Hello. I am currently finishing the build of a small cabin in zone 4 (barely) it is south coastal british columbia. General summer temps 25 to 35c general winter temps 0c to 10c It has a single slope roof built with 2×6 rafters. I have built 2.5″ continuous venting with spacer straps against the roof deck and used 2″ rigid foam between the joists to create the bottom side of the roof venting. The rigid foam has been screwed into the spacer straps and then sealed around the periphery with expanding foam above the wall top plates and sealant around the foam sheets. Since the 2×6 rafters don’t leave a lot of insulation space, I have added milled lumber and glued and screwed these strips to the bottom of the rafters to allow more space for insulation. I have allowed an additional 3.5″ cavity below the 2″ foam so i can install fiber or rockwool insulation. Batts as a second layer of insulation. The layers are then roof sheathing->2.5″ continuous vent->2″ rigid foam->3.5″ insulated Batts.

My question is in regard to using some form of vapor retarder.  the space is conditioned with a ductless split heat pump but we often do not use a.c. in summer as we have quite a canopy of trees above. The majority of the time the room will be either ventilated with outdoor air / fans or in the shoulder seasons/winter will be heated. A.c. will be used but only on the hottest days where it becomes uncomfortable.

My concern comes from using a vapor barrier above the ceiling drywall. Since the rigid foam is sealed into the rafters and wall plates, I assume this creates quite an effective air and vapor barrier. What i don’t want to do is create a vapor sandwich between the 2″ rigid foam and the finished ceiling drywall.

My question is then, what should I use below the 3.5″ Batts and above the ceiling drywall? My brain tells me using poly here would be a bad idea. I was reading about possibly using a smart vapor retarder that would keep the room vapor out of the cavity for the most part but also allow the cavity to dry in case of any water or vapor ingress? What about building wrap? It is pretty vapor permeable but would it let too much room vapor into the cavity? Should I use nothing at all? My worry here is that excessive room air MIGHT work its way up into the cavity. The ceiling drywall will be well sealed but there are 5 shallow recessed light boxes in the ceiling, though i plan to seal them very well to attempt to keep any room vapor out if the cavity.

What would be the suggested material to use below the insulation Batts and above the ceiling drywall to keep room vapor out of the cavity but also let it dry to the inside in case of water or vapor ingress? Is well sealed drywall / ceiling light boxes ample without any further barriers? Smart vapor retarder?

Also, between fiberglass and rockwool, i assume rockwool is preferred but I absolutely hate the stuff. It is difficult to install properly and makes an enormous mess in addition to the cost being 2x fiber Batts but if it’s worth it, I will deal with it and go with rockwool as much as I despise the stuff!

Thanks for your time.