The first post was so fun I couldn't wait to try another. Even though we haven't started on our next home, I thought I would take a stab at building a heat storage tank. It is based on structural insulated panel construction. I figured if homes could be built out of SIPs then tank sides would work also. The tank will be 8' long, 4' high and 4' deep. The foam is 3.5" thick and has an R value of 24. It is amazing how hard the foam gets when it sets up. My first panel uses OSB as skins but I think aluminum sheet would work just as well and would look better (at 10X the cost of OSB). I've attached a photo of the first piece. I'm planning on using an HDPE liner and building an insulated top. After looking at the cost of currently marketed pieces, I figure I will have a sturdy, well insulated tank that will cost 1/4 the amount of other tanks. We'll see how it works!