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Passive ventilation

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 convection—the upward movement of heated air because of its lower density—but 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.

The 2012 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 area—down to a minimum ratio of 1/300—if 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.