sound insulating internal steel I beam

[markharro]

We have a beam in a wall between two rooms. I want to fill in the space with some form of insulation to contribute towards overall acoustic insulation between the the rooms.

 

What would be best for this. I have some leftover PIR that would fit but I suspect I am better using some acoustic wool batt?

[Mr Punter]

Yes, mineral wool is far better than pir for this.