Building a detatched studio/drum room

[Gedidrum]

Hi, we hope to find some in with regards using sand in the cavity of a wall!
I am about to begin construction (from scratch) on a 3m x 4m outbuilding which will house my mini studio and drum kit and we were all set to use 2 layers of blockwork (breezblock) with a 100mm cavity, possibly filled with Rockwool, until our builder and upstairs neighbour suggested this...
a bed of only 6"(yes, inches) of concrete with a base of brick, with the main 3walls consisting of...

outer skin = cedar cladding
waterproofing of some sort
2" ply layer
100mm cavity filled with SAND
2" ply layer

...the inner wall will be professional acoustic treatments (double staggered acoustic boards/resilient bars/acoustic sealant etc.
He's also suggested using sand in the roof with a corregated metal roof, on top of which we'll have some planting (soil and wildflowers I think).

I suppose the question I'm posting here is...How on earth do we gaurantee the sand does not take on any moisture AND how do we ensure the studs don't pop out under the weight of the sand? Bearing in mind I found this site due to a post in 2003 about a similar issue (are you still here Henrik?) and have read up quite a bit (I think!) on soundproofing. Obviously the sand option is a cheaper one, but also a more hazardous one taking the possibility of mould into account as well as the chance of things collapsing. Also, finally, would we need a stronger footing for all this weight of sand?
The builder has years of experience and is NOT a cowboy, but the sand issue makes me nervous!
Thanks in anticipation.
Gedidrum

[TexRoadkill]

Originally Posted by Gedidrum

how do we ensure the studs don't pop out under the weight of the sand?

Get an engineer to design the building.

In the end you will probably find that the sand is too heavy for realistic use.