I am considering constructing a storage container, for thermal storage(unpressurized), similar to our hot tub. The hot tub is circular in shape, made of 2x6 western red cedar, held together with metal rings and holds approximately 650 gallons of water. There is no liner. The cove and bead edges of the wooden slates coupled with natural swelling of the wood keeps it from leaking. We have enjoyed the tub for the past 8 years and it has held up quite well. The only difference will be the water temperature. As a hot tub the water is rarely warmer that 102 degrees compared to the 180 degrees as thermal storage for the heating system. I would think it wood would be a better insulator.I am not sure if it will hold up to the temperature. Does anyone in the "room" have any thoughts? Thanks for the help.