Should I be concerned with these wall and roof insulation details?
I am a weatherization contractor located in coastal MA, climate zone 5. After reviewing countless articles here and from Joesph Lstiburek’s website, I’m concerned that the plans I received for a new construction project are not optimal and may pose problems down the road for the homeowner. Specifically, I’m looking to see if my concerns are accurate or if the plans are acceptable. My questions/concerns are (images for walls and ceiling as attached):
1. The plans call for double stud walls with 2×6 exterior and 2×4 interior framing (24″ O.C.) for a total of 12″ of dense packed cellulose. A continuous layer of polyethylene vapor barrier is called on the interior wall, covered by 1/2″ GWB. My concern is excessive moisture levels in the exterior sheathing (either 1/2″ plywood or Zip System) and potential failure in years to come. To further heighten my concerns, no rigid foam is called for on the exterior sheathing nor are furring strips. So my question is: Are my concerns accurate? If not, great, but an explanation why would be appreciated. If they are, and assuming cost is not an option, what would be an ideal assembly? Two that came to mind are below.
Option 1 – Joseph Lstiburek’s ideal double wall
Option 2 – Rigid foam on the exterior, per Martin Holladay’s recommendations. https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing
If option 2 and given the double stud wall assembly is 12″, minimum exterior foam would need to be in the neighborhood of R16-R18, correct?
2. For the roof assemblies, the plans are calling for an unvented cavity containing 18″ of dense packed cellulose. Once again, I’m concerned about moisture levels in the exterior sheathing (Zip System). The plans do call for a smart vapor barrier on the interior; however, no exterior rigid foam or furring strips are called for. The main roof is covered in galvalume (sections of 5 and 8 pitch) as well as a flat roof, covered in an EPDM rubber membrane. My first question here is if this is even code, and my second is this is extremely risky, correct?
My understanding would be to use closed cell foam to treat the underside of the roof sheathing or a combination of air permeable insulation (open cell spray foam, but not cellulose) with R20 rigid foam on top of the roof sheathing.
Thank you for reading my post and providing useful feedback.
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