You cant find any foamboard, geez is Google broken again? The best is
foilfaced polyisocyanurate R 7.2 per" The white is R 3, Pink or Blue
board is R 5". Look at Dow, Owens Corning, John Mansville. Use
construction adhesive in a caulk tube. Polyiso outgasses the least
because of the foil face.