I’m narrowing down my foam choice for my exterior insulation retrofit. I’m in climate zone 7/8 so I know I need R15 min with my 2×6 wall and plan to achieve this by installing two layers of 2″ thick foam. My house wrap will be installed on the OSB sheathing. A couple of questions I have:

Is foil-faced foam okay to use in cold climates?

I will have a rain screen between my siding and the foam. The air gap will be 1.5″ because I am installing metal board and batten siding which requires horizontal lattice to support it. So the rain screen will be 3/4″ (1×4) and the lattice will be 3/4″ (1×4).

I have access to R-Tech EPS foam boards. The spec sheet shows an effective R-value of 11.6 with a dead air space between 3/4″-3.5″. Will my rain screen gap effectively serve as the dead space and increase the effectiveness of the foam board as the specs suggest?

Lastly, is XPS foam improved from what it used to be with respect to global warming potential through its production process? Owens Corning Foamular claims “Zero ozone depletion potential with 70% less global warming potential than our previous formula”. What are the numbers for EPS?

R-Tech and Foamular specs are attached.

Thanks for any help and feedback!