Insulating a garage roof

[Skyedriver]

I know this subject has been touched on before but:
glassfibre roof on roofing grade chipboard on 5x2 joists.
limited headroom
Usually you need a 50mm air gap to stop condensation but, if I was to bond kingspan/Reticel etc to the underside of the chipboard, would I suffer condensation and rotting wood/chipboard. there wouldn't theoretically be any air to condense.
Only have the option of airflow from one side hence the question.

[JR]

I'd just do it. If it rots away in 5 to 10 years time then replace the roof with a plastic coated, insulated profile steel one. Alternatively you could introduce a detail to have ventillation at the non-vented end, say mushroom type devices in the roof.