Dense product to place on top of adjoining walls?

[JKami84]

we have taken ceiling down in our garage and are insulating walls and ceiling to use this space as a utility. Our garage joins our neighbors garage and we share the same roof. We are separated by a 9inch wall (double skin they seem to call it). if you look in picture once the wall ends at the top, there is a void where the joists sit. I can basically reach over to my neighbours ceiling if I wanted. Although we will pack some insulation in ceiling and some of this will sit part way on the adjoining wall... Is there anything very very dense I can "stuff" on top of the wall for further sound proofing? E.g. cutting to shape the thickest celotex type insulation there is or something.

[nod]

Yes it’s normal to use a fire batt 

(FRboarsd) It has a stone wool core and a hard outer shell 

About 20 quid for a 600x1200 sheet 

simply cut to size with a hand saw Then seal around with an acoustic sealant 

[JKami84]

@nod