Double Stud Assembly Layering
Our builder recommended a double stud wall instead of 2×6+5. We are building a single level cabin in northern Alberta, Canada. As we are looking to use metal siding, and our build package jumped 20% since our initial quote a couple months ago. Our builder suggested this with no change in labour cost and we will adjust dimensions to work for the same interior. The two systems are the same cost for materials with 2 layers of insulation right now.
Double Wall assembly (Inside to outside)
Gypsum – 6mm poly – 2×4 16oc R14Rockwool – GAP – 2×4 16oc with R14Rockwool – 1/2Plywood – WRB – Siding – Tied together with a 2×8 on top
1. GAP size, I was thinking of leaving 1/2″ between the assemblies to match 2×8 for a total R~25. Does this work and breathe properly? We plan to tape and seal Poly, Sheathing and WRB completely for a low air change value.
I then read “Lstiburek’s Ideal Double-Stud Wall Design” He Implemented the mid wall sheathing and Dense blown cellulose. We will be using batts due to ease and value.
2. Is the mid-wall sheathing required for vapour issues? If so, could this be replaced with a Vapour permeable WRB? Sheet Goods are the highest cost per sqft in the wall.
3. Is it worth it to build 1 layer wider insulation and aim for the R40? 3.5in gab to be filled with R14.
4. Is the rain screen required for condensation? Alberta is fairly dry.
Thank you, I have been pouring over the Q&A for days end throughly enjoying the perspectives.
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