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:
Here’s a summary of the information covered in the above four articles:
The two researchers who have investigated this issue in the greatest depth are Kohta Ueno and Joseph Lstiburek of Building Science Corporation in Westford, Massachusetts. In July 2016, I reported on one of their research projects: a field study near Chicago showing that installing a diffusion port at the top of unvented rafter bays insulated with fiberglass or cellulose failed to prevent damp sheathing (especially near the ridge). The data suggested that in Chicago, the best way that builders could stay out of trouble was to follow the conventional vented approach.
Following up on that Chicago study, Ueno and Lstiburek obtained funding to conduct a three-year study of similar roof assemblies in Massachusetts. In the Massachusetts study (unlike the Chicago study), some of the rafter bays included an interior “smart” retarder — that is, a vapor retarder with variable permeance. The researchers wanted to know whether the inclusion of a smart retarder (in addition to a diffusion port at the ridge) would be enough to keep these rafter bays safe. The Massachusetts research was funded by the U.S. Department of Energy’s Building America Program and a consortium of insulation manufacturers.
Ueno and Lstiburek recently published their findings. Interestingly, the data gathered in Massachusetts reinforces and confirms the GBA advice summarized in the bullet points above. Of course, the main reason that the building community can be confident of this advice is that…