TM 5-852-4/AFM 88-19, Chap. 4
The stack height required to produce this draft head is
using a mechanical blower system to increase air
circulation (however, see discussion of disadvantages of
latter systems in (k) above. In order to minimize air flow
effectiveness between different parts of the foundation
duct system, the number of ducts connected together
through plenums into a single pair of intake and outlet
stacks should not exceed three to five. Thus, an
average foundation of this type may have numerous
intake and outlet stacks. A chamber extending the
length of the structure and open to the atmosphere along
its length as shown in figure 4-24 may provide an
acceptable alternative. Plenum chambers should be
designed so as to permit ready access to the ends of the
ducts for cleanout or other maintenance.
insulation may be used to control heat flow for the
To control frost penetration and heave.
To reduce rates of thaw of permafrost and
(1) The general properties of insulating
materials which are pertinent to construction under cold
climatic conditions have been reviewed in paragraph 2-
6d. Insulation used in foundations must satisfy the
If the stack height is too high for the structure, a
following performance criteria:
greater thickness of foundation insulation could be used.
Provide required thermal insulating properties.
In this example the effect of increasing the insulation
Provide adequate bearing capacity for static and
thickness by one-half would result in lowering the stack
dynamic loads which may be imposed.
height by five-eighths.
Resist loss of thermal insulating properties and
The first approximated stack height is next
bearing capacity with time under the effects
incorporated in the calculation of the length of straight
of moisture, ice and cyclic freeze thaw.
duct l S, and the newly obtained le is used to recalculate
(2) Insulation can reduce quantity of heat
the friction head hf. By iteration, the final calculated
flow but cannot prevent it entirely. Insulation should not
stack height is found to be 26.5 feet.
be depended upon by itself to prevent thaw of permafrost
The stack height is an important variable as an
under a continuously heated building or to prevent frost
increase in stack height will increase the duct airflow.
penetration under a continuously refrigerated warehouse
Circulation of air through the ducts results from a density
or other structure.
difference between the air inside the duct and that
(3) Where comfort is involved, as in
outside the building; a pressure reduction at the outlet
quarters buildings, the insulation thickness should not be
end due to the stack effect; a positive pressure heat at
less than that required to maintain floor surface
the inlet end when wind blows directly into the intake
temperature at satisfactory comfort levels under design
stack opening; and a negative pressure head at the
minimum winter temperature conditions.
outlet when wind passes over the exhaust stack opening.
temperature must also be maintained above the dew
If reliance is placed upon wind-induced draft for part of
point of the interior air under these conditions; moist
the required winter cooling, minimum wind conditions
floors are not only unpleasant for personnel but may
should be assumed in order to assure freezeback even
present hazards of slipperiness or sanitation. Much of
in the least favorable winter. Vents should be cowled to
the problem of cold floor discomfort in cold regions
take advantage of any available velocity head provided
originates from cold air drainage from inadequately
by the wind and as previously noted should be positioned
insulated ceilings, exterior walls, windows and doors.
to minimize snow infiltration. If sufficient air cannot be
However, discomfort may be experienced even where
drawn through the ducts by natural draft, consideration
these factors are absent, such as in interior rooms,
may be given to such alternatives as placing, the exhaust
stacks at the center of the building with intakes both
sides, in order to reduce the effective duct length, or