Q: Insulation recommendations for cathedral ceilings?
A: Cathedral ceilings are tricky to insulate. Most building codes require them to be vented to remove the inevitable condensation caused by warm, moist, air rising to the top of the ceiling and penetrating the ceiling through light fixtures, skylights, cracks and just from diffusion through the drywall. The moisture in this air then condenses when it hits the cold roof surface. If this does not evaporate you will get mold and rot. You can now get vapor barriers especially designed for this situation. One supplier is Majrex in Switzerland. I have not used this myself but I intend to use it when converting unfinished space in our rental property to finished space. With a proper vapor barrier you can either add insulation behind the barrier or in front of it (lowering the ceiling). Check out Martin Halladay’s posts on this topic on Green Building Advisor.