Hello,
I’m sure this question will be received with a long sigh, but I have to ask.  A good friend of mine just bought a camp on Miles Pond in Concord Vt. He texted me with a question about insulating his walls. He has 2×6 stud walls with tyvek stapled to the outside of the studs and wood siding on the exterior. He wants to fill the stud bays with rock wool insulation and then put up shiplap paneling inside. He asked me if he should put up a vapor barrier and my reflex answer was to say yes. I grew up in north eastern Vermont and that was standard practice in the 70’s and 80’s but I remembered an article I read here a few years ago about the use of vapor and air barriers that made me pause and tell him I would look into it. I build mostly timber frame homes with stress skin panels and have not used plastic sheets in a wall in over 20 years. would you put anything over the unfaced rock wool batts before the shiplap? If so, what product would you recommend. I have read several of your articles, blogs and comments regarding wall construction and the use or non-use of air and vapor barriers so I don’t need links to those or a long explanation of the building science consensus. My understanding is that I should not recommend Plastic sheeting. If he were doing a drywall that would be a sufficient air barrier. But air will flow almost freely through the shiplap so should I recommend something for an air barrier. I think the camp will be mostly a three season camp with the occasional winter use and it has a wood stove. 
Thank you,
Luke