Trying not to over-ventilate attic with continuous soffit board?
CZ3, 2065 square foot vented attic. Simple gable (36×60). If I follow Joe’s advice to split the net free vent area (1/300) to 60 or 65% at soffits and 40 or 35% at roof I’d be aiming to put approx 600 square inches on the two 60 foot sides (300 square inches per side). And then the remaining 400 square inches on the roof.
I look at vented soffit board info and I see that one particular brand provides 10 square inches of vent area per linear foot. If I were to use that on both long sides of the gable (60 feet each side) I would end up having doubled the vent area I needed for the entire attic and I wouldn’t have even touched the “35-40%” in the roof-which I would still need to install.
Why I think it’s important? This is a two story structure. 1st floor garage/shop. 2nd floor living space. I’m paying close attention to the air barrier between floors. Sealing floors/type x drywall in garage ceiling, dense pack cellulose between joists…. But because I know no installation will be executed perfectly, I’m subscribing to Joe’s “slightly pressurized attic” approach to minizine the pulling of air between floors into the attic. I’m also using a vented rainscreen that will vent into the soffit/attic. Its just 0.75 inches wide. I’m not calculating it into the 1/300 number…
Do I just seal up every other soffit vent opening on the vented soffit board before install? That would leave me with approximately 5 inches of vent area per linear foot. Or do I just install solid soffit board and add the appropriate amount of vents panels (maybe 5 or 6 vents per side) along each side to get the right vent area per side? How do you use continuous soffit venting and not over do?
Oh wait…I think I hear my builder grinding his teeth! No one at the site meeting this morning bought into my suggestion about splitting the vent area disproportionately between soffit and roof. I’m just the crazy lady client…But the trusses come next week and I intend to continue the conversation then…
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My take on this: Joe's recommendation should be interpreted as "no more than 35-40% at the roof." Having too much at the soffit isn't a problem. More at the soffit reduces the problem that Joe is worried about.
And overall having too much ventilation isn't a problem either.
In addition, since you are paying attention to air sealing, you are way ahead of most buildings anyway.
See below for other experts with other opinions. And AFAIK, there is little data to support any of it.
https://www.greenbuildingadvisor.com/blogs/dept/musings/all-about-attic-venting
I'll aim for 400 square inches in the roof and not worry about the vented soffit being 1200 square inches? A total of 1600 square inches when I only need 1000 per code? That puts my ratio at 75% soffit and 25% roof. still ventilated. still pressurized. I get the concept. I just never thought of that interpretation.... But maybe unvented soffit is cheaper..
Ended up going with smooth soffit...and LP doesn't make vented smooth soffit....so I will use individual vents. Looks like I will end up with a total of 1486 square inches of total vent....38% exhaust at the roof and 62% intake at the soffit. That will be 14 soffit vents out of a total of 60 rafter bays. I'm planning on rigid foaming the top plate and building a 2" baffle space in each of the 60 bays for a few feet (just above insulation level).....although my first thought was to only baffle the 14 bays...but I'm sure that I'll be advised that it would be wiser to allow ventilation along the entire eave length...with or without the soffit vents. I do have a 3/4" rain screen that will be pulling air as well. thanks to all for the brain power!
Inger, did you consider something like Coravent? It should work well with your smooth soffit and be a lot better than individual vents.
I did... but dismissed after I saw that my builder failed to order/install raised heel trusses. Now I'm looking to insulate that top plate (while still allowing a generous vent channel) with a few layers of R-5/inch foam.. The outer edge of the 2x6 top plate will be R-15. The inner edge will be at R-20 plus some (the loose cellulose will add a bit). I think the individual vents along with my rainscreen should be fine. I guess what I'm saying is that I'll be there anyway....can easily add a sturdy baffle.....Thanks
You can put the Coravent on the outside edge of the soffit.
Inger,
When you say your rain screen will also be pulling air, I assume you mean it vents at the top into the soffits. That creates a concealed space for fire to spread from the wall cavity to the roof. Recognizing that, our code here in BC precludes connecting the two.
Malcolm: That is correct. vents into the soffit. Oklahoma has it's own reasons why one should not vent the attic at all (blowing embers, etc). But I'm installing a vented attic just the same. I don't believe I'm out of compliance with the building code here.
Inger,
Barring fire, I bet connecting the two really improves the effectiveness of the rain-screen cavity.
Good luck with your build!