Replies

1. 
   user-2310254 | Sep 16, 2019 12:21pm | #1

   I will give this post a bump to help you secure some knowledgeable input. If you are concerned about the environmental impact of foam, consider buying reclaimed material. The reclaimed foam also will be more affordable than new material.

2. Expert Member
   Dana Dorsett | Sep 16, 2019 05:48pm | #2

   The IRC code minimum wall for US climate zone 3 is 2x4/R13 + R5 continuous insulation. For mass walls (CMU walls would qualify) a continuous R13 (no penetrating studs) would qualify.

   Going by the book 3/4" of foil faced polyiso with seams taped, trapped to the CMU with a 2x4/R13 studwall works, and there is no need for an interior side vapor retarder tighter than latex paint on wallboard. Treating both the CMU and the framing with a spray-applied borate treatment (eg Bora Care, PenaShield, etc.) and putting a bead of polyurethane sealant under the bottom plate of the studwall as a mechancal barrier to insect traffic. If the subfloor is a slab, install an inch or more of foil or plastic faced EPS that extends under both the wall foam and bottom plate, leaving ~3/8" of clearance between the EPS and structural wall as a drainage space can work pretty well as a capillary & thermal break from the ground moisture & temperature. Polyiso can potentially take on moisture if in contact with damp-ish concrete, but EPS will not.

   To do it completely foam-free takes 1.5" of rigid rock wool (pretty pricey) plus the studwall, or 3" of rigid rock wool and a studwall. Using steel furring instead of wood studs takes the termite risk out of it, which might also work OK in your case for limiting the amount of loss to the interior.

   IRC 2018 for zone 3 for mass walls is R8 continuous insulation if done on the exterior. That could be 2" of EPS finished with EIFS, or 1.5" of foil faced polyiso strapped to the wall with steel furring, mounting a siding onto the steel furring, or 2" of rigid rock wool + steel furring + siding.

   If insulating on the interior using only fiber insulation, install 6 mil polyethylene or some other vapor barrier on the CMU wall first, which will limit the peak moisture content of the finish wall and studs. Any R5 or higher foam board between the studwall & CMU would be sufficiently vapor retardent to limit exterior moisture drives, but rigid rock wool is extremely vapor permeable on it's own, and can also wick moisture from the CMU toward the interior.

1. 
   Jacob_Z | Sep 19, 2019 03:23pm | #3

   Hi Dana,

   Thanks for your response.

   It seems like you are suggesting that this wall will need a vapor barrier (taped foam board, 6 mill poly, etc), but you also said that rigid rockwool could be applied directly to the CMU, which is pretty vapor open. Did I misunderstand you?

1. Expert Member
   Dana Dorsett | Sep 19, 2019 04:46pm | #6

   The 1.5" RIGID rock wool in a continuous layer can be applied directly to the CMU, as one way to insulate with no foam of any sort. That would be in lieu of the R5 continuous foam layer. The studwall would still need to be insulated. With 3" of continuous RIGID rock wool it would meet code without insulating the studwall.

   This is not completely without risk- yes, it's very vapor permeable, which could be an issue for the finished interior side of the wall in summer if air conditioning to well below the outdoor dew point temperature. Installing a vapor barrier between the rigid rock wool and CMU would mitigate the summertime risk, but can lead to some amount of wintertime condensation/accumulation in the wall if the indoor humidity is too high. Putting the vapor barrier between the continuous rigid rock wool and the insulated studwall would be fairly condensation-safe in any season in zone 3.

   With CMU walls my strong personal preference is to insulate on the exterior (foam or rigid rock wool) behind siding + rainscreen, taking advantage of the thermal mass of the CMU by keeping it inside the thermal boundary of the house. But that can also be more expensive, and depending on the house design may be (nearly) impossible. My second choice would be insulating on the exterior with EPS using an EIFS finish to the foam in lieu of siding.

1. 
   Jacob_Z | Sep 19, 2019 05:48pm | #7

   Thanks for clarifying. I would certainly prefer to insulate on the exterior, however it is not feasible on this project due to some random local zoning codes.

3. 
   Jon_R | Sep 19, 2019 04:27pm | #4

   Go with Lstiburek's recommendation, including the "vapor semi-permeable foam" part.

4. 
   Deleted | Sep 19, 2019 04:28pm | #5

   Deleted