Replies

1. 
   paulmagnuscalabro | Sep 27, 2022 11:40am | #1

   Hi kgbarber,

   Short answer, that layup seems reasonable and reasonably robust. I work up in Bozeman, Montana, and that's a pretty similar layup to what we often use up here. A couple notes/thoughts:

   If you go with Rockwool for your exterior insulation, I wonder if it wouldn't be easier to put your WRB (looks like this is also your primary air barrier?) right on the exterior sheathing? Rockwool drains pretty well, and it might just be easier to roll out the WRB if it's on the sheathing - plus you'll be dried in a little faster.

   Do you have any additional wall insulation, other than the 1-1/2" - 2" of rigid on the exterior? Up here (granted, pretty sure we get a bit colder here) we'll typically have some exterior + a flash coat of CCSF + batts (I know, CCSF isn't the best environmentally, especially in new construction, but it's what the industry is used to around here - it's changing but slowly).

   Does the drainage cavity product provide enough air flow for the assembly to dry if it gets wet?
   I would think so, but this might be another vote for Rockwool at the exterior - gets you a little redundancy.

   Do you think I need the drainage cavity product on the interior as well?
   If you are getting that much water on the interior, you probably have other problems.

   Am I over-thinking this and will it ever even get wet from vapor?
   As long as your sheathing stays warm, I don't think you'd have any trouble with warm wet air condensing on it. If you Google it, there's a chart showing how much continuous R-value you need for your climate zone for a safe wall assembly. Others on this site are far more knowledgeable than me about this topic, though.

   Should I forget what the cavity drainage manufacturer says and provide a 1″-2″ air gap free of mortar similar to a brick wall? Are we being stupid by not doing it out of concrete/CMU?
   Again, hopefully someone more knowledgeable weighs in here, but probably an air gap isn't a bad idea. I think it'd just be a bit more labor.
   We typically only go up against concrete / CMU at & below the foundation walls. Once we get more vertical, we're putting stone on wood walls.

1. 
   kgbarber | Sep 27, 2022 12:03pm | #3

   Thanks for the input! I figured someone would weigh in on my WRB location. We prefab wall panels that are vapor open and construct them this way, but in this case we will likely not prefab these and in that case would put the WRB on the sheathing as is more traditional. We are planning on dense packing the studs full of cellulose currently, but that is up for discussion. I think if I closed my eyes long enough the GC would just spray foam the cavities.

2. 
   kbentley57 | Sep 27, 2022 11:41am | #2

   It's an interesting problem, and an intriguing one! I look forward to following this one along. I'll comment on a few thoughts, but I wouldn't take them to the grave.

   1. Drop the stud wall. It's prone to long term failure here, because (when) water gets in, there's essentially no way to repair it down the road, without a major demo of either the interior/exterior wall. I thought about a 'double exterior' wall type setup, but it's really just a big thick wall on both sides without much actual support. With drywall and other plasters, the repair is much easier from the inside, and those materials are much more permeable than 4" of stone with cement mortar.

   2. We're accepting that the client is willing to pay for whatever solution you come up with. We wouldn't be discussing a double stone veneer otherwise. So with that, lets get extra thicc. What about a double wythe cmu wall, separated by ~ 6" void space, tied together with truss reinforcement / bond beams / rebar.

   Something like this arrangement, from outside to inside:

   1. Stone veneer
   2. Air space
   3. Drainage plane / fluid applied WRB
   4. 6" CMU outer wythe (quasi-structural)
   5. 4-6" void, filled with SPF (to your insulation requirements), rockwool, or other
   6. 8" interior wythe (structural)
   7. Air gap
   7. Stone Veneer

   Most of the 6" wont require grouting, while much of the 8" probably will. It's sort of like an inside out ICF wall. It has the advantage of decaying gracefully, where there's essentially nothing to rot out from the inside. It provides a 100% coverage of insulation, it's thick, and has drying capability in both directions.

   It's beautiful, strong, and expensive AF.

1. 
   kgbarber | Sep 27, 2022 12:06pm | #4

   I like your very last sentence haha.

   Although money is not a primary consideration, the client is involved and doesn't want to spend needlessly. I think if we went CMU, we would just rely on a continuous layer of rockwool rather than a double wythe setup. In that case I think we would just start looking at ICF.

3. 
   Malcolm_Taylor | Sep 27, 2022 12:20pm | #5

   kgbarber,

   I would read everything I could find by Building Science Corp on reservoir claddings.
   here is one: https://www.buildingscience.com/documents/building-science-insights-newsletters/bsi-061-inward-drive-outward-drying

1. 
   kgbarber | Sep 27, 2022 01:01pm | #6

   This is a great read thank you. Do you think they would need to tape the seams of the XPS if I were using that as a "vapor impermeable sheathing"?

1. 
   Malcolm_Taylor | Sep 27, 2022 01:39pm | #7

   kgbarber,

   No. The only reasons I can think to tape it would be to avoid bulk water intrusion - and I can't see how it could ever get that deep into the wall assembly.