Attic ventilation: Is it ok to have WAY more intake than exhaust?(ratio= 1224. sq. in: 180 sq. in?)
Hi there! I know that for attic ventilation, Joe Lstiburek recommends having more intake than exhaust ventilation, but is there such a thing as “too much” intake vs exhaust? I have a hip roof with only about 10′ of ridge over a 40′ long x 28′ wide attic (not counting the 1′ overhangs) and am about to replace the roof, and also replace my old plywood soffits with vented vinyl soffits. Using the vented soffits all around all sides of the home, along with a ridge vent would give me an intake of at least 1224 sq. in to an exhaust of only 180 sq. in.
The total ventilation would exceed the attic floor area divided by 150, so does that mean it should be ok to have “unbalanced” ventilation?
I am in southern Maryland in climate zone 4, btw. I do have lots of insulation (just had more blown in) but there is no vapor barrier under the insulation.
Thanks so much in advance!
(By the way, I LOVE listening to Joe Lstiburek’s videos- I’ve learned so much and he is so funny and a great teacher! I learned that I should definitely disconnect my old electric attic vent that I had installed 20+ years ago the last time I reroofed, thinking it would help with the small length of ridge vent, LOL!)
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Replies
I don't see a problem with the extra intake venting, but you're probably pretty low on exhaust venting compared with the square footage you're trying to cover with your vent system. You could potentially add some roof vents on the "non ridge" part of the hip roof. Some others here might have some other ideas too.
Bill
BTW his name is Joe Lstiburek, not John 😉
So you have ~ 1,404 sq inches of venting NFA? Be sure to check the NFA because it’s not going to be 100% open for airflow. If your numbers are correct, you have 87% intake and only 13% exhaust. It’s definitely more than the ideal 60/40. I think here in Ontario this would not be code compliant as OBC requires min. 25% for each intake and exhaust, so your exhaust is like half of what is required.
I will leave it to the experts here but it seems like you could benefit from balancing it out a bit. My attic was recently built with only 35/65 which is at least code compliant but lacking intake. I spoke to Joe L himself and he thinks it should still be 60/40 despite my builder insisting with the airtight ceiling it will be fine with this ratio. If you want to be safe, 50/50 is good and then 60/40 is ideal.
Ah- thanks for responding- and thank you for correcting me on Joe L's name-- I will edit my original post to correct it, if it will let me! I wasn't sure if the intake to exhaust ratio applies IF you have twice the required ventilation OVERALL (i.e. 1 sq. ft for every 150 sq. ft of attic vs the standard 1 ft for every 300 sq. ft of attic.) I mean I could reduce the amount of ventilation at the soffit (I was going to go with a soffit that either had 10 sq. in. NFA or another with 12 sq. in. NFA/sq. fot per sq. ft, and I have 144 L.F x 1' wide soffit all around if you count the overhangs!) I could intentially go with something with less soffit venting NFA if need be. I just don't want to use box vents for the exhaust instead of the ridge because I have had such issues with them leaking before. I'm not sure what other options I'd have. I was thinking about those turbines until I heard that Joe Lstiburek doesn't recommend them due to them possibly creating negative pressure and pulling air from the house.
If you haven’t done it yet then maybe you might as well try to get it more balanced? It’s probably not a huge deal but again where I am it would not be code compliant with your current numbers. It might not lead to problems, as Akos said below the air sealing is more important. But you should check your local code requirements!
Thanks, Bill! I have had bad experiences with a box type vent that leaked no matter how hard we tried to fix it, so I am leery of those. Do you know of any other vents that aren't as prone to leaking? Thanks!
I would wait and see if you have an actual problem. Take a peak in the attic after a bit of deep freeze in the evening, if you don't see any frost on nails, you don't have issues.
Generally a house with a well sealed ceiling needs very little venting and no amount of venting will get you out of trouble if the house leaks like a sieve. I would focus your efforts on air sealing instead of adding more venting.