I know its a late reply but from from the site below I got this ref your Rockwool flexi which would seem to give an approximate average density of around 40 kg/m³


"Rockwool Flexi density can vary depending on the specific product. The Flexi range of Rockwool products are designed to be flexible and easy to install, while still providing effective thermal and acoustic insulation. The density of the Flexi is around 33kg/m³ up to 45kg/m³ depending on the product. "


https://www.tradeinsulations.co.uk/i...l/flexi-slabs/