What size roof vent do I need for my attic?
Step by step
- Measure the attic floor. Use the footprint of the space below the attic, including a garage if it shares the same attic. Leave out vaulted areas with no attic above; those need their own vent channels.
- Pick the ratio. Use 1/300 if you will have balanced high and low venting, 1/150 if proper intake is not possible.
- Convert to square inches. Square feet times 144.
- Split it. About half to exhaust and half or a bit more to intake. Shingle manufacturers commonly want intake at least equal to exhaust.
- Divide by the product rating. Per vent for box vents, per foot for ridge vent.
Choosing the vent type
If the ridge is long enough to supply the exhaust in one continuous run, a ridge vent is usually the cleaner choice. If it is not, as on many hip roofs, box vents placed within a few feet of the ridge fill the gap, or a manufacturer's hip vent may work. Use one exhaust type per attic.
When the attic is split
Garage attics, bonus room knee walls, and areas separated by firewalls or ceiling changes are often separate air spaces, and each needs its own intake and exhaust. That is why some roofs end up with a ridge vent on the main house and a few box vents over a garage wing.
Check the intake side
Exhaust is only as good as the intake feeding it. If you cannot get enough soffit intake, scale the exhaust down to match rather than adding more exhaust, which would just pull air from the house. See ridge vent vs box vents and roof ventilation basics.