spray foam for under a jetted tub

I installed a jetted tub about 15 years ago. It came attached to its own wooden supports on top of 3/4" plywood. I basically just had to put in place and screw the plywood down. Since that time, I've been fighting the caulking around the edge of the tub and the wall tiles. It seems that when the tub is filled with water and someone is soaking in it, the extra weight causes the tub to settle down. This has caused very small hairline cracks in the caulking.
I heard someone say to get some spray insulating foam and spray it under the tub between it and the plywood. I'm not sure if there is a specific type of spray foam that would work best. When I went to Lowes recently, they had two different types, small gap and wide gap. I'm not sure if either would work here. I think the spacing between the tub and the plywood is about 3-4 inches.
Someone else mention using a very dry mortar and then packing it in with the tub empty and letting it cure, I'm not sure if this is a good idea either.
Any ideas?
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wrote:

use the regular stuff it will try to push the tub up to the ceiling. I used it when I helped re-install a friend's tub (his brother, a plumber, had installed it level instead of having the drain end down 1/8 inch or so - so it never drained properly). The foam also keaps the bath warm a lot longer.
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tub to lift several tons. If it doesn't blow the sides out first.
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On Dec 12, 7:24 pm, snipped-for-privacy@snyder.on.ca wrote:

Thanks for the replies. I'll try to fix it this weekend.
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rlz wrote:

Nothing wrong with mortar, it works fine.
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dadiOH
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Oren wrote:

After the tub was set.
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dadiOH
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