How much attic ventilation do I need?

Common questions

How do you calculate attic ventilation?

Divide the attic floor area by 150 to get the minimum net free area (NFA) of ventilation required. For example, a 1,200 sq ft attic needs 1,200 ÷ 150 = 8 sq ft of NFA. If at least 40% of that NFA is placed near the ridge and a vapour barrier is present in the attic floor, the guideline allows a 1:300 ratio (half as much), so the same attic would need 1,200 ÷ 300 = 4 sq ft of NFA.

What is the 1 in 150 rule for attic ventilation?

The 1:150 rule says the total net free area of attic vents should be at least 1 sq ft for every 150 sq ft of attic floor area. It is the widely-used standard guideline for attics where the ventilation openings are not balanced between low (soffit) and high (ridge) positions. Most residential attic ventilation designs use this ratio as a starting point.

What is the 1:300 ventilation rule?

The 1:300 rule allows half the ventilation area of the 1:150 rule, but only when at least 40% — and not more than 50% — of the total net free area is located in the upper half of the roof space (near the ridge), and a vapour barrier is installed across the attic floor. This balanced arrangement creates a more effective air path from soffit to ridge, allowing the reduced total area to work as well as the larger 1:150 area.

How many soffit vents do I need for my attic?

A standard 16 × 8 in screened soffit vent has about 64 sq in (0.44 sq ft) of net free area at a 50% NFA screen factor. The calculator uses the exact figure (64 sq in = 0.4444 sq ft per vent) and shows the number of soffit vents that would cover the full required NFA on their own. In a balanced soffit-to-ridge design, install roughly half that count at the eaves and match it with the ridge vent length shown. Continuous soffit strips have a different NFA per linear foot — check the manufacturer's label.

Want the full walk-through? Read the attic ventilation guide →