I have a post and beam barn with non-pressure treated sill beams. I noticed that at times there will be condensation on the sill beams and underneath the barn, an area which seems to stay moist. I want to spray a protective material onto the sill beams but do not know what to use. Additionally, the ground under the barn is moist and I am considering laying down an impermeable membrane to prevent water from evaporating up out of the ground. Is this a wise decision, and if so, what type of membrane would you recommend. I have already tried to improve ventilation under the barn by removing some of the surrounding ground level dirt, but this has only seemed to increase the condensation problem.

Any advice would be much appreciated.