the control
layers, the foundation is kept at a
constant temperature, and the insulation system does not interfere with
the inward drying of the assembly.

FIGURE 1 Soil Gas Control. Approach for basement
foundations.

Roof Flashing

Vent Stack

Sealant at
All Slab
Penetrations

Concrete Slab
Polyethylene Vapor
Barrier
Granular
Drainage Pad
(No Fines)

Perforated Drain Pipe Added to " T " in Order to Couple
Sub-Slab Pressure Field to Vent Stack

Exterior basement insulation is
completely compatible with the
traditional approach for foundation
water control.
Unfortunately, exterior basement
foundation insulation can have signiﬁcant application problems that
often make it impractical to use. The
ﬁrst is the difﬁculty in protecting the
insulation layer during construction
and subsequently during its useful service life. The second is insect
control. Exterior insulation can be
an " insect interstate " that provides a
direct pathway into the structure.
These factors have resulted in
primarily locating insulation layers
on the interior. However, locating
insulation layers on the interior
often conﬂicts with the traditional
approach of foundation water
control-namely inward drying.
Constructing frame walls, insulating the resulting cavity and covering
with an interior plastic vapor barrier

MARCH 2021

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ASHRAE Journal - March 2021

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