Insulate garage roof from underneath

Only if you want to create condensation problems

Yes use celotex or kingspan or similar 100mm preferably.cut between joists try and leave 50mm gap at top for airflow.

Yes, if you have a flat roof, cut the insulation in tight as possible between the joists, you may want to bang some nails in a little bit, so the insulation stays up in between the joists, if you have actually pitch rafters and your ply is fixed down to the inside of your rafters to make a floor then I'm afraid you will have to prize the plywood up and then insulate ,especially if there is a ceiling underneath already.

I think it depends what purpose the garage will serve once the job is completed. If you are working in there or converting it in to a living area then you may want to consider an internal roof installation. Create a wooden frame, insulate over the top of the frame to maintain that air gap, clad over the top of the insulation.

Celotex in between the joists must be flush and then back across the opposite direction to stop thermal bridging 150mm in full should be achieved but check with building inspector you may get permission to use less if the rafters aren’t that deep The thickness must be between the rafters or first or you could develop problems

Yes, you can insulate the roof from below, provided adequate ventilation and condensation control are maintained