Inside the tank I lined the walls with Polyiso foam insulation board. I had extra board left over from the house build in the crawlspace. Actually, just enough insulation to finish the tank, which was lucky. There are two layers, each two inches thick, for a total R-26 on the walls. There is no additional insulation on the floor, just the existing crawlspace insulation of R-26. I lined the tank with EPDM rubber, folding the corners, and stapling the top edge to the wood f