ASHRAE Journal - June 2020 - 80

COLUMN BUILDING SCIENCES

typical manifestation of this is a tile roof over batFIGURE 7 "Flow Through" Approach. The "flow through" approach is limited to hot dry
climates such as Las Vegas and Phoenix. The roof cladding needs to be "back-vented," and
ten strips over a roofing felt over plywood or OSB
the roof sheathing and roofing paper/roofing felt/roofing membrane need to be vapor open.
sheathing (Figure 7). Any moisture that accumuThe most typical manifestation of this is a tile roof over batten strips over a roofing felt over
lates on the underside of the roof sheathing (the
plywood or OSB sheathing.
"first" condensing surface) can pass through ("flow
through") the sheathing and the roofing felt, into
the air gap under the tile roof, and be vented away.
New materials allow the roofing paper or roofing
felt to be replaced with a vapor permeable fully
adhered membrane, or a semi vapor permeable
taped OSB sheathing. Netted fiberglass, fiberglass
batts, and mineral wool batts are common insulations that are used in the rafter cavities.
The final way to control condensation or moisture accumulation on the underside of the roof
sheathing is remove attic/roof assembly moisture via vapor diffusion rather than air change
between the attic/roof assembly and the exterior.
When using fiberglass and mineral wool insulation
while installing standard roof vents near the ridge, but
to construct conditioned unvented attics or uncondicovering the vent opening in the roof deck with an airtioned unvented attics, moisture control is provided
tight but vapor open layer (Figure 8, Figure 9, Figure 10 and
by letting water vapor exit the roof at the roof peak
Figure 11).
via a mechanism called vapor diffusion. A "vapor difTwo types of vapor diffusion port openings are typifusion port" allows water vapor to exit-but prevents
cal-strip port openings and round port openings-
inward or outward airflow. The vapor diffusion port is
depending on structural requirements. Strip port
"vapor open" but "airtight." The vapor diffusion port is
openings are analogous to the omitted sheathing used
also "watertight," preventing entry of rainwater. This
in typical vented roof ridge vents. However, standard
mechanism works because air containing water vapor
ridge vent openings can compromise roof deck dia("humid" air) is less dense and more buoyant that "dry"
phragm ("shear") resistance. Using round openings in
air. As such, the water vapor ends up at the peak or ridge roof sheathing addresses this issue. Round openings
of attics. When it gets there, the water vapor passes to
are typically used at roof hip assemblies to maintain the
the exterior, through a vapor permeable but airtight and roof deck structural diaphragm. A standard roof vent is
watertight layer.
extended downward along the hip 2 to 3 ft (0.6 to 0.9 m)
There are limitations to this approach based on climate (Figure 12). Similar approaches are used for dormer roofs.
and roof slope. The approach is limited to hot-dry and
The diffusion port approach is limited to IECC Climate
hot-humid climates (IECC Climate Zones 1, 2 and 3).
Zones 1, 2 and 3. It should not be used in colder climate
Additionally, the roof must be sloped to allow moisture
zones.
buoyancy to work; the roof slope must be greater than or
The vapor permeance of the diffusion port covering
equal to 3:12 (vertical/horizontal).
should be greater than 20 perms. The ridge vent area
In attic spaces and sloping rafter roof assemblies,
should be approximately 1:300 of the ceiling area and the
one of the reasons moisture accumulates at the ridge
roof needs to be sloped a minimum of 3:12 or greater.
is hygric buoyancy; he other is thermal buoyancy.
Where the insulation is installed on the underside of
Installing a "vapor diffusion vent or vapor diffusion
the roof deck and the insulation is "air permeable," then
port" at the upper part of attic spaces and sloping rafHVAC supply air should be provided to the attic space
ter roof assemblies allows this moisture (in the vapor
from the interior of the house to remove moisture, by
phase) to exit the attic and roof assembly. In practice,
providing "conditioning" to the attic space, thereby
this involves eliminating the lower soffit attic/roof vents, treating it similar to a room or occupied space. At least
80

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ASHRAE Journal - June 2020

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