How do I avoid condensation issues with an insulated garage roof?
Based on your comment I think what you will need to do is replace the insulation.

As I said in my comment condensation is caused when warm moist air hits a cold surface, be it a window or roof. If your insulation was sound then this would prevent this happening, but it sounds like it letting the air through to the roof.

I'm reluctant to suggest what to replace it with as I haven't seen the roof - so it might be worth getting someone in to quote for the job and ask them what they intend to do.

If you do decide to replace then putting a semi-permeable membrane up as well will help keep the moisture off the cold surfaces.