Mostly I’ve seen the 1:300 rule specified for attic ventilation, but I’ve also seen 1:150 mentioned. E.g. HVI says to use 150 when there’s no vapor barrier and 300 when there is.
I’m putting in continuous soffit vents on Friday (hopefully) and it’s looking like we’re going to use the standard 2" wide variety which has about 9" NFA per foot. But when I do the math that only gives me 1:300 just barely.
I found that there’s a 3" wide vent (Lomanco 140) which has 11" NFA and that would give me closer to 1:150. But I haven’t seen it, and more importantly it’s special order and I’m not sure I’ll be able to use it by Friday and thus whether I should postpone the job (risky… getting carpenter back soon!)
So my dillema is whether to scramble to try to get 3" (near 1:150) or just settle for 2" (1:300) which would be MUCH EASIER.
As you may know my attic has a moisture problem. It’s not fantastically insulated and even if I rip off the floor this spring, reinsulate my 770 sq.ft and put a new floor on, I’m still limited by my 2x6 joists so I think that’d be R-19 max.
So even then my attic will not be insulated to the “modern spec”. So does that mean that 1:150 (and 3" wide vent) are important?
I realize there isn’t a perfectly easy answer to this. Some of it may be a little subjective but I thought I’d ask for people’s opinions anyway.
Thanks again (as always)!