The current building codes let you build moisture-safe unvented roofs using spray foam or rigid board foam. But what about just using cellulose or fiberglass instead? Our team has been studying this issue over many climate zones for years. The current Building America/Department of Energy-sponsored research has a test hut here in Massachusetts, looking at a variety of assemblies, over the past two winters. Cellulose vs. fiberglass? Diffusion vents vs. no diffusion vents? Interior vapor control membranes--fixed perm or variable-perm? Effect of interior relative humidity? All things that we've examined. We will present the latest data, and provide our current thinking on what assemblies are the most likely to work.