Proper attic ventilation prevents moisture damage, ice dams, and overheated shingles. This calculator tells you how much net free area (NFA) you need and how many vents that translates to, based on your attic floor area.
Attic Details
sq ft
Vent System
Standard (1:150)
Scenario: Your attic floor is 1,500 sq ft and you only have soffit vents with no ridge vent, so code requires the 1:150 ratio.
Result: Total NFA: 1,500 / 150 x 144 = 1,440 sq in. Half for soffit intake: 15 vents (at 50 sq in each). Half for exhaust: 40 linear ft of ridge vent (at 18 sq in per ft).
Balanced Ridge + Soffit (1:300)
Scenario: Same 1,500 sq ft attic, but now you have balanced intake and exhaust, qualifying for the 1:300 ratio.
Result: Total NFA: 1,500 / 300 x 144 = 720 sq in. Half for soffit intake: 8 vents (at 50 sq in each). Half for ridge vent: 20 linear ft (at 18 sq in per ft).
The standard is 1 sq ft of NFA per 150 sq ft of attic floor (1:150). With balanced intake and exhaust, you can use 1:300. A 1,200 sq ft attic needs 8 sq ft (1,152 sq in) NFA at 1:150, or 4 sq ft (576 sq in) at 1:300.
NFA is the actual open area of a vent that allows air to pass through, after subtracting the screening and louver material. A vent rated at 50 sq in NFA has 50 sq in of actual airflow opening. Always use the NFA rating, not the vent's overall dimensions.
Ridge vents are preferred because they provide continuous exhaust along the entire roof peak. Box vents (also called turtle vents) are simpler but less effective. Never mix ridge vents with powered attic fans -- the fan will short-circuit the passive airflow system.
More ventilation is better up to a point. The real risk is unbalanced ventilation -- too much exhaust without enough intake can pull conditioned air from the house through ceiling gaps, increasing energy costs. Keep intake and exhaust roughly equal.
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