The most common way to provide attic ventilation for asphalt shingle roof assemblies
is by nonpowered, passive ventilation. This method relies primarily on natural air
convectionthe upward movement of heated air because of its lower densitybut
may also take advantage of wind-generated pressure differences.
Natural convection is responsible for initiating the upward flow of air through
an attic. This air current can be maintained to aid in continuous circulation of
air through the attic if intake vents placed low in the attic make colder air available
to replace the heated air exhausted through vents placed high in the attic. Convection-assisted
ventilation is most effective when approximately equal amounts of ventilation opening
areas are placed at the soffits or eave and at or near the top of the attic space.
This is shown in the figure below and is referred to as "balanced ventilation."
Figure: Passive ventilation of attic space. Soffit or eave with
ridge ventilation in a balanced configuration is shown.
Passive ventilation can be configured with wind-assisted exhaust vents. These products
employ wind speed to induce pressure drops across the vents so air is forced or
"pulled" out of the attic. They are capable of significantly increasing the volume
of air exhausted from an attic if adequate intake vents are also provided.
Wind-assisted vents should not be used in combination with other passive vents placed
high on a roof because air flow in windy conditions may bypass the intake vents,
largely negating the benefits of ventilation. In calm air, wind-assisted vents function
just like convection-assisted vents. The popular varieties of wind-assisted exhaust
vents are turbines and shingle-over ridge vents.
International Residential Code (IRC) requires cross-ventilation under
roof sheathing over attics with flat floors and where there are open spaces between
the ceiling and roof deck in buildings with vaulted (cathedral) ceilings. These
requirements apply to new construction and reroofing projects.
2012 IRC section R806, "Roof Ventilation," prescribes a minimum area of ventilation
openings that is a 1/150 fraction of the area of the space ventilated. The code
permits less ventilation opening areadown to a minimum ratio of 1/300if a vapor
retarder of 1 perm or lower transmission rate is included on the warm-in-winter
side of the ceiling in cold climates and/or at least 40 percent and no more than
50 percent of ventilation opening area is allocated to exhaust vents located not
more than 3 feet below the ridge or high point and the balance of ventilation is
provided by intake vents at or near the eaves. A vapor retarder is a material used
to appreciably reduce the flow of water vapor into a roof assembly.
The code also requires a minimum clearance of 1 inch between the ceiling insulation,
roof sheathing, and intake vent openings to allow unobstructed air flow.
When passive attic ventilation is used, NRCA recommends the minimum amount of net
free ventilation area (total unobstructed cross-sectional area of ventilation openings)
of 1 square foot for every 150 square feet of attic floor area. Furthermore, NRCA
also recommends the amount of ventilation opening area in passive systems be allocated
evenly between the eave (intake) and ridge (exhaust).
Where average January temperatures are 30 F or lower, NRCA suggests using a vapor
retarder on the warm-in-winter side of attic space insulation (ceiling side). A
vapor retarder has to be uninterrupted to be effective. Breaks or openings, such
as unsealed recessed ceiling light receptacles, attic hatches and utility chases
in walls defeat ceiling vapor retarders because water vapor is transported with
interior air escaping into attic or ventilation spaces.