Insulating a cathedral ceiling

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. This is especially problematic on north facing roofs that do not warm up in the sun. You can now get vapor barriers specially designed for this situation. One supplier is Siga 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 add insulation behind the barrier. Check out Martin Halladay’s posts on this topic on Green Building Advisor (GBA). One of his posts states: “GBA gets more inquiries about rotting cathedral ceilings than any other type of building failure. That’s why I’m conservative about recommendations for cathedral ceilings. It’s important to get these details right. If you screw things up, everything gets damp and begins to rot.” See below information on condensation problems in general.