If the cottage is built on solid rock then the problem is definitely not one of seepage. You do not want to put a vapor barrier just underneath the floor joists; this would only collect moisture in the winter and further damage the joists. You do want to install a vapor barrier, but on the unfinished basement floor. You should cover as much of the floor as possible, but it does not have to be completely covered. The amount of moisture entering the crawl space by evaporation is proportional to the surface area covered by the plastic. If 80 percent of the floor surface is covered by a vapor barrier, condensation will be reduced by 80 percent. Weigh down the vapor barrier with sand or some heavy rocks.