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I recently printed off 15 pages on Koolthern K7 Pitched Roof Board, and on page 12 under Sitework it lists 3 methods for installation between rafters based on the depth of the rafters compared with the depth of the insulation. Fixing for insulation less than the depth of the rafters should be done by nailing battens to the inside of the rafters so that the insulation is flush with the bottom of the rafters and should fit tightly with all gaps sealed with expanding urethane sealant.
Have a look for yourself, as I am no expert. Indeed, I read this to be contradictory.