1) Installed heat pump (14 seer). Home was originally cooled via window units. The indoor portion of the system is installed in the crawlspace.
2) Replaced insulation between floor joists in crawlspace. Original insulation had fallen down in spots and was moldy in many areas.
3) Installed plastic vapor barrier (4-mil cross-laminate poly) over entire ground surface in crawlspace. Overlapped and taped seams. Ran plastic 4-6 inches up crawlspace walls.
4) Sealed interior crawlspace walls with UGL waterproof paint. Also painted UGL on outside walls in one corner where improper grading led to excessive moisture.
5) Replaced all windows. (This was done more for energy efficiency, rather than humidity control).
6) Dug trench down to footing around house in above mentioned corner. Placed Akwadrain sheet drain against foundation. Installed solid pipe to carry downspout output away from structure. Backfilled in trench with gravel up to 6 inches from surface, then remainder with dirt.
7) Temporarily sealed crawlspace vents and installed crawlspace exhaust fan in remaining open vent to create negative pressure in crawlspace.
Before accomplishing tasks 6 & 7, the humidity had been running around 48 - 51% with the A/C running (thermostat set for 70 degrees) and a Sears 50-pint dehumidifier running full-time in the center of the home. The dehumidifier would normally need to be emptied every 1.5 days. After steps 6 & 7, the humidity has been running 53% - 56% and up to 61%, if the dehumidifier is off (it fills up in a little less than a day now.) Outdoor humidity was in the 73% - 100% range both before and after that crawlspace sealing, so I don't think that is a factor in the humidity increase.
My suspicion is that sealing the crawlspace vents is the culprit. There seems to be two schools of thought with regards to keeping them open in the summer in the southeast. Some state that the vents must be kept open for proper air flow, others state that the vents are only pulling in humid air from the outside. The latter group suggests sealing the crawlspace and creating negative pressure with a vent fan, thereby allowing the cooler, drier air from inside the house dry the crawlspace. Oddly enough, I've notice a few "hot spots" throughout the house and think that attic air is now being pulled down into the living area. I've re-opened one of the vents to see what, if any, difference it makes.
It may be that humidity levels in the low 60% range are ok for a home in North Carolina, but I would like to have a few second opinions.