I dont use rockwool myself but here is some info on it. Hope it helps..

Hydroponics - Rockwool as growing substrate
Weedhound Reviewed by Weedhound on . Plant problem I would like some advise. -indoor System NFT/hydroponic -specific medium - rockwool -Hydro/NFT system type -Water source rain water passed through uv anti bactieral filter Rating: 5