Traditionally, crawlspaces have been equipped with vents to allow fresh air exchange between the underfloor space and the outdoors, and floor systems were insulated with fiberglass batt insulation in the joist cavities. But in hot, humid, and mixed climate zones, the building-science justification for that approach has never been solid, and these days, the state-of-the-art solution is a sealed, conditioned crawlspace, with a vapor barrier protecting the underfloor space from ground moisture, and with the insulated thermal boundary moved from the framed floor to the wall perimeter. Codes have evolved in recent years to allow this approach, and the latest codes include prescriptive details for the ground vapor barrier, wall insulation, and conditioning system.
In the hot, muggy climate of South Carolina, a conditioned crawlspace is particularly advantageous. Even in dry Northern climates, ventilation is only marginally effective at removing ground moisture from an under-floor crawlspace; in the humid South, crawlspace ventilation openings that connect to the outdoor air often make things worse. So...
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