Rockwool is pretty much fiberglass... or nuevo asbestos...
Rockwool is a hazardous material, there is no practical way to clean it up, especially from soil and installation exposes the workman, which exposes the laundry system and home. Cancer of the skin, lungs, digestive tract etc., it schwizzle sticks the nucleus of cells! Fun stuff and our homes are packed full of it, composite shingled roofs shed it constantly into air and water and it is constantly disturbed and rains almost microscopic particles indoors where it is inhaled and causes havoc in our cardio-pulmonary systems. Thanks industry!
Styrene is found in almost all human blood samples, propably from drinking vessels...
coffee cups and those ridiculous party cups.
I removed all fiberglass from our cinder block and concrete home and was at the same issue with styrene as you are with your tank. We ruled out the best performer; poly
spray foam because we saw extreme reports of
people having to abandon their homes.
We did use styrene panels in the end as a temporary measure. The fire retardant chemicals are my main issue. They are known hormone disruptors. It is a vapor. We sealed it in its integral foil and mylar and hope for the best.
We also live in a very humid climate and have extreme temperatures, so insulation is a must.
Living in new zealand, could you use wool? It is moisture resistant, bug resistant except maybe moths, and insulates like any other batts. If you build a tight box on a riser to keep it out of the mud, cotton (or recycled denim) or wool batts could be kept well. As a positive, wool is natural, has no chemical additives and has good insulation properties even when damp.
The only problem i see with crete, air or foam mixed, is thickness. Airy or foamy concrete is still massive and conductive.
For a small
project, wool is great, i doubt most people here or anywhere could afford a whole house insulated with wool. You could also skim coat it with clay or concrete to seal out bugs and mitigate moisture.
Our domestic
hot water tank is a 48 gallon poly barrel (was for condiment food stuffs) and is insulated with multiple wraps of reflective double bubble. Works great minimal hazard or migration.