As I work in the building industry I forsaw the need to have terrific insulation and vapour control. I applied these components from the very beginning of the build and have just come through a very cold winter having used the shed most days and evenings with only 1 small electric radiator, set to minimal output providing all the heat required. For a shed to be used as a garden office heat and the lack of damp are the number 1 requirement and I've built a winner.
Tech spec Raft base 8' x 2', 4' insulation 4' studwork with rockwool, drylined to inside,2' loglap timber to outside. Double glazed upvc windows and doors with integral ventilation, Fully insulated pitched roof. Vapour barrier to all walls.