How much net free airflow (vents) do I need in attic


I live in Indianapolis, IN, and I have been getting new roof estimates. I have quotes that say I need 13 exhaust box vents (50 in nfa), and quotes that say I need 41 of the same box vents. Why such a big difference. My attic has 3525 sq ft. I have seen the formula for minimum nfa as either 1:300 or 1:150 ( IRC Code). Which do I use. Can’t find local building code. Can anyone help? Thanks, Brad.

I’m in Indianapolis as well. Can’t imagine 41 turtle vents, I’d cross that guy off the list. Why not ridge vent? I assume if you have room for that many turtle vents, you have enough ridge for ridge vents. Depending upon which ridge vent you use, you need 50 to 80 feet. That’s to satisfy 1/300.

Thanks for reply. I have hip roof with only 47 ft of ridge. Company suggesting 41 is big name reputable co. Puzzling?

Didn’t you also get an estimate that wanted to use power vents?

Yep, same person that said I needed 41 (50 nfa) box vents or 3 pc 1200 power vents. House has 3525 sq ft attic space. I think he is using 1:150 nfa standard.

Just because they’re big doesn’t mean the sales rep you drew isn’t a moron. They were obviously going to stack turtle vents on all slopes. Horrible proposal.

Hasvanyone checked to make certain you have adequate intake with soffit vents and also checked to make sure your attic insulation isn’t blocking the air flow path? Power vents are likely your best option unfortunately. 2 large would be fine. There are also alternative products such as hip ridge vents. Unfortunately, these architects that design the steep pitch hip roofs don’t consider ventilation when doing so.

Thanks for the reply. As for the soffit/ intake vents I have been told I have enough. I have 44 and they measure 9.5X11 inches so about 103 sq in nfa. This particular person (suggesting the 41 vents) says they are clear and 90% have baffles.