Climate Zone 7 Colorado
I'm struggling to understand the use of Rigid foam on exterior of a home, new construction. I understand the benefits and want to use it. I would like to use from the interior wall out, 5/8" drywall, smart vapor retarder, blown in BIBS, Zip System sheathing, polyisocyanurate per the code R-15, for a 2X6 wall all seams taped. Apply wood siding over foam.
Questions: Should my wall cavity be filled with insualtion to meet current code 2012 R-20 or can I use a portion of my R-15 Rigid to makeup the required R-20 cavity and R-5 continuois sheathing?
The 2012 Code suggests R-20 with R-5 continuos sheathing which does not comply with the essential information from Table N1102.5.1 that applies to foam-sheathed walls:
Minimum R-Value of Foam Sheathing
Marine Zone 4
R-2.5 for 2x4 walls; R-3.75 for 2x6 walls
R-5 for 2x4 walls; R-7.5 for 2x6 walls
R-7.5 for 2x4 walls; R-11.25 for 2x6 walls
Zones 7 and 8
R-10 for 2x4 walls; R-15 for 2x6 walls
Questions: If I use a flash and fill in the cavity instead of BIBS, let's assume 5" of closed cell spray foam and the rest blown in BIBS, does that allow me to use less exterior rigid polyisocyanurate than the code suggest for my climate zone 7 = R-15 for 2x6 wall.
Also if you use venetian plaster on the interior as a finish on top of the drywall are you essential creating a vapor barrier? I'm assuming my wall make up is going to breath to the interior unless I use the flash and fill wall cavity assembly, which the 5" of closed cell foam is also acting as a vapor barrier?
Posted Fri, 03/22/2013 - 12:45
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