Home interior air/vapor barrier selection

This is a homeowner's question, not news, so there's no policy or program update here. It's one person asking for advice on their own build, but the details are worth knowing if you're renovating or finishing a similar structure.

The homeowner bought a post-frame (pole barn style) home in West Virginia that was already framed and enclosed but never finished inside. They're insulating the walls with rockwool (a mineral-wool insulation) to around R-29, and want to add a barrier on the inside face of the wall that stops air leaks but still lets some moisture dry toward the interior, since the metal siding outside blocks drying to the outdoors. They're weighing a few "smart" vapor barrier products, membranes that adjust how much moisture they let through depending on humidity, rather than a plain plastic sheet.

They also have a vaulted, vented attic ceiling finished in tongue-and-groove pine, which is a wood plank ceiling with visible gaps that can leak air. They want to know if the same type of smart membrane can go behind that pine to block air movement without adding a layer of drywall or plywood first.

For a homeowner, the takeaway is general: when metal siding or roofing blocks outward drying, the choice of interior air and vapor barrier matters more, and matching the material to your climate zone and wall assembly is worth getting right before drywall goes up.