As seen in BILS3, conventional insulation materials containing air will not be usable. As an alternative material we identified an open porous filter material which is basically a PU-foam with PPI20 (pores per inch). The material data are promising: A matching temperature range for our purposes and it is immediately wetted when under water.
A first sample was built using this material and was brought to water - for the first time in a lake. The test sample is only the shell, it is not yet operational and it is not filled with "storage water", The experiment confirmed that the material is easily wetted and also emptied again when taking the storage out of the waterr. However, as the image shows, the density of the wet material is still lower than pure water, meaning that the storage is swimming at the surface in horizontal position.
Next step therefore. Make sure that the some weight is added to the insulation.
More about insulation experiments