Quantcast Weather Resistance

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MIL-HDBK-1012/1
a) Strength, weight, corrosion resistance, shielding, Radio-
Frequency Interference (RFI) attenuation, equipment mounting requirements,
fire criteria, and compatibility of dissimilar metals shall be considered when
selecting frame and skin materials.  Container framing may be formed, rolled,
or extruded sections of aluminum or steel.  Interior and exterior skins are
often of aluminum since it is lightweight and corrosion-resistant and provides
a significant amount of RFI protection even if RFI gasketing and cut-off cores
are not provided.  Any of several plastics may be used as a skin material for
applications such as radar windows; steel may be used if adequate finishes are
applied.
b) Flooring shall be aluminum plate or plywood-metal lamination,
so that metal surface is exposed top and bottom.  Vinyl tile is typically
added to the top surface.
c)  Between interior and exterior wall skins, a thermal barrier,
such as wood or thermosetting plastic laminate conforming to National
Electrical Manufacturers Association (NEMA) Grade L or LE, is required. Any
remaining space in the wall shall be filled with fiberglass or polyurethane
insulation.
d) Roof construction shall be similar to wall construction, except
that the exterior skin shall be a single sheet of aluminum alloy laminated to
a hardboard backing to resist stresses from personnel walking on the roof, as
well as transportation-related stresses.  The laminating adhesive shall be
flexible to allow for thermal expansion of the roof sheet.
8.10.2.2  Weather Resistance. Exterior surfaces of the container shall be as
airtight and watertight as possible.  Water vapor passing through any opening
or non-vaporproof barrier in the outer shell will condense in the insulation
or in cavities in the insulation space. As a result, the thermal conductivity
of the insulation may be much higher than its original value, so that required
interior conditions are not achieved or a much greater load is placed on the
equipment.
a)  Some moisture may enter the space by diffusion through cracks
or permeable surfaces. The major amount, however, is brought in by
infiltration.
Because of the difficulty of making the outer skin completely leakproof,
additional protection is necessary.  Three methods are possible:
(1) using foamed-in-place plastic insulation,
(2) lining the inside of the exterior skin with a non-
permeable vapor barrier, such as aluminum foil, coated with a plastic binder
that can be sealed at the joints, or
(3) coating the interior surface of the exterior skin with
vapor-sealing compound. Care in installation is important, especially around
wiring, piping, and framing.  The use of the first method is encouraged.
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