The base ratio (R806.2)
2018 IRC R806.2 reads: the minimum net free ventilating area shall be 1/150 of the area of the vented space.
required vent area = vented attic area ÷ 150
Here "vented attic area" is the floor area of the vented space in square feet, and the result is net free ventilating area in square feet. The other fractions that appear as distractors (1/100, 1/200, 1/50) are not the R806.2 base ratio. Using 1/100 or 1/50 gives too much vent area; 1/200 gives too little.
Worked example in square feet
A vented attic has 1,500 sq ft of vented space.
1,500 ÷ 150 = 10 sq ft
Traps:
- 5 sq ft divides by 300 instead of 150.
- 15 sq ft divides by 100.
- 100 sq ft multiplies the correct result by 10 in error.
Converting to square inches
There are 144 sq in. in 1 sq ft (12 in. × 12 in.).
A 1,200 sq ft attic:
- 1,200 ÷ 150 = 8 sq ft
- 8 × 144 = 1,152 sq in.
Traps:
- 8 sq in. forgets the conversion entirely: the number is right but the unit is wrong.
- 960 sq in. multiplies 8 by 120 instead of 144. A square foot is 12 × 12, not 12 × 10.
- 1,728 sq in. uses 1/100 (1,200 ÷ 100 = 12 sq ft, × 144) instead of 1/150.
Method to use every time
- Divide the vented area by 150 to get square feet.
- Multiply by 144 if the question asks for square inches.