I-joist roof with 1 1/2 in. vent channel
swooley
| Posted in General Questions on
We’re building a 2:12 shed roof running north/south, with no penetrations, hips, valleys, etc., and are using 16” I-Joists. It’s all cathedral ceiling.
I’d like to simply attach plywood/osb to the upper I-Joist block to make a 1.5” ventilation channel, venting the roof fully from soffit to ridge.
With a tight ceiling and I-Joist cavity filled with R-49 fiberglass batts (can’t find anyone who will dense pack cellulose), is a 1.5” ventilation gap enough, or do I need to go unvented with rigid on top of the roof deck instead?
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Replies
swooley,
That happened to the assembly we discussed in this thread? Shallower trusses?
https://www.greenbuildingadvisor.com/question/cathedral-ceiling-and-conditioned-attic-same-thing
Malcolm,
The story is a long one, but as one might expect the GC that got us to that point of “what are we going to do with the roof” also made a number of structural mistakes—undersized footers and inadequate ICF wall thickness and reinforcement. Per structural engineering reports, it has to come down. Lawsuit is ongoing.
Meantime, we’re basically starting over. Same general plans, but the ability to hammer out these details ahead of time and a new GC who is one of the good ones. It has been night and day.
We’ve talked either unvented 2x12 roof with rigid on top or vented 2x12 roof with r-38 hd fiberglass (10.25”) in the cavities. He also just got quotes for 16” I-Joists, which he’d prefer to use because the I-Joists are so straight and we’ve got a long span with little tolerance for uneven framing members.
The I-Joists excited me because of their lower thermal bridging and more room for cavity insulation. They’re also super easy to vent since you can just attach under the top block and you have a ready-made “baffle”.
But it all comes down to whether or not 1.5” is enough. It’s like I’m staring at the perfect system with this one variable that might disqualify it, so I’m hoping that 1.5” gap is just enough to say yes to that assembly.
Though it does look like your recommendation was 6” previously, so this may be moot. Is there any kind of math that dictates how big of a vent gap is needed at various roof slopes? Maybe if we vent all soffits?
swooley,
That's great you have ended up with a GC you can work well with.
2/12 is in that grey area of being a bit risky but not very. Get the rest right - primarily a good warm side air barrier - and it should be fine with an 1 1/2” vent channel.
If it were me I'd use this assembly: Run sealed house-wrap over the I joists, followed by 2"x3” strapping parallel to the top flanges, and then sheathing. This allows you to insulate the whole rafter bays, provides excellent drying into the vent cavity, and avoids the complication of baffles. https://www.greenbuildingadvisor.com/article/building-a-vaulted-high-performance-and-foam-free-roof-assembly
Another alternative to increase the roof venting would be build the baffles as you suggest, strap with 2” x 4”s perpendicular to the I joists under the sheathing, and vent all the soffits.
You can also use a "smart" membrane on on the bottom of the I-joists to minimize vapor migration up into the insulation and sheathing space. Detail that well as an air barrier and your 1.5" is plenty. Pay special attention to lighting penetrations. If you're going to have lots of lighting in the ceilings, criss-cross strapping underneath the membrane gives you lots of room to work and you can run the wiring in any direction.