Impermeable insulation and vented cathedral ceiling (Zone 6)
I have a question which is driven by the 2009 IRC code requirement for unvented cathedral ceilings; specifically the requirement that at least 1/2 of the total R-value be made up a an impermeable insulation layer, next to the sheathing. Ostensibly, this is required to mitigate condensation on the boundary between the impermeable insulation and the permeable insulation. My basic question is: why isn’t this required for a vented cathedral ceiling assembly? If a “baffle”/ventilation channel is employed in a cathedral ceiling assembly (assume 1 1/2 or 2 inch space) between the roof sheathing and the “permeable” insulation (e.g., fiberglass, rock wool, cellulose) why isn’t that “baffle”/ventilation channel boundary also required to be made up of an impermeable layer at least 1/2 the total insulation value of the roof assembly? Won’t the same condensation issues arise between the baffle interface and the permeable insulation? Or, is this boundary assumed to be leaky enough that any condensation will be taken care of by the vented air stream?
I’m specifically asking because I want to use a 1″ polyiso material for the baffle/ventilation channel, in a vented cathedral ceiling assembly, making sure that there is an effective air seal between the vent channel and the permeable insulation.
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