the styrofoam board insulation is very good, and, I've seen it as little as an 1/8" thick, too ... can be pricey ... if at Home Depot, check out their 'tileboard' ...it's 1/8" thick, masonite-type material, made for damp areas, shower rooms, etc. ... one side is a flat white, porcelain-like hard finish ... I took a light meter to Home Depot, and checked its reflectance, it beat the other surfaces they had ... it was $8.47 for a 4'x8' sheet, in Kalifornia, just the other day ... it's what I use :thumbsup: