Bora-Care should be sprayed on all exposed sides of the raw, unfinished wood that you’re trying to protect. If will not absorb properly into adjacent wood that has not been treated.
Yes Boracare can be used as a foam in wall voids. You should only be concerned about getting the foam near the wood and studs. Typically when a company foams walls, they drill holes at the top of the ceiling on either side of the studs and foam down the stud from the ceiling. This way you have gravity working in your favor and you are getting the foam against the wood which is all that matters. You will need a foaming agent and a poly foamer. You will mix 2 gallons of hot water with 1 gallon of Boracare. For raw, untreated wood that you're able to access, you would apply Bora-Care by itself at a 1:1 ratio.