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.

1:300 and 1:150 are the same formula, stated differently.

1:150 means one foot in/out (like 1/2 foot in, 1/2 foot out per 150 sq feet of attic floor space)
1:300 means one foot in, one foot out. (like one foot in AND one foot out for every 300 feet of attic floor space)

They work out the same. This is how is was stated to me from my GAF representative among others in the mfg side over the years.

Here’s a quote from a document from Home Ventilation Institute (HVI.org):

“HVI guidelines recommend one square foot of ventilator net free area for each 300 square feet of attic floor space. If no vapor barrier is used, the net free area of ventilation should be doubled.”

The way I read that is that if you double 1:300 you get 2:300 in other words 1:150.

I’m having trouble with the attic venting formula, so I’d also appreciate more info on the topic. Either the formula is overkill or all of the houses in my neighborhood are waaaaaay under ventilated. I just fixed a bad attic mold problem and want to make sure I’ve got enough venting now.

The most common formula I’ve heard & read is /300 if there is a vapor barrier and /150 if not. That’s total NFA. You need to split that between intake and exhaust. All the houses in my neighborhood have round soffit holes. Do the math and you’ll see that it’s not easy to get that much intake by drilling 2" diameter round holes in the soffits! (So the continuous cuts RPR is doing seems like really the only way to go). BTW, regarding screening, I’ve read that 1/8" wire mesh screen reduces the NFA by 20% and 1/16" reduces it by 50%.

If you’re interested, here’s the math on an example using a 48’x24’ rectangular house…

Little less than 1200 sq-ft attic floor. No vapor barrier above the ceiling drywall. That means 8 sq-ft total vent NFA is needed – 4 for intake and 4 for exhaust. 4 sq-ft = 576 sq-inches.

A 2" diameter hole provides about 3 sq-in. 1/8" screen means you need to divide that by 1.25, so the NFA per hole is 2.4. To get 576, you’d need 240 holes. With 2’ spaced rafters, that works out to 5 holes per soffit space! That seems like too much. Or, you need a continuous cut. Am I missing something?

My gray area is that the old insulation I have does have a vapor barrier (paper) but it’s very old insulation and only 3-4 inches thick so it’s not doing a great job. And even if I reinsulate it’ll probably only be 5-6" thick (R-19?)

So that’s why I was wondering if I should be shooting more for 1:150 (more ventilation) because more hot/moist air is getting up there in the winter than if I had a new house with R-38.

I DID FIND Lomanco 140 3" wide aluminum soffit vent this morning… VERY LUCKY.

So, since that increases my NFA from 9" to 11" per foot, a 22% increase, I’m probably going to go w/ that.

As a result my Cobra ridge vent (14.1" NFA) will probably be undersized so later on I’ll probably ask my roofer to upgrade it to something better. In my mind, he should just do this for free since I’m in this moisture bind because he didn’t advise me to have soffit vents… seem reasonable?

here are 2 products (really they are the same thing, the tamko is a private label) that have a 18" nfa, but they have to be installed with hand drive nails, but the nails are included, hopefully your roofer has a hammer.

air leaks let moisture through, poor insulation lets radiant heat through.

If you re-insulate, I would strongly consider spray foam…double the R (2# OR BETTER) per inch, its own contiguous air seal, no critter food value, does not promote mold growth (no mold food). The cost difference is quickly repaid in energy savings. The more energy rates rise, the bigger the savings.

1/300, or 1/150 has to do with weather there is soffit venting or not. No soffits means 1sq ft (144 sq inches) of venting for every 150 sq ft of attic space, if Soffit venting is there 1sq ft (144 sq inches) venting per 300 sq ft. NOTE: when you purchase vents check the square inches for those vents, they range from 38, 49, or as high as 100 inches per vent, so do the math

That is funny. When I first came on here I responded to a lot of old posts also. I learned to check the dates and now I get to laugh at the other guys doing it.

It was just a funny time warp for me! I needed advice on this back in 2007 and thankfully ended up solving all my roof issues. Had pretty much forgotten all about that crisis, and all the soffit/venting knowledge I had to quickly accumulate, which for the most part was all discarded all these years later.

Just want to thank you for clearing up what I had been wondering about. Your math process is helpful. Others like me may not have known that vents on the gable sides can disrupt the airflow from soffits to ridge vent and that limits the location of soffit vents to the sides parallel the ridge vent.