Thinking a little more about your problem, while I realise that good ventilations is important I have an engineering workshop on the rear of my garage built this year that is accessed from outside the garage so has it's own door. The ceiling is insulated with 100mm foil covered hard yellow foam with an air gap at the top vented to the outside, although there is no heating in there when not used there is no mould showing on the ceiling and even this morning the ceiling is slightly warm to the touch.
Another method of insulation is a blanket of about 17 layers of foil and silvered bubble wrap material, around 25mm in thickness. This works on both walls and ceilings and is used in my loft conversion and comes in well above current building regs, there are no rads up there just the door left open when the house is occupied.
But as others have said, you need to get rid of condensation with ventilation first, I know this goes against the grain as you pumping out expensive heat but........
Hope this helps
Paul.