CLT walls, insulation, and dew points – GreenBuildingAdvisor

Hello,

I am looking for clarification/help/advice about where a dew point can be within a CLT wall assembly. I am designing for a cabin in climate zone 6. IBC has required insulation R value of 15/20 of continuous insulation (ci) for Mass walls (EC 402.1.3). However, if you only put R-15 of insulation outside a 3-ply CLT (4-1/8″), that places the dew point threshold of 55deg F (based on 70deg interior and -9deg winter dry bulb) inside the CLT, running risk of mold and delamination. But if I try to keep the dew point in the exterior ci, it then requires over 6″ of the exterior ci (Rockwool 80 at 4.2/in. is the best that I have found that is vapor permeable) because of the inherent R-value of the wood (4.125″ x R1.25/in.=5.16 for the 3ply CLT), bringing the overall wall assembly R-value to over 34. Is that typical for CLT wall assemblies in colder climate zones?

Oddly, if I increase to a 5ply CLT (6-7/8″), that requires even more insulation because the additional thickness of the CLT pushes its exterior surface further out past the dew point, requiring more exterior ci to pull the dew point out further within the overall assembly. Is this counter-intuitive result correct? 

The attachment is screenshots of my calculations for both the 3 and 5ply. Thanks in advance for any feedback provided.