The best replacement for foam board is foam core.

The best alternatives to foam board are rated as following:
★★★★★  foam core
★★★★★  fiberglass insulation
★★★★  spray foam
★★★★  gator board
★★★  depron
★★★  tyvek
★★★  styrene
★★  cork board
★★  mat board
★  styrofoam
★  foamex
★  coroplast
★  house wrap
★  foam board
  poster board

Your alternative for foam board is not listed? Suggest!
Speaking about foam board, one probably shouldn't forget about radiant barrier.

Other search terms related to foam board and their ratings are:
★★★★★ radiant barrier
You have in mind a companion for foam board and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of foam board in all years. Click on graph for more.
foam board
foam core
fiberglass insulation