How many vents do I need? Attic and crawlspace vent calculator.

Divide the attic floor area by 300 if the vents are split high and low, or by 150 if not. Multiply by 144 to get square inches of open vent area, then divide by each vent's net free area. An 1,800 sq ft attic at 1/300 needs 864 sq in, which is about 14 O'Hagin Fire & Ice comp vents, 7 high and 7 low.

Most LA homes can use 1/300 if 40 to 50% of the vent area sits within 3 feet of the ridge and the rest is in the bottom third of the attic. The code's other condition, a ceiling vapor retarder, only applies in climate zones 6 to 8, and Los Angeles County is zone 3B. If you can't split the vents that way, use 1/150.

One square foot of vent for every 150 square feet of crawlspace floor, with a vent within 3 feet of each outside corner. A 1,500 sq ft crawlspace needs 1,440 sq in: about 36 Vulcan 6x14 vents or 62 BrandGuard 6x14 vents. A Class I vapor retarder on the ground cuts that to a tenth.

Compared with bare 1/4-inch mesh, yes. Compared with old louvered vents, not always. A standard louvered 6x16 foundation vent has about 24 sq in, close to BrandGuard's 6x14 at 23.4 and well under Vulcan's 6x14 at about 41. On O'Hagin roof vents, 1/8-inch mesh cuts open area by 10% compared with 1/4-inch.

Pick your roof type in the calculator. O'Hagin's own calculator uses the same code formula: attic area divided by 150 or 300, times 144, divided by the vent's net free area, with 40 to 50% of vents high and the rest low.

O'Hagin says its system needs no under-eave or soffit vents: intake vents go low on the roof, in the bottom third of the attic, and exhaust vents go within 3 feet of the ridge.