Jolene,

We have used premium whitewood ship lap siding in many bathrooms without having an issue with mold or moisture. The key is to have good ventilation from vent fans in the bathroom near the shower area to remove steam and moisture from the air. Also you may want to use an elastomeric paint on the wood, which will give additional moisture protection.

If this still does not satisfy your oncern you could consider using a pvc 1 x 6 board, such as AZEK. This is not a true shiplap but you can simulate the look by installing it with a gap between the boards to simulate ship lap. This will be quite a bit more expensive, but would give you the ultimate peice on mind for your concerns.

Thank you are your question!