blockwork shed

[lloydy b]

Hi ,i'm thinking of building a single skin blockwork shed to use as a music room/I'm going to clad the outside with upvc cladding and fix insulation backed plasterboard to the the inside ,the size is 3.0x2.7 so do you think i need a column for support half way thru the 3.0 metre wall?and is it good enough to keep the damp at bay?

many thanks Lloyd