Hi Paras,
You will need to define the material "Rockwool" in Material properties and then assign it to the appropriate geometry of insulation.
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)
Regards,
Saumadeep Choudhury
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