What PPF said.

We installed over 3/8 dry wall in my sons house. We also primed the dry wall before we installed the ship lap.

Yes to drywall. It may not be authentic, but who's going to see what's behind your ceilings? Lots of charming old things are improved by modern techniques.

The gyp board also reduces the creepy crawly things. If that's important.

Finish the drywall to an acceptable Level 4. By that time, you’ll realize that you aren’t located in Waco and the 15 minutes has long since passed for shiplap. And you’ll be 60K ahead too.

Really ship lap is done no matter what the material just drywall to nice smooth finish and save the money for something else.

Thanks every one, I will have drywall hung and finished before installing the ship-lap.

Sophie Wheeler- This said Cypress Ship-Lap was the actual hull of my Grandfathers shrimp boat he built in the 20's. He made a fine living in the boat until the 80's when he had a steel-hull shrimp boat built. My farther, brothers, and I took that old boat apart board by board saving every thing that was salvageable. After refinishing the boards we refinished the house I grew up in with the cypress ship lap on the ceiling. It was enough to do the entire house plus some. That house was destroyed in a hurricane a few years ago, my father and I tirelessly re-salvaged every piece of that cypress that we could save. It has been sitting in my shed since then. Its safe to say when I finish construction of my new home, and hang that cypress ship-lap on the ceiling, it will not cost me 60k and It will be there long long after the trend has faded.