Green Build Blog: Can Cellulose be Used in an Unvented Roof?
All building assemblies — but particularly “hot” roofs — require a dedicated, comprehensive air barrier. Dense-packed cellulose, while less air permeable than other loose insulation, is NOT airtight enough to qualify as the dedicated air barrier. Be sure that the rigid insulation used on the interior is air sealed to become a dedicated continuous air barrier. “Hot roofs” that are not fastidiously air sealed, particularly at the eaves, can fail. I find it hard to imagine how tedious and expensive it would be to wrap all of the framing and underside of the roof sheathing with peel-and-stick membrane. Since the roofing is vapor-impermeable and not back-vented — and since most of the roof sheathing has peel-and-stick membrane on the top side — the roof sheathing and framing would be encased between two vapor-impermeable membranes, with no drying potential at all.