I am in the process of planning the wall design for an addition we'll be adding to our Indiana home this summer. I'm having a contractor do the envelope and I will finish/sub out the interior. We are planning to use CraneBoard vinyl siding as our exterior. Like many of you, I've been reading through countless ways to insulate the structure. I've looked at going with 2X4 or 2X6 walls. I have a competitive bid for cc foam applied to 2X4's and I am getting bids for spray cellulose in 2X6 walls. I've also considered doing BIB in 2X6 walls myself. Our contractor said the 2X6 route would add $600-800 to the build. This starts to offset the cellulose savings over 2" of cc foam (does not take into account "greenness"). I also considered doing 2" of exterior XPS plus BIB or spray cellulose in 2X4 walls because thermal bridging comes up as such a hot topic, however that much XPS starts to add up. I finally saw a mention of ripped XPS strips added to the exterior side of the studs and then the OSB is hung on the outside. This appears like a cost effective and efficient way to go because it breaks the thermal bridge, only uses two sheets of XPS, and adds 2" of wall cavity which means I can add even more cellulose. Does anyone have experience on this approach? I assume it would meet code but I need to check for sure.

The sheathing is the structural membrane of the assembly, and adding a layer of foam between the two can compromise the structural rigidity of the wall. there are other ways to brace the wall, but be very careful in what you do.

Thanks for the input Bob. What are thoughts on I adding 1.5" foam strips to the inside of the wall to go behind the sheet rock?

i have no issues with that, but adding even thin EPS will increase your insulation value for not much more money & no more time. Either means longer drywall screws and electrical box extensions.