Preserving the Exterior Look of an Old Barn

I’m working on restoring a 70 to 80 year old barn. I would like to keep the original outside walls, that are in good shape. You can see through the vertical wood walls as with any barn in this age range. There are gaps of 1/2 to 1.5 between each board. I’m looking for a material that can be placed on the inside of the walls to prevent moisture(rain) from penetrating without compromising the exterior old look or rotting it. The material would have to run to the floor to keep water out. I plan o