Sizing Vents

Most codes specify a net-free vent area (NFVA) of 1/150 of the attic floor area if the ceiling has no vapor retarder. (Or, in In Climate Zones 6, 7 and 8 you can have 1/300 as long as a vapor retarder is included. Ventilation is more effective where there's an airtight ceiling plane to prevent warm, moist interior air from getting into the attic.)

In a home with a complex roof, it's a good idea to exceed these minimums.

Typical Net-Free Vent Areas (NFVA)
Vent Covering Vent Area Multiplier
1/4-inch mesh hardware cloth 1.00
1/8-inch mesh screen 1.25
#16 mesh screen, with or without plain metal louvers 2.00
Wood louvers and 1/4-inch mesh hardware cloth 2.00
Wood louvers and 1/8-inch mesh screen 2.25
Wood louvers and #16 screen 3.00

For site-built or manufactured but unlabeled vents, this multiplier tells how many square feet of vent area is needed to give 1 sq.ft. of net-free vent area.

NFVA is the combined area of all unobstructed vent openings. Most manufactured venting products are labeled for NFVA. (Soffit vent strips and ridge vents usually express this as square inches per linear foot; fixed-sized units tend to list total NFVA for the entire unit.) For site-built, or for manufactured but unlabeled vents, start with the required NFVA then use the multipliers in the table to enlarge it to compensate for screens and louvers.

The building code doesn’t address—or ask roofing professionals to factor in—the actual volume of space under the roof. The volume for a 1,000-square-foot attic under a 12:12 pitch roof is not the same as the volume under a 5:12 pitch. Recommend practice calls for increasing the ventilation by 20% for roofs with a pitch from 7:12 to 10:12. For roofs steeper than that, we recommend increasing ventilation by 30%.

Vent Types and Location

Install ridge vents that have an external baffle. As wind passes over the roof ridge, the airstream jumps over the vent's baffle, causing suction as the air lifts upward — the same way an airplane wing works (see illustration below). Called the Bernoulli effect, this driving exhaust force works regardless of wind direction. Without an external baffle, either a single- or a double-sided roof vent can allow outside air to come in and short-circuit the venting process.

Roof Vent Location
Roof ventilation should be divided about 50/50 between low and high locations. For the best performance, use a soffit vent and baffled ridge vent to allow for both intake and exhaust.
Roof ventilation should be divided about 50/50 between low and high locations. For the best performance, use a soffit vent and baffled ridge vent to allow for both intake and exhaust.

Ventilation should be balanced, with the NFVA area split evenly between low and high vents, preferably soffit and ridge vents. If balance isn't possible, it's better to have more intake vents. This puts the attic under a slightly elevated air pressure, forcing air out the exhausts on the leeward side of the house.

Never install a ridge vent without soffit vents. In windy conditions, a lone ridge vent could pull moist air from the house into the attic.

Avoid gable end vents. Gable-end vents do not pull air evenly from all parts of the attic. Use them only when soffit vents cannot be installed.

Soffit baffle. In high-wind regions where wind-driven rain and snow entering the attic may be a concern, install a soffit baffle, too, as shown in the illustration below.

In areas prone to wind-driven rain and snow, moisture can be blown into soffit vents that are at or above the level of the wall top plate. To prevent this, detail the soffit vent as shown.
In areas prone to wind-driven rain and snow, moisture can be blown into soffit vents that are at or above the level of the wall top plate. To prevent this, detail the soffit vent as shown.

Venting Cathedral Ceilings

Venting Cathedral Ceilings
In a cathedral ceiling, leave a minimum 2-in. space for airflow between the insulation and the underside of the sheathing.
In a cathedral ceiling, leave a minimum 2-in. space for airflow between the insulation and the underside of the sheathing.

Handling Roof Vent Interruptions

Venting Skylight Rafter Bays
Where roof penetrations such as skylights or chimneys interrupt the airflow from soffit to ridge, cut notches or drill holes in the rafters to let air flow around the obstruction.
Where roof penetrations such as skylights or chimneys interrupt the airflow from soffit to ridge, cut notches or drill holes in the rafters to let air flow around the obstruction.

Ice Dams

Warm air rising from the house in winter can warm the sheathing and melt snow on the roof. This snowmelt can then flow to the eaves or other cold area and refreeze, causing an ice dam. Additional snowmelt can then pond behind the dam, back up beneath the shingles.
Warm air rising from the house in winter can warm the sheathing and melt snow on the roof. This snowmelt can then flow to the eaves or other cold area and refreeze, causing an ice dam. Additional snowmelt can then pond behind the dam, back up beneath the shingles.

A tight ceiling plane along with good attic insulation and ventilation will help prevent ice dams by keeping the roof sheathing cold.

Detail the eaves with a full thickness of insulation but do not allow the insulation to plug the ventilation channel near the eaves.

Keep heating ducts out of the attic space, since most heating ducts leak at the joints, and will increase attic temperatures.

The Cold Roof

The cold roof is appropriate for some cathedral ceilings in very cold and snowy regions. It consists of two distinct roof layers separated by a 2- to 3- in. airspace and capped by a large, site-built raised ridge vent with an overhang large enough to keep out snow.

In a double-layer “cold” roof the upper vented layer removes warm air that escapes through the insulated lower layer. The outside roof surface remains at the same temperature as the outside ambient air, preventing the freeze-thaw cycle that normally occurs on standard roofs.
In a double-layer “cold” roof the upper vented layer removes warm air that escapes through the insulated lower layer. The outside roof surface remains at the same temperature as the outside ambient air, preventing the freeze-thaw cycle that normally occurs on standard roofs.