insulation for stud walls

[whitehart]

Hi all

i am thinking of putting Rockwool flexi in stud wall before plastering for insulation and sound proof qualities, if the timber is 75mm wide does the insualtion need to be bigger (to be stuffed in) the same thickness or smaller so theres an air gap??

I am sound proofing my party wall too with the same product, again should it be packed tight or left with an air gap??

thanks in advance

[welsh brickie]

pack it tight, 100mm thickness is sufficent