Scott Stewart wrote:

This summer, I have been attempting to rectify moisture problems in a
40 year-old brick veneer ranch home (one storey) with a crawlspace.
This home is located in North Carolina where high humidity is the
rule. Below are the steps currently taken:

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.

Thanks!
Scott

I think you're on target about the fan pulling hot/moist attic air down.
If you add up all the sq inches of penetration holes to the attic,
it will be a sizeable gap even if there is no unusual construction
opening.
Unless you get the humidity down to 50-55%, you will encourage
mildew growth.

Jim