Post-Frame Construction Wall Assembly
Just want to say I really appreciate this community and GBA you guys are great.
I’m designing a single story 46×46 post frame build. The roof will be comprised of trusses with a 2′ energy heel spaced 2″ on center packed with loose fill cellulose. Intello Plus will be stapled to the bottom of the trusses and 5/8 drywall under that.
The wall assembly from inside to outside.
latex paint 5/8 drywall
Non load bearing 2×6 stud walls 24″ on center with 5.5″ Rockwool batt
Intello Plus fastened to the posts of the post frame taped to the slab floor and taped to the ceiling intello plus for continues air barrier.
5.5″ rockwool batt between the 8′ on center posts of the load bearing post frame.
2×6 girts 24″ edge to edge with 1.5″ rockwool friction fit between.
15lb felt paper
2×3 rainscreen strapping 24″ on center
LP Smartside lap siding
I’m in the warm side of zone 5 South Eastern Pa.
A couple questions.
Given that I air seal the Intello properly considering I put it in my opinion the easiest place to do so, does anyone see any issues with vapor drive condensing on the first condensing surface of the Posts and girts?
The metal siding on Pole Barns give them shear strength will the 2×3 rainscreen straps 24′ on center be enough to make up for the metal?
Any and all other issues I may completely be missing?
Really appreciate the input!
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Replies
Montypbbc,
No continuous surface, no condensing surface of interest. The posts and girths are just like the studs on an exterior wall. That's not where the trouble usually occurs, it's on the interior face of the sheathing. Your wall will dry well enough to remove any small amount of water that has plans to hang around the wood structure and cause mischief.
I'm sorry I don't know enough about the shear requirements of pole structures to comment usefully on that.
Thanks very much Malcolm.
With everything I've read/listened to, what you said seems like the most likely outcome to me as well, but maybe someone on here will have some further insight.
Montypbbc,
With good air-sealing and rain-screens, even double walls with sheathing don't experience moisture problems. Yours are safer still.
Without solid sheathing as backing, you might also consider using a sheet WRB like Tyvek instead of building paper.
Good point, I have been mulling that over a weave WRB is probably the way to go, and would make flashing and taping rough openings easier too I would think.
Thanks Again.
I agree with Malcolm that you shouldn't have any moisture-related problems. It would be best if your WRB was airtight but the assembly is good as is. As for shear, I would want more bracing than the steel siding and/or strapping. Not for everyday winds, but for 100 mph+ wind events. You can use cables, Simpson straps, let-in bracing or knee bracing--you don't need full sheathing. More info here: https://codes.iccsafe.org/content/IRC2015/chapter-6-wall-construction#IRC2015_Pt03_Ch06_SecR602.10.
I too am designing a similar post frame house. For peace of mind, you could simulate your wall in this tool - it will show the true R value and drying time of the wall:
https://www.ubakus.com/en/r-value-calculator/
Regarding the rain screen straps and siding. Doubt that these are enough to substitute for the shear strength of the metal. However, would venture that if you have the structure engineered, the engineer can find other ways of bracing the structure.
Am planning something similar. We seem to be on the same page with the "Mooney" wall. Climate zone 6 (MN).
Inside to outside:
5/8" drywall
2x4 stud wall with rockwool batts
1/2" air gap
Intello membrane, completely sealed with tape
2x8 girts on 2' centers with rockwool batts
Commercial Tyvek or Mento membrane, with butyl tape where metal fasteners go in
Standard post frame steel, screwed in.
Trying to answer the question of how well the wall will dry through the small open channels in the wall steel. Believe there might be a way of configuring the Ubakus tool to answer this question.
Regarding the energy heel. I have my design drawn up in CAD. If you are planning on having roof purlins flush to the top of your trusses (with joist hangers), there won't be enough room for full R60 cellulose (~20") and good wind dams (1-2") at the eaves. Purlins on edge on top of the trusses would buy a little more space. This detail is slightly different on post frame, the roof purlins can get in the way. I have gone to close to 3' for the energy heel to make this detail clean.