Can any one out there tell me the proper way in the hot humid south to the venting of a unheated crawl space with A/C and heating ducts  in the crawl space.  I just had a HVAC contractor change out my system of 20 years old.  He found trouble on my first installation where the returns on a inside wall were drawing hot humid air out of the attic.  He also mentioned that the vents under the house should be closed in the summer.  He also mentioned that he has been on jobs that the mold and mildew caused him to turn down the job as he would not let his men go under the house for health reasons. He also said that the ducts are insulated and people let the hot air in through the open vents and causes the ducts to sweat.  He recommended that plactic 6 mills thick should be put on 100 percent of the ground under the crawl space.  There was a ariticle in the Sunday USA paper several weeks ago saying how many people are disabled from the mold and mildew.  There are many different versions of what to do in books but which way is the correct way?  Thanks – Larry