GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Climate Zone 7 Colorado

MONTANARED | Posted in GBA Pro Help on

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:

Climate Zone
Minimum R-Value of Foam Sheathing

Marine Zone 4
R-2.5 for 2×4 walls; R-3.75 for 2×6 walls

Zone 5
R-5 for 2×4 walls; R-7.5 for 2×6 walls

Zone 6
R-7.5 for 2×4 walls; R-11.25 for 2×6 walls

Zones 7 and 8
R-10 for 2×4 walls; R-15 for 2×6 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 2×6 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?

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. GBA Editor
    Martin Holladay | | #1

    The minimum code requirements for foam insulation aren't thick enough to keep you out of trouble.

    I suggest that you fill your stud bays full of insulation and also include R-15 exterior rigid foam insulation.

    In theory, you can count the contribution of closed-cell spray foam on the interior side of your wall sheathing as part of the minimum R-value of the foam required to keep your stud bays above the dew point. (You can add the values of the exterior foam and the spray foam.) However, it's a bad idea to sandwich your OSB sheathing between two layers of (relatively impermeable) foam. Choose exterior rigid foam or interior spray foam -- but not both.

  2. Expert Member
    Dana Dorsett | | #2

    I'm hoping you mean 0.5" , not 5" of closed cell foam? or maybe you meant R5 of flash-foam?

    At 0.5" it's still to vapor open to be protective with only R5 on the exterior, but 5" is way too vapor closed. With R15 on the exterior you can skip the flash-foam in a 2x6 wall. At 1" it'll be about 1 perm and R6, which would get you to an adequate dew point and still have some drying capacity, but with fiberglass fill you don't have much margin- cellulose would be better. If you're above 10,000' of elevation that can help a bit on the dew-point issue too- the volumes of available moisture will be ~25% less than at identical dew points at sea level.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |