TM 5-852-4/AFM 88-19, Chap. 4
because the floor must necessarily be colder than the air
protected against moisture infiltration and condensation
above it whenever there is downward flow of heat
or seepage by an overlying slab and/or other means, any
through the floor, out of the building; otherwise heat flow
closed-cell type insulation which has high integral
would not occur. It is therefore most important for
resistance to moisture absorption may be used. Unless
ventilated foundations of all types that adequate floor
continuously frozen, installation of any type insulation
insulation be employed, together with a heating system
where it will be below the water table should be avoided.
that delivers enough heat near the floor to counteract so
Instead, alternatives should be sought, such as
far as possible the effect of the heat transmission
construction of the facility on a well-drained granular
through the floor, as well as the effects of downdrafts
embankment where the insulation can be protected
from cold exterior surfaces. Systems which heat the
against moisture by the shelter of the structure itself
floor itself can eliminate discomfort from heat loss
and/or by embedment in high quality impervious concrete
through the floor but thus far have not been widely used
when appropriate.
because of cost.
(7) When insulation is used in a facility
(4) Some typical insulation designs which
such as a hangar floor, where live loads occur, it must be
have been used in actual structures in Alaska and
placed at sufficient depth so that concentrated live load
Greenland are shown in figures 4-15, 4-19 through 4-22
stresses will be reduced sufficiently to be within the
and 4-24 through 4-28. In general, the insulation
allowable bearing values for the particular insulation
amounts shown in these figures have provided only
used. At the same time the maximum allowable depth of
marginal comfort under winter conditions.
insulation placement established by the live load stress
(5) When insulating materials are used in
in combination with the increase of overburden pressure
foundations, conditions may be extremely adverse for
with depth must not be exceeded.
(8) Insulation of ventilated and ducted
satisfactory performance of these materials.
In
foundations.
insulation is installed below ground level under wet
conditions, its value may be reduced or lost as a result of
(a) For
structures
with
open
absorption of moisture (para 2-6d). If exposed to cyclic
airspaces such as shown in figures 4-13 through 4-23
freeze-thaw, progressive physical breakdown of the
and ducted foundations such as shown in figures 4-24
insulating material may occur, again with increase of
through 4-28 insulation should be installed above the
moisture content and with loss of thermal insulating and
airspace or ducting system. This will not only minimize
strength properties. In addition to use for controlling flow
the amount of heat which must be removed by the
of heat from buildings into the foundations, insulating
foundation ventilation system but will provide excellent
material may be used below ground level for a number of
protection for the insulating material against moisture
miscellaneous purposes such as to reduce the thickness
and freeze-thaw effects. Heat losses may be computed
117
of fill required to prevent freezing and thawing
by the procedures of the ASHRAE Guide .
temperatures from penetrating into underlying frost-
(b) For ducted foundations, trial
susceptible soil, to thermally protect buried pipelines or
computations for various alternative duct system and
utilidors, or to control freeze and thaw penetration under
insulation design combinations will yield data on
and around bridge piers, grade beams, culverts or paved
insulation requirements for foundation thermal stability.
surfaces. In any such applications where detrimental
Computations of thicknesses required to maintain
moisture, ice, or freeze-thaw effects may be
comfortable floor temperatures and to prevent
encountered, great care must be exercised in specifying
condensation will provide additional input. From these
type, placement and protection of insulation.
data, a decision on insulation thickness can be made.
(6) Only closed-cell types of insulation
(c) For open airspace type ventilated
should be used underground. Cellular glass may be
foundations, comfort and heating economy will usually
used under high ground moisture conditions if it will be
determine insulation thickness requirements.
(9) Insulation of slab on grade and basemented
either continuously frozen or continuously thawed.
Under these conditions no separate moisture barrier or
foundations.
protective membrane for the cellular glass is required.
(a) These types of construction are
Cellular glass should not be used where it would be
suitable for use in seasonal frost areas and thaw-stable
subject to cyclic freeze-thaw in presence of moisture.
permafrost without detrimental ground ice. Because of
Where cyclic freeze-thaw in presence of moist (but not
the susceptibility of insulating materials to moisture, ice
immersed) conditions is anticipated, it is recommended
and freeze-thaw effects, sufficient elevation above the
that only foam plastic closed-cell types of insulation,
natural terrain should be provided in such foundations,
protected against absorption of moisture by a self-
together with drainage, so that exposure of insulation to
membrane or by sealed heavy polyethylene sheeting or
these adverse effects will be minimized. It should be
equal, should be used. In soils of permanently very low
noted that even where permafrost foundation materials
moisture content, not subject to cyclic freeze-thaw, and
4-46
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