Replies

1. 
   creativedestruction | Dec 08, 2020 07:29am | #1

   R7.5 ci might not even be enough with R23 in the cavities. Zone 5 you want about 30% outboard which is more like R10. Code may require that you include a class 2 vapor retarder inboard of the stud cavities. Intello, Membrain, or asphalt saturated kraft would suffice.

2. 
   charlie_sullivan | Dec 08, 2020 09:32am | #2

   With rock wool on the exterior, you don't really need to follow those rules. Your sheathing will be warmer and dryer than without the mineral wool board. So you are fine. On the other hand, intello on the interior will help and give you more margin, so you might consider it even though it's not strictly needed. It can be a good air barrier, and you need a good way to do that on the interior in any case.

3. 
   JMMR | Dec 09, 2020 06:42am | #3

   Thank you Jason & Charlie,
   Charlie regarding your comment that the sheathing will be warmer and dryer with out the exterior rock wool. Not sure I understand, dryer yes but with the exterior insulation would it be warmer or colder?

1. 
   alex_coe | Dec 10, 2020 04:06am | #6

   Exterior insulation will keep the sheathing warmer.
   Why not just use the 2" thick R8? It's a nice sweet spot in terms of value and performance.

4. 
   Jon_R | Dec 09, 2020 08:39am | #4

   IRC allows a Class II or Class III. But with low perm Zip and R23 fill, don't push either limit (.1 perms and 10 perms). I'd use something around 1 perm. +1 on something with variable perms (even paint) to add resiliency. Always test air sealing and consider interior side air sealing.

   https://www.continuousinsulation.org/content/2021-ibc-and-irc-adopt-improved-vapor-retarder-requirements

1. Expert Member
   Dana Dorsett | Dec 10, 2020 09:42am | #7

   >"But with low perm Zip and R23 fill..."

   ZIP isn't significantly lower perm than any other OSB product, and with high-permeance rock wool on the exterior next to a 3/4" rainscreen gap the drying rate to the exterior is pretty good even in the dead of winter.

   Bottom line, even though the R-ratio isn't enough to keep the average temp at the ZIP above the indoor air's dew point, there isn't really an appreciable risk in a zone 5 location since there's effectively zero exterior moisture drive and plenty of exterior drying capacity in that stackup. At the cold edge of zone 6 it MIGHT need a lower-perm interior (even then, maybe not), but not in zone 5. A smart membrane interior vapor retarder would only be for the "belt AND suspenders" folks. Any accumulated moisture over the winter can dry (in both directions) during the spring shoulder season well before the average temp at the ZIP will support mold.

1. 
   creativedestruction | Dec 10, 2020 02:38pm | #9

   "would only be for the "belt AND suspenders" folks"

   Or for those needing to satisfy the code.

5. 
   JMMR | Dec 09, 2020 07:38pm | #5

   Thanks Jon, that’s a great link you posted

6. 
   Deleted | Dec 10, 2020 02:37pm | #8

   Deleted