Stephen, I know that the closed cell foam used in our addition has a fire retardant in it and it's even ok that it's exposed in the attic area which is conditioned space within the envelop. But it's absolutely true that doing retroactive wiring and other things is pretty darn hard with spray foam in the wall, especially if the bays are filled. You can't just "fish" something. For anything beyond just a couple of feet, the best way would be to cut a channel in the drywall to gain access to the full extent of the wiring to be added, cut out a channel in the wall for the wire, do the installation and replace the strips of drywall with the required tape/mud, etc. Alternatively, pre-installing conduit can be helpful for some "anticipated" changes/additions. Unlike cellulose with a binder blown into walls and smoothed before covering (or other similar materials) closed cell foam is very rigid. Open cell foam is less so and closer to the cellulose with binder filler, but still not fun to retrofit through without fully exposing it. Honestly, fishing through fiberglas bats under drywall isn't a piece of cake either!
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The most expensive tool is the one you buy "cheaply" and often...