There is no boundary condition to model Rockwool, what you need to do as we said and I hope it is clear, is to define the material “Rockwool” in Material properties and then assign it to the appropriate geometry/body representing the Rockwool.
To check how to define a material in the “Engineering Data” tab (as shown in the figure below), you can take a look at the following Ansys Help Link: 6.4. Material Properties (ansys.com)
To add, we need to use perforated material properties (e.g., DB, JCA or Miki material models) for the rockwool which is modelled.
The perforated materials are under engineering data. The material is then assigned as normal/usual to the corresponding body in Mechanical.
For a tutorial on using these see:
All the best