Post Frame Sheathing, Air Barrier, and Insulation – Questions & Options
Hello, and thank you for all the resources this community has provided. I am an experienced interior remodel contractor building a barndominium/shop-home for myself and family in Northern Idaho (zone 6 – wet spring/fall & dry summer/winter). This has been a long road saving, planning, and paying cash as we go. The number one goal of this build is to provide a solid working shop/office space for our business, with a 2 bed living space attached.
This structure was engineered for a strong 130psf snow load, we plan to have hydronic radiant cement floor, and wood stove heat. Exterior finish will include a 3′ metal wainscot (to protect from snow), 5/8″ Fir T1-11, and one horizontal Cedar ship-lap accent wall. Until I stumbled onto this site and other building science information, my plan was to attach Tyvek directly to the horizontal commercial girts, and then siding finishes on top of that, with R21 fiberglass between all stud bays.
Now however, I am stuck trying to decide the best practice to make this home last as long as possible and be “entry level” high performance. My current options (idea’s) include:
1) No exterior sheathing, 2.5″ of Polyiso foam attached directly to the studs, Tyvek applied to the foam with “outie” type buck extension windows, and then 1×2 furring strip rain screen. Fiberglass R21 would be still be used between stud bays. ***side note – I found a polyiso supplier with a great deal on factory seconds 1/2″ and 1″ foam board. So this would be 3 layers in total to create the 2.5″. I could also budget to do 3″ with 3 layers of 1″.
2) No exterior sheathing, Benjamin Obduke Hydrogap house wrap attached directly to studs, and R21 mineral wool insulation between studs.
3) Huber zip system sheathing (or CDX with fluid applied WRB) and fiberglass R21 between studs.
Basically, we only have the funds to do ONE of these options. If money was not an issue, it would be an easy decision to do zip system, innie flashed windows, and rockwool exterior insulation with a rain screen… On a building this size however, these materials would be $15-20k line items which just isn’t in the budget. I do worry about the lack of permeability in the layered polyiso, but also the thermal transfer in all these post’s and beams (seems like there is more wood in these walls then not).
I am not overly concerned about shear strength as this building is already over engineered, and I have added blocking with 2×4 let-in bracing as double suspenders for my belt.
My wife and I live a “rural” life and have been developing this property our-self over the last 5 years. I do not want to be excessive, but I do worry about the interior health (mold) and long term performance.
Please let me know any thoughts, ideas, and/or opinions. Again, I really appreciate all the resources this membership has already provided. Thank you all for your time,
Augustus Balogh
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Replies
Is R21 really all you need in your area? I'm in Zone 5 and require R24.
I can't tell you what the best option would be but I can tell you how my own barndominium project is going. My building is 40'x100', slab on grade, 6x6 posts 10ft oc, exterior girts, 7/16 on that, tamlyn drainwrap on the sheathing, and metal on the wrap. The main benefit of that is that I felt like I needed spray foam to reach my desired air tightness and target r-value. However spray foam is very expensive so I wish I had done something else. Looking back, since I sheathed and taped the building my air tightness probably would have been fine and I could have always gone to aerobarrier as a backup. I made interior studs 24" oc, notched and wedged between my headers which worked well but took a lot of time for me to make all those cuts. What I think would be best is to not do a barndominium at all but since I see you have already started, I would plan on interior girts which would give you between a 5.5" and 8.5" cavity along your walls. Fill it up with blown in cellulose for R24+ which is super cheap. sheath and tape some 7/16 on the exterior, 5/8 drywall interior.
I apologize my reply is all over the place, but I just noticed your construction isn't standard post frame. It might be difficult to do blow in. Your headers are also crazy deep so I think if you can afford it, Zip-R (or similar, eg standard 7/16 with exterior rockwool) is the way to go to stop all that thermal bridging.
Sorry my thoughts were all over the place, just woke up, but good luck on your project!
I think with that amount of solid blocking, I would want some sort of exterior insulation. I would also want sheathing at some location in the wall to keep critters out given your rural location..
I would price out using nail base panels. Zip R is probably the most expensive option, but you could also look at a regular nail base panel with osb and adding tyvek to that.
Thank you for the responses! I have priced out various nail base panels and with 100pcs needed, the price starts at $7k with very little R-value in return. I also don't like the idea of any potential off gassing from spray foam, not to mention price.
The metal wainscot with rat guard will start below top of interior slab, so I think pest control "should" be sufficient.
We have no building codes here, so what R-value we really need, is a bit subjective to whatever building performance outcome we are personally looking for. I do plan on keeping this shop/home for many years, but it will be an ADU guest home long term - as we plan to keep saving for the next "main" home as soon as this one is done.
With money and time being my two main constraints with this build - again, paying cash and not hiring help - I have done more reading over Christmas break and come up with another budget/time friendly option...
Stephen Bonfiglioli method - Apply Hydrogap or Tamlyn Drainwrap (I personally like BO's tape system better being acrylic vs. butyl), then 1" foam and strapping from interior for a 7.25" R-30 depth. I could also rip down 1.75" EPS to fill in gaps at all beam/header locations.
Thanks again,
Augustus