I can't tell you what worked for me since I didn't insulate at all, but I can tell you that you definitely should insulate
What I'm planning for my rebuild this summer is a layer of aluminum foil and some perlite under the burn tunnel, then foil and bubblewrap under the actual bench (min 4" cob between bubblewrap and duct). No idea how the bubblewrap will do though.
Hi Ronald; I used a 4" layer of cob heavy with straw on the ground before putting my duct work down. This seems to have worked well for me in the greenhouse, i imagine that you could put rigid foam insulation down first if you wanted to.