The pink styrofoam stuff.... Not the fiberglass rolls..... I just built the frame out of wood then lined the inside and bottom with the 4 by 8 sheets of this stuff. I didn't line the inside with any plywood..... I just used a 2 by 4 skeleton with the styrofoam layed in directly on the 2 by 4s. Basically I built the frame then instead of sheeting it with plywood I used the styrofaom insulation..... Then the liner. When walking barefoot on the stuff it feels heated.....LOL When I did my test pond in the winter it was 500 gallons and despite my basement being like 60 degrees I was still able to keep the 500 gallons above 80 with only 2 250 watt heaters. The 150 gallon ag vat took 4 heaters to stay above 80.