TM 5-852-9/AFR 88-19, Vol. IX
Metal hoods with plywood lining and baffles should be provided to prevent frost accumulation from
restricting air flow through the intake hood. During the winter, a slight positive pressure normally occurs in
a heated building; therefore, mechanical pipe chases for plumbing in buildings should be sealed to prevent
the condensation, in cold attics and on the undersides of cold roofs, that is caused by moisture rising to these
areas. To prevent condensation, vent pipes, ducts, rain leaders, and chimneys should be insulated where
sections go through the cold attic. All joints in walls and roof should be sealed to prevent snow, wind, and
condensation infiltration, particularly if a negative pressure could develop in the building.
b. Vapor retarder. The vapor retarder should be installed on the warm side of the insulation. Vapor
retarder materials are normally required to have a permeance not to exceed 0.5. To obtain a continuous vapor
retarder, care must be exercised to assure that all joints, corners, and penetrations are completely sealed with
properly specified mastic or sealants. Well detailed designs and careful supervision during construction must
be provided to obtain maximum retarder continuity between walls and roof. The design should strive to
provide a 100 percent vapor retarder. The degree of success is a function of both the material used in
construction of the basic structure, and the skill, diligence, and supervision of the workmen applying the
vapor retarder. Therefore, design provisions should always be made to assure that the insulation behind the
vapor retarder can breathe, that is, rid itself of moisture before excess trapped moisture can develop which
would lead to deterioration. There is an economic balance point where the cost of attaining 100 percent vapor
retarder has to be weighed against the potential damage aspect from nonattainment. The vapor retarder
should consist of a not less than -ounce copper sheet, or 2 to 3-mil-thick aluminum foil adhered to heavy
kraft paper with glass fiber reinforcing spaced not more than 1/4 inch in each direction, or 4 to 8 mil
polyethylene sheet. Polyethylene sheet does not meet the flame spread and smoke development rating listed
previously. It may be used, however, if covered by properly designed gypsum wallboard or a fire resistant
material. The polyethylene material is considerably less expensive and easier to install than the other vapor
retarders, resulting in fewer and better sealed joints and providing a more effective end product.
c. Insulation. Insulation should be provided in walls, ceilings or roofs, and elevated floors to provide
the required heat transmission "U" value difference between the interior and exterior surfaces (see paragraph
4-8). Three general types of insulating systems currently being used are: rigid board which is normally used
on top of roof decks, in refrigeration plants, and as perimeter insulation at foundation walls; batts or blankets
between furring or framing members in walls and ceilings; and insulated panels in sandwich construction. For
information on insulation for roof decking see TM 5-805-3 and TM 5-805-14. For typical installation of
vapor retarder and insulation see figures 2-6 and 2-7. All exterior foundation walls shall have properly
designed (2-inch minimum) perimeter insulation board extending from the top of the footing to the bottom
of floor slab. At doors, a preformed joint filler should be used between slab(s) and wall. Depending upon the
wall construction, the insulation may be on the exterior or interior face of the wall. If used on the exterior,
it shall be protected with metal or other durable finish materials and shall be extruded polystyrene conforming
to ASTM C-578, Type IV which has a low moisture coefficient. Type I is acceptable on interior surfaces in
areas where the fill does not retain moisture and the water table does not rise up to the insulation level. All
insulation shall meet flame spread and smoke development rating requirements, or shall be covered on the
interior by properly designed gypsum wallboard, or a fire resistant material.
(1) Spaces between wall framings and door and window frames should be fully and carefully filled
with insulation. This not only provides continuous insulation, but minimizes cold air penetration and
infiltration.
(2) Blanket-type insulation should be fastened down near the eave vents to prevent it from curling
back or being lifted which can block the air flow and reduce the insulation value in these areas. Wire mesh
baffles between rafters are often used to maintain vent space (see figure 2-8).
(3) All interior air spaces in the wall cavity should be filled with insulation or blocked at intervals
to minimize the chimney effect of air flow within the cavity. This construction helps reduce moisture
condensation and freezing in the cavity. Where the air space is not completely filled, all types of insulation
must be adequately supported to assure that settlement does not occur.
d. Ventilation. In addition to an effective vapor retarder and sufficient insulation, a building needs
adequate ventilation to eliminate indoor pollution, reduce excessive humidity, and bring in fresh air.
(1) Attic, roof, and under-floor ventilation. Usually, a small amount of frost or vapor condensation
accumulates on the underside of the cold roof even though the ceiling is protected with an effective
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