If you do the roof make sure that you are venting the underside of the roof decking or you will end up with a hot roof.
I would insulate the walls with it because you can stop thermal bridging in the walls this way and insulate the attic with blown in cellulose insulation, you can also cellulose insulate the wall cavities if you want to pull the fiberglass, but a foam shell around your house is going to help you a lot, make sure that it goes down to the ground as well, but for below grade you are going to need a foam that is made for ground contact, the foam you have is most likely not if it's foil faced.
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