Cathedral Ceiling with exterior foam, interior ceiling air barrier importance?
Hi all, trying to understand the importance of a fully detailed drywall air barrier on a cathedral ceiling that has exterior rigid foam insulation.
I’ve read lots of GBA articles and threads, and I know that it is specifically prescribed to have a continuous and well detailed drywall air barrier in this case but I’m trying to understand the reasoning for it.
Assuming you have a cathedral ceiling buildup in z0ne 3 with the following from outside to inside:
Metal roof
Roofing Underlayment
Single layer of 2″ R9 nail base exterior insulation
Ice & Water Shield underlayment
5/8″ plywood roof deck
R30 fiberglass insulation in 2×8 rafters
Drywall
How important is the drywall detailing in this instance?
Since there is enough exterior rigid insulation for condensation control, does having ceiling penetrations only affect the R value of the fiberglass due to interior airflow through the insulation or can it actually lead to condensation issues on the roof deck?
If the purpose of the exterior insulation is to keep the roof deck warm enough during the cold months to avoid condensation, would it still work in the case of a “leaky ceiling”? If not, what changes in the case of a “leaky ceiling” which could lead to a failure?
Thank you
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Replies
These hybrid roofs need one solid air barrier. In your case that can be the peel and stick over the original deck but make sure you seal up the soffit area for air barrier continuity down to your walls. This would be similar to how you seal up rim joists area (spray foam or rigid sealed in place with canned foam).
These assemblies can work without any interior cover, so the air tightness of the drywall doesn't matter as long as you take care of the air barrier at the roof sheathing plane.
Since this assembly can only dry towards the interior, the one you don't want is any interior vapor barriers. So no poly under the drywall.
great answer, agreed fully.