I went with Homasote over 1/2" plywood. Screwed the plywood to my framework and then screwed the Homasote to the plywood sub-base. I can't imagine using Homasote in an un-supported configuration? I remember using this stuff in the sixties, the new stuff seems to be a much better product. I took the advice of several forum members and sought out the "real" trademarked Homasote (Home Depot).
Never have used foam on a model railroad, detest the stuff!