COLUMN BUILDING SCIENCES
FIGURE 8 Vapor Diffusion Port (Strip Port Openings) Shingle Roofs. Standard roof
vents located at the ridge, but with the opening in the roof deck covered with an
airtight but vapor open layer.
FIGURE 9 Vapor Diffusion Port (Round Port Openings) Shingle Roofs. Holes near
ridge through sheathing to maintain structural diaphragm, covered with an airtight
but vapor open layer.
FIGURE 10 Vapor Diffusion Port (Strip Port Openings) Tile Roofs. Standard roof
FIGURE 11 Vapor Diffusion Port (Round Port Openings) Tile Roofs. Holes near ridge
vents located at the ridge but with the opening in the roof deck covered with an
airtight but vapor open layer.
through sheathing to maintain structural diaphragm, covered with an airtight but
vapor open layer.
50 cfm (24 L/s) for each 1,000 ft2 (93 m2)
of ceiling area is required. This airflow
does not need to be continuous. The typical 30% duty cycle of air-conditioning
system operation has been found to be
effective. Alternatively, a dehumidifier
can be installed.
FIGURE 12 Hip Roof Assemblies. Round openings are typically used at roof hip assemblies to maintain the roof
deck structural diaphragm. Standard roof vent extended downward along the hip 2 to 3 ft (0.6 to 0.9 m).
Unconditioned Unvented Attics
In hot, humid climates, an increasingly
common problem in vented attics is condensation ("sweating") on attic ductwork
and mold growth on attic sheathing and
framing members. In addition, "burying" ductwork under attic insulation can
greatly improve thermal performance,
but has the potential to create moisture
J U N E 2020
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ASHRAE JOURNAL
81
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