Yes the wood needs to be sealed. Yes it will get condensation and dust on it. Is there an hvac duct at the very peak of the vault? Anything above the highest hvac supply will be 90° and 90% humidity because warm damp air rises.

No HVAC supply at all. The ERV intake is at a gable end, about two feet below the peak.

Are you aware of what odor molecules are? They are tiny little fragments of a substance, floating in the air. And they will penetrate porous surfaces. Wood is a very porous surface. It also shrinks and swells with changes in humidity, which you'd like to minimize as much as possible.

So, if you want a poop smelling ceiling that swells and bows up, or shrinks and has giant gaps, then by all means, do not finish the wood.

If you like the wood look, consider wood look porcelain. Many building codes do not allow for large pieces of tile over the shower, bit smaller tiles may provide a look you want.

There are exactly ZERO building codes that prohibit any tile sizes in showers, anywhere. Large format tile and large gauged porcelain panels are done every day on ceilings. It just needs the right build assembly and thinset.

And that whole wall assembly is a hot mess.

@Charles Ross Homes the ducts are in an interior wall. Its a narrow 1.5 story house (the second floor walls are only 5’ tall), so the vaulted ceiling runs the length of the house, resulting in each partition wall having a gable. Unfortunately, there was no practical way to locate the exhaust directly over the shower - its about 5’ away from the shower door.

@Minardi The house dates to 1865. Trust me, its considerably better than it was before. It’s an interior, non-bearing wall.

I tested two finishes and decided to use General Finishes High Performance Dead Flat. It didnt change the appeaeance of the wood at all, and even maintaned the raw wood’s natural irridescence. Pretty remarkable product.