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How to handle unvented cathedral/vaulted ceiling

ohal3030 | Posted in General Questions on

I know there is a lot of healthy debate here, but let me share my situation.

Climate zone 3. 40 year old house I just bought.

I am about to start remodel which includes ripping the cedar wood planks off my cathedral ceiling and replacing with drywall. The current construction is Cedar lap siding, craft paper barrier, backed fiberglass insulation filling the cavity 2×8 cavity, roof sheathing, asphalt shingles. No airflow/ventilation.

The way the room is constructed putting in vents is not an option. There is a large skylight dormer on one side and the other side does not terminate to the outside. So there are no soffits. The current construction has not seemed to fail but I haven’t fully demoed yet. 

I am tempted to just take the cedar planks down, put a smart vapor barrier up like membrain seal up the drywall as tight as possible. Second best option is to spring for closed cell spray in.

I’d appreciate all thoughts. Hopefully ideas that do not require me to tear my house down and rebuild.


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    BILL WICHERS | | #2

    If you can’t vent the assembly — and skylights are especially problematic here — your safest option is spray foam unless you can put up exterior rigid foam (and a lot of it). I would use closed cell spray foam to get up close to your target R value, then fill the rest with open cell to get a full fill of the rafter bays. There is less to go wrong when the assembly is completely filled with insulating materials leaving no room for air to circulate above the drywall.

    I would also install the drywall airtight, paying particular attention to careful detailing of the skylight wells. I’ve had issues with “sneak path” airflow in skylight wells before that causes moisture problems in the immediate area of the skylights.


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