Need some advice on insulation/ventilation for a new A-frame build in zone 4A. My preference would be to leave a tongue and groove roof deck exposed to the interior and insulate from the top of the deck, but the cost of rigid board or spray foam versus batt insulation has killed that option.

My “A’s” will be built with rough cut 2×12’s, so achieving R38 with Rockwool batts is no problem (I see that zone 4 requires R49, but all the state-level literature I have seen says R38, so I’m rolling with it). I understand I will need a 1-2″ ventilation channel, along with soffit and gable/ridge vents. My final hang up is moisture and air control. I know I need an underlay between the metal roofing and the deck… does that need to be water tight, or will a felt paper or similar be sufficient?

Additionally, I’m reading I need a vapor barrier between the T&G ceiling and the batt insulation, but I have seen some awful cases of moisture pooling up in wall cavities when that’s done and don’t like it at all. Am I missing something?

Lastly, and as less of a question and more of a rant, it seems building codes have destroyed the simplicity of the A-frame. The simple and cost-effective framing seems to be more than negated by the high cost of insulating a roof-dominated structure. The fact that I’m forced to cover my rafters and T&G roof decking because exterior insulation would cost $10-15k+ on even a small house just hurts my soul. I haven’t found any loopholes around this given the unique design, but if any exist, I’d be thankful to hear them!