Insulate under a metal roof

I recently bought a cabin that has a metal roof mounted to purlins spaced every two feet. Between the metal roof and the purlins it appears there was some sort of vapor barrier plastic installed that looks like bubble wrap. The attic is currently ventilated with two gable vents on each side of the stucture. We would like remove the drywall cealing and the blown in insulation that is in placed now and create a catheral ceiling by installing tongue and groove boards over the roof joist, but would need to install insulation between the T&G and the metal roof. Could you make a recomendation on how best to do this.