Dusty earth means its drying to the crawlspace. Scratch the earth and you may find moist soil underneath. Try laying a large rectangle of plastic down. The earth will get wet under the plastic moisture is wicking up. An exception would be if there were several inches of course washed gravel or concrete are underneath. An air-conditioned floor will have condensation on the bottom above the insulation, when the outdoor dewpoint is +75^F outdoors. In dry climates like Colorado, this less or no problem.