Insulation below Copper Bathtub

Question, we are ready to install our Copper "drop in" tub, and was curious if anybody else has worked with copper tubs?

The bottom of the tub has some flex, so we need to use foam or something to insulate and build some support for the tub. Our first thought was to use some foam underneath the tub, fill the tub with water and let it set hard. My buddy remembers some type of marsh mellow texture stuff, but can't find it now. We did build the support frame about 1/2 inch high.

Does anybody have any other ideas?

Thank you
Robin

Anybody have any ideas? Is spray foam the only option we have for underneath? The bottom of the tub has a little flex, so we can't just lay it flat on the concrete. So far spaying foam and filling the tub with water for 24 hours is the only solution we came up with.

Barbourdg said:

Anybody have any ideas? Is spray foam the only option we have for underneath? The bottom of the tub has a little flex, so we can't just lay it flat on the concrete. So far spaying foam and filling the tub with water for 24 hours is the only solution we came up with.

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Normally a deck tub like that we set with mortar mix. Small piles placed below that spread out to conform to the tub when it's dropped in. You can place some plastic sheeting over the mortar piles too.

We also use plaster under tubs. Anything that will spread uniformly and dry solid. I don't think foam is good for this.