"The Natural Philosopher" wrote in message
...

How can I say it loud enough

THERMAL INSULATORS DO NOT NECESSARILY MAKE GOOD SOUND INSULATORS AND VICE
VERSA.

Since this is an internal wall, thermal insulation is IRRELEVANT..unless
you want to create a refrigerated room, in which case anything inside will
die of hypothermia, which will silence it anyway.

The BRE did say how to make a sound proof stud wall...
You need two stud walls about a foot apart and hang mineral wall from the
ceiling to the floor in the centre.
Not as good as using blocks though.