which were available or affordable to the project.
it is large. The floor's temperature is reduced, but
The designers decided the concept of an SIF
the frost boundary does not advance very deeply
would be the most practical solution to this
at the foundation wall because of the large heat
unique project.
flow contribution from the floor. On the other
hand, if the floor insulation has a high thermal
resistance, heat loss is restricted and thereby
DESIGN CONSIDERATIONS
maintains higher floor surface temperatures. As
For a typical SIF structure with slab-on-grade
a result, the frost boundary advances deeper
construction, about 10% of the heat loss from in-
down the foundation wall and thus the founda-
side the building is through the floor (Torgersen
tion depth must increase. A balance must be es-
1976). The actual amount varies according to the
tablished that results in both a comfortable floor
effectiveness of the insulation in the rest of the
surface temperature and acceptable frost pene-
structure. The heat loss through the floor passes
tration depth.
through the ground below and rises toward the
The next step is to determine the requirements
outside air by means of an approximately semi-
for foundation wall insulation. Great care should
circular path. The effect is to reduce the frost
be taken to ensure interruption of all possible
depth near the foundation compared with the
cold bridges at the junction of the floor, the foun-
frost depth in undisturbed ground. The heat flow
dation wall, and the outer wall. Insulation placed
from a floor will take the path with the least
on the outside of the foundation wall requires
amount of thermal resistance, whether it be the
protection above grade from physical and ultra-
connection between the floor and foundation
violet radiation damage. Insulation installed on
wall (where a cold bridge could form), through
the inside of the foundation wall does not require
the foundation wall, or through the soil under the
this protection; however, with inside insulation
floor and foundation.
there have been cases of the outside foundation
In an SIF structure, the insulation must be
wall becoming cold enough for the soil to ad-
placed so that the majority of the heat loss
freeze to it. If this outside soil then heaves, the
through the floor is guided to the underside of
foundation can be lifted or jacked. Furthermore,
the foundation. This can be done with floor, wall,
walls with inside insulation are more susceptible
and ground insulation, as shown in Figure 3. The
to cold bridges and in some cases are subject to
type of insulation to use, how much, and where
loads from the floor or foundation, and thus re-
to place it depend on many variables, including
quire an insulation with a higher compressive
(but not limited to) climate, soil type, moisture
strength.
conditions, building type and use, snow condi-
Insulation placed horizontally outside the
tions, and exposure.
foundation is called ground or wing insulation
The starting point in an SIF design is to select
and is usually used only in colder climates with a
freezing index greater than 2250 F-days (NBI
the necessary floor insulation. If the insulation
has a low thermal resistance, the heat loss across
1986). The horizontal insulation inhibits the
release of the soil heat, which is stored in the soil
during the summer half of the year and is availa-
Freezing
Effect
ble to retard the downward advance of frost. For
maximum thermal efficiency, ground insulation
should be placed as near the ground surface as
possible (Farouki 1992). Because of the three-
dimensional heat-loss effect at corners, addition-
al ground insulation is usually required. Howev-
er, this could result in damage by inadvertent
digging or vegetation. Care must be taken to pro-
Cooling of
tect the insulation from physical and moisture-
Unfrozen Soil
related damage. In areas where the insulation
Heat Flow
Cooling of
Frozen Soil
may experience loading, an insulation with suffi-
Freezing Heat
cient compressive strength must be specified.
Released
32F Isotherm
The three most common types of insulations
Figure 3. Heat flow lines around a shallow insulated
used in SIFs are mineral wool, expanded or mold-
foundation.
ed polystyrene (EPS), and extruded polystyrene
3
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