Hi
my garage is single block work with a first floor room above. Top of my list before taking delivery of my 4/4 last month, was to stick 50mm cellotex with solvent free adhesive to all external walls of my garage, then over clad with 12.5mm WBP Ply (plugged & screwed)
I know I haven't experienced a winter yet, but I'm pretty confident I shouldn't need any kind of dehumidifier / heating.
