Hello, I am working on a project that involves removing old batt insulation and some reframing of a 2×6 cathedral ceiling with skylights. I have read about both the vented and unvented cathedral ceiling s approaches and I understand the merit of both approaches. The ceiling I am working on has both soffit venting and continuous ridge venting and most of the rafter bays have uninterrupted communication between the soffit and ridge venting, but not all of them. Some of the communication channels are blocked by skylight framing so they either have a soffit vent or ridge vent but not both. In this situation I wonder if it would be best to treat all the bays the same or to treat each bay as a unique situation. In other words, should I create a airflow channel in the full length bays with both lower and upper venting, and fill the other bays full of rigid insulation eliminating the airspace. What are your thoughts on this matter.

I also wonder if it would be better to create a communication port between the uninterrupted full length bays and the interrupted bays by drilling a 1″ hole, just beneath the roof sheathing, through the rafter to allow air to flow into the adjacent uninterrupted bay and out the ridge vent. In this situation I would create a air flow channel in every bay.

I have left over 1 1/2″ Polyiso rigid insulation I plan on using for this project. If I create air flow channels I will end up using two layers of 1 1/2″ insulation, and If I fill the cavity with insulation I will have a total of 5 1/2″ of polyiso insulation, without airflow.

Of course I will drywall over the insulation and rafters creating an airtight seal.

I live in Washington State very near Puget Sound.

Thank you in advance for your expert advice.

Cameron