Hybrid unvented cathedral ceiling ratio – GreenBuildingAdvisor

Because this is about long-term resistance to moisture accumulation, and not an arbitrary thermal goal, I go with conservative insulation values. CCSF will age to around R-5.5/in, and dense-packed cellulose can be as high as R-3.6/in. That suggests 8″ of foam and 16″ of cellulose.

If I had that much space to work with, as did a client I visited today, I’d use a vented roof, unless there was a good reason not to. 8″ of foam represents a lot of embodied carbon emissions and a lot of cost. I’ve used as much as 36″ of cellulose at client request, but the ROI drops off after about R-40 and after R-60 you might save a few dollars a year, literally, while spending thousands more than necessary.