Attic Ventilation Sizing
Net free area from your attic floor area at the code ratio you qualify for, split into intake at the eave and exhaust at the ridge, with the vent count and linear feet each product needs.
A 1,200 sq ft attic at 1:300 needs 4 sq ft of net free area: 2 sq ft of intake at the eave and 2 sq ft high on the roof. At 5 NFA per ridge foot that is 5.8 ft of ridge, and 2 soffit strips at 41 NFA each covers intake.
How the math works
- Code baseline (IRC R806.1): 1 sq ft of net free ventilation area per 300 sq ft of attic floor area, and where the attic is a conditioned space the same rule applies to the ventilation of that space.
- 1:150 and 1:100: 1 sq ft per 150 where a vapor-permeable insulation layer sits on the cold side of the ventilation and not less than 50% of the net free area is in the top 3 ft of the roof, and 1 sq ft per 100 with a fully ventilated roof assembly (battens, vented deck, or vented tile) plus that cold-side layer and 50% intake at the eave.
- The 50/50 split: intake low at the eave (soffit) and exhaust high (ridge, box, gable, turbine) so the whole attic cavity is swept. High-only vents work by wind, not stack, and a blocked soffit is the usual reason a ridge vent does not pull.
- Product NFA: continuous ridge vent is typically 1.5 NFA per ft low-profile, 5 NFA per ft standard, and 7 NFA per ft for a high-capacity profile; a box vent is about 12 NFA, a gable louver 24 NFA, a turbine 4 to 6 NFA, a 12 in circular soffit vent 41 NFA, and continuous soffit vent about 4.25 NFA per ft.
- Where the intake goes: baffled rafter vents (rafter socks) keep the intake clear of insulation to the ridge line, and the intake must not be blocked by a roof-to-wall intersection, a dormier cheek, or a bath fan that dumps into the attic.
How many square feet of attic ventilation do I need?
At the 1:300 minimum it is your attic floor area divided by 300, in square feet: 1,500 sq ft of attic is 5 sq ft of net free area, split half intake at the eave and half exhaust high. Product NFA is the conversion, and NFA is not the same as the hole you cut.
Is a ridge vent enough by itself?
No, and this is the most common failed attic in the country. A ridge vent is exhaust; without intake at the eave it pulls from the leak gaps in the roof. Pair it with continuous soffit vent on the same wall lengths, with rafter baffles keeping the intake clear.
What is the 1:150 attic ventilation rule?
Half the area ratio (1 sq ft per 150) where a vapor-permeable layer is on the cold side of the ventilation and at least half the net free area sits in the top 3 ft of the roof. Read the shingle and underlayment vapor permeability before you claim it, and 1:100 needs a fully ventilated deck.