When it came to the roof insulation, my original plan was to use 100mm Celotex, and have a 50mm air gap above, creating whats known as a 'Cold Roof'. Soffit vents would then be placed at each side of the shed, to allow airflow on the upper side of the celotex, preventing condensation.
BUT. I messed up. I went to fit the 100mm celotex, to find my rafters were only giving me 120mm gap, meaning my air gap could only be 20mm, which would not be enough.
Because of this I opted for a kind of 'hybrid warm roof'. A breathable membrane is attached to the roof from the inside, coating it completely. The 100mm celotex is then placed between the rafters. At this point there will be little to no gap between the Celotex and the plasterboard that'll be added later.
I sealed up any gaps with expanding foam, and used Celotex Foil Tape to completely seal the gaps around the celotex. This means no air can get above the celotex, thus there should be no condensation.
The only downside to this method is that an air vent has to be placed somewhere inside the shed to avoid condensation building up on the ceiling. I'll likely place a small vent above the window or door, assuming I can find one that is small enough.
If this was a house build, this method would probably not work, however since its a one room shed conversion, it shouldn't cause any problems with thermal bridging so long as sufficient ventilation exists inside the room.
So far I've got the celotex up and secured, I've now just got to place the plasterboard on the ceiling and can then make a start on the stud work walls.
I had to wait until now to start the stud work as the building has a sloped roof, and it would have been near impossible to work out how high each wall joist needed to be without having the roof completely finished.
Once I have the studs in place, I'll be calling an electrician in to get the power fitted, as I assume this will need to be done before I add wall insulation.
The end is in sight...weather permitting I should be able to get the stud work done over the next week. That being said, given the weather over the previous few days I'm pretty doubtful. I came very close to going out and buying one of those white plastic gazebos so I could continue working!