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
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.
Venting Cathedral Ceilings
Venting Cathedral Ceilings
Handling Roof Vent Interruptions
Venting Skylight Rafter Bays
Ice Dams
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.
