Insulating under roof

I have an outbuilding that I need to partition a section off and insulate so it can be used as an office. It is concrete base, brick walls and corrugated cement fibre board roof which is fixed to the purlins. The roof is in good order so I want insulate and board under it, build the partition wall and insulate the floor. I am uncertain of the best way to insulate the roof, I don't want to remove the roof but am concerned that insulating directly under it may create a condensation and damp issue. What is the best way to do this, also I am not keen on spray insulation.

Only insulate under the roof if you are going to heat that room, otherwise as you say, you will suffer condensation issues.

I will heat the room when in use so I will need to insulate it. So if that is the case what is the best method and what materials are best for the job?

Gareth Birkett said:

I will heat the room when in use so I will need to insulate it. So if that is the case what is the best method and what materials are best for the job?

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I would imagine building regs would want about 150mm Kingspan (or equivalent) installed. Some within the rafters leaving an air gap to the felt, then build the rafters up with timber to take more Kingspan and then screw on 50mm insulated plasterboard (it's thirty odd mm Kingspan with 12.5mm plasterboard stuck to it).

What are the purlins made of, what size are they and how far apart? How much headroom do you have?
Are you looking for a decorated finish to the ceiling? (Plasterboard is heavy, there may not be a lot of spare capacity in the roof structure)