Can felt paper be put under planks nailed directly to studs?

I have seen that done as it helps hide what is behind the boards when they expand and contract with weather and moisture changes. I haven't tried it as I don't have shiplap but it sounds like a good idea to me.

Sometimes planks will shrink after being installed and open up so that you can see underneath. Some kind of paper, the same color as the planks would make it more invisible if they do happen to open up. One thing to stop the shrinkage is to coat the back of the boards with a coat of varnish or varathane. I know this is time consuming, but it really does help. Lay some boards out on 2X4's and then roll the back of them with a roller and varathane. Let dry and then install.

I think that sounds smart. Be sure to back prime your ship lap prior to installing.

You cannot nail planks directly to studs without a fire barrier. This is building and fire code and homeowners insurance regulations. You need to screw at least 1/2" drywall to the studs which acts as a fire barrier. Then you can cover with anything you choose.

Thank you William. I did not know this