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relative humidity.  In warm, humid climates, e.g., Hawaii and
Guam, however, vapor flows downward through the roof toward an
air-conditioned interior.  Placing a vapor retarder on the deck
and below the insulation is harmful in a tropical climate.  It
prevents venting of water vapor to the interior, entrapping it in
the roof sandwich where it may condense in the insulation (refer
to Section 4).
This difference in predominant vapor-flow direction may
influence the proper choice of roof system:  conventional
assembly with insulation sandwiched between the deck below and
membrane above, or a protected membrane roof (PMR) assembly, with
membrane and insulation positions reversed, i.e., with insulation
above the membrane.  In a cold climate where the predominant
vapor-flow direction is upward from a heated interior, the
membrane in a PMR doubles as a vapor retarder. This is an
efficient arrangement.  In a humid tropical climate, however,
where the predominant vapor-flow direction is downward, the
conventional roof assembly's position with the membrane above the
insulation may be more efficient, with the membrane again
functioning both as waterproofing and vapor retarder.
Solar Radiation.  Solar radiation embrittles asphaltic
materials and thermoplastic sheets.  Plasticizers are lost
through the combined effects of heat and sunlight.  Shielding of
the membrane and flashings is important.  Aggregate, mineral
granules, or heat-reflective coatings may be used as shields to
increase the membrane's service life and cut cooling-energy
Wind Velocity.  Wind velocity has an impact on roof
design apart from the obvious requirement of better anchorage (or
heavier ballast) in high-wind areas. Loose aggregate ballast
should not be used in high-wind areas.  "Windstripping" of
aggregate-surfaced built-up bituminous membranes has been noted
in such diverse locations as Guam and Grand Forks, ND.
Roofing Material Selection.  See Table 1 below for
selection of low-slope membrane roofing.  See Table 2, in Section
9, for selection of materials for steep roofing.


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