I am in the process of insulating and enclosing my garage that has a cathedral ceiling with 2x10 roof rafters. CA First, you should install proper vents against the under side of the sheathing from the soffit vents to the open space of the attic. Ventilation is crucial in today's well-insulated homes, helping to insure good indoor air quality. Also, this is the time to install a ridge vent if needed.
Vapor barriers keep moisture from penetrating and condensing in the insulation.
Be sure to seal every seam in the vapor barrier after insulating. Moisture can penetrate through even the smallest gaps. This Old House and the This Old House window and This Old House Online are trademarks of WGBH Educational Foundation. Neither WGBH Educational Foundation nor PBS endorses any product or service mentioned within This Old House Online or any web sites linked to or from This Old House Online.