The 50/50 works everywhere except for the coldest climate zones (basically close to or in the polar circles), it's about the safest advice you can give. However in reality you just need to ensure you have enough exterior insulation to keep condensation at bay for your wall assembly. But the exterior is not always the most cost optimal place to put insulation, after all fiberglass batts are cheap. As long as you are hitting your clear wall insulation targets, and have the appropriate amount of exterior insulation it really doesn't matter where you put the insulation.
As for recycled foam, try craig's list often commercial roofers will put it up on there after they replace a flat commercial roof. Also a google search for "recycled rigid foam insulation" brings up a lot of re-sellers.
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