Best DIY sound insulation for a generator box

[woodman-46]

Hello
I am making a weatherproof container for my smallish generator so that it can obviously be outside but protected from the elements. I have already planned in an inlet fan to supply plenty of air and an exhaust vent to clear that away.
I want to sound insulate it on the inside but wondered what would work as an inexpensive insulation in this case. It would not be right up against the generator but have several inches clearance all round
Any suggestions welcome
Thanks

 

[Deluks]

A box in a box, with dense rock wool in between the two.

 

[woodman-46]

Thanks.

 

[aveatry]

Yes with the rock wool but also need to separate the generator the ground with rubber.

 

[freddiemercurystwin]

Rockwool RW5 or 6.

 

[jj4091]

Surely all the noise will escape through the exhaust won't it.