Climate zone 2 – all brick, one-story 2400+ sq house.

We are to choose between 2×6 studs with full fill open cell spray foam insulation on the inside part of the wall VERSUS a combo of 2×4 studs with 3.5″ nominal fill open cell spray foam PLUS 1″ of some sort of exterior rigid foam insulation (trying to figure out if we want the premium Rockwool Comfortboard 80 or just the Dow Board/Owens Corning pink Foamular XPS board — we have a stiff budget, gulp).

 

I would be happy with choosing the XPS rigid foam combined with the nominal 3.5″ spray foam on the inside of the exterior wall BUT knowing that XPS foam can have a thermal shift due to shrinkage over time plus the attraction ants and termites have to it makes me want to rethink and choose Rockwool instead, which is more expensive.   Plus, we are not sure if the framers available here are familiar with the details of putting on Rockwool Comfortboard exterior insulation and combining that with window installs and other fenestrations. Unfortunately, I am still working on finding out the price comparison on these products mentioned.  It sounds like Rockwool products have a super long lead time right now though. If I go with Rockwool it’ll probably be the thinnest they have (1.25′) as we are in hot and humid Houston. 

My builder thinks that the open cell spray foam on the inside of the exterior wall is more than enough for our needs and that it’s overkill to try to add to that but with my research it seems like air leakage is a big deal to have to manage besides bulk water management.  Good thing is, we as the homeowners have the last say on what ends up in our future home. 

Please help out with the best cost-effective strategy for our build. (CZ 2), or maybe the most practical route for us. Thanks