For years, building scientists have investigated whether fiberglass insulation or cellulose insulation can be used to insulate an unvented cathedral ceiling. In general, the answer has always been no, because this approach can lead to damp roof sheathing, especially in cold climates.
I’ve written at least four articles on this topic in recent years:
- “Can Unvented Roof Assemblies Be Insulated With Fiberglass?” (August 2015)
- “Smart Vapor Retarders for Walls and Roofs” (January 2016)
- “A Researcher Looks at Insulated Roof Assemblies” (July 2016)
- “Vapor Diffusion Ports” (August 2018)
Here’s a summary of the information covered in the above four articles: