Hi, wet shed,
assuming the base is concrete, and reasonably think, (say, 4" on 6"crushed clean hardcore, 9"deep/square at the edges) then simply knock it down and start again using either 4"conkers or real bricks.
Then you can clad it inside with 4x2" and cover that with insulated plaster board, not forgetting to fill the cavity between block/brick with 100mm.fiberglass.
If you then install a wooden floor on 4x2" and insulate similarly there and in the roof, with a cheap second hand double glazed window and a substantial reclaimed door, you will then find you have a good serviceable workshop/shed which will be warm in the winter, cool in the summer, and soundproof.
If you have had the forethought to install electricity, then you are set for life.
Hey presto....
Best regards Alan D (chessspy)