ASHRAE Journal - May 2020 - 76

BUILDING SCIENCES
COLUMN

Joseph W. Lstiburek

Concrete Solutions
BY JOSEPH W. LSTIBUREK, PH.D., P.ENG., FELLOW ASHRAE

Concrete slab-on-grade foundations should be easy to insulate-but they are proving
not to be easy to insulate. We were here more than a half decade ago ("Slab Happy,"
ASHRAE Journal, April 2012) and again a couple of years ago ("Hot and Wet But Dry,"
ASHRAE Journal, June 2016). It is apparently time again to go over what is working and
what is not.
The absolute easiest slab-on-grade foundation to
insulate is a stem wall*-except when it is not easy. Some
structural engineers hate stem walls, for good reason.
Other structural engineers hate stem walls, not for good
reasons.
First, for good reasons. When we have bad soil conditions, stem walls are difﬁcult to make work. You are
going to end up with a monolithic slab/grade beam that
may also be post tensioned. They are a bear to insulate,
but it can be done. Note this is a change in my opinion of
a couple of years ago when I pretty much said it was not
possible to do so practically. I still think it is a pain in the
hind quarters. We will talk about how to do them later.
Second, for not good reasons. When we have good soil
conditions, it is easy to make stem walls work-structurally. But some folks refuse to believe it and are making
folks structurally connect stem walls to slabs. This is
* A stem wall is a short foundation wall cast on a footing that surrounds the perimeter of the slab and supports the above grade
walls and sometimes-but not always supports the slab-and
sometimes-but not always-is structurally connected to the slab.
†You

connect them when your structural engineer tells you to
connect them. Don't mess with your structural engineer. Chat with
your structural engineer and ask why-and there is usually a very
good reason. Structural engineers love it when someone talks
to them because things are typically very boring for them. Pretty
much they have ﬁgured out how to make everything work. Not like
mechanical engineers. Things are not boring for mechanical engineers because nothing works, and we typically blame the architect.

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not necessary-and it results in major thermal bridging
unless you insulate them externally. And wait until we
talk about termites and rodents and external insulation.
More about this later. Can we structurally connect stem
walls to slabs and not get major thermal bridging without external insulation? Yes. And we will go there, but
ﬁrst the easy stuff.
The easy way to do things is to insulate a stem wall on
the inside and thermally decouple the concrete slab
from the stem wall. In this approach the stem wall is
structurally disconnected from the slab. The stem wall
is supported by a footing that is bearing on the soil. The
concrete slab is supported by soil-it is bearing on the
soil. Amazing, both the stem wall and concrete slab are
supported by the same thing-the soil. They do not have
to be connected to each other, except when they have
to.†
Internally insulated stem wall foundations typically
are insulated with two types of insulation-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. Mineral wool/stone wool or ﬁberglass needs
to be of sufﬁcient density to support the slab-typically
around 8 lb/ft3 to 10 lb/ft3 (130 kg/m3 to 160 kg/m3).
Joseph W. Lstiburek, Ph.D., P.Eng., is a principal of Building Science Corporation in
Westford, Mass. Visit www.buildingscience.com.

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ASHRAE Journal - May 2020

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