I spoke with the home owner about this a few months ago. He didn't have a vapor barrier under his floor or enough gravel between the ground and his earth floor. He has since corrected this by ripping up his old floor and putting down a vapor barrier, a layer of insulation and then a thicker layer of gravel before putting down the earthen floor. He also installed a french drain around the house to keep water away from the walls. These improvements have worked.