It all started with a moldy musty smell in our master bath near our whirlpool tub. Removed the front panels and realized the cutouts in the subfloor for the plumbing were never filled in around the pipes--just wide open space between the crawlspace and tub deck. Warm air was blowing up from the crawlspace and bringing that musty smell with it. DH used spray foam insulation to fill in the open spaces the next day and smell is better but not fully gone. Then the HVAC guy we've used for years in previous houses came over to see about installing system for upstairs expansion (builder used a different contractor). He went under the house and the exterior of the ductwork insulation was literally dripping wet and puddles were forming on the vapor barrier. The main part of the system was also drenched in condensation. His recommendation was to close the crawlspace vents and use a couple dehumidifiers under there for now to try and dry things out. We closed the vents immediately and 24 hrs later it seemed to slow things down some.

We understand how condensation works, what I don't understand is if the ductwork is insulated, would it really get cold enough on the outside of the insulation to form condensation when it comes in contact with the moist hot air of outside? We don't understand closing the vents--we always had the vents open in spring/summer/fall to keep the air moving under the house and closed in winter to try and retain heat for the plumbing. I searched online and saw differing opinions on closing the vents in this type of situation. Is there anything else that could be causing this issue that we need to look for? We have lived in this area for 30+ years, always had open vents spring/summer/fall & closed in winter and have never had this problem in any other house (including a new build 12 years ago)--what has changed? Any other advice or suggestions (other than retrofitting the crawlspace to a cleanspace type area)?

Thanks for any advice you can give!

This is your problem. If you let the hot humid air into your house, you will get condensation when it hits a cold surface. Keep the vents closed and get a dehumdifier. You want the indoor humidity to be in the 45%-50% range.

My builder (not for our current basement house) recommended sealing the crawlspace - even if that just means closing the vents - and run a dehumidifier. After that you do rigid foam at the vents and then upgrade the plastic on the ground. Ideally you tape and overlap the seams and run it up the walls. Perhaps you have some leakage into the crawlspace or something that really increases the humidity.

Condensation forms with really humid air or really cold delivered air. So if you are running your house at 65 degrees, the delivered air is really cold. Since I assume you haven't changed what temp you live at, the difference with the newer house is just greater humidity in the crawlspace. I suppose it is possible the ducts aren't sealed well if you see the condensation mostly at the connections.

You might also consider a problem with the drain around the house. Another thought is to have the builder fix the problem....

if you have cold air leaking out of the ductwork
and at best 2" of duct insulation, then surround the
duct with unconditioned air..they condensate.

you don't mention type of ducts...flex or hard pipe
ducting.

flex only leaks at either end where it is attached..
unless it has holes within the run.
hard pipe leaks at every joint & connection.
ductwrap is installed over the leaks.

if ducts are not mastic sealed..and plenum connections
to equipment are not mastic sealed..it all condensates.
duct tape isn't a sealant and hvac caulk only seals
if it is actually in the place where the leak is..not
next it or around it..where the leak is actually located.

closing the vents and putting dehumidifiers is
an ongoing expense, it will reduce condensation
but you pay for equipment and cost to operate.

adding a vapor barrier to the ground of crawlspace
and properly draining system and now dehumidifiers
becomes the next items you need to explore.

best of luck.

24/7 usage
one stand alone dehumidifier centrally
located can dehumidify about 2000 sq ft.

I have a whole house dehumidifier that costs
me about $20 a month.

don't you hate those open ended answers.
but sometimes depends is the answer.

The short story is that raised homes with open crawl spaces in humid areas worked fine until mechanical cooling (AKA air conditioning) was added. You combine high dew point with cold ducts in the coolest part of the outside. That coolest part of the outside is often the dampest as well.

What has changed? Your luck has changed. In hot humid areas air conditioned houses with vented crawlspaces are always on the knife-edge of condensation down there. Ducts that are cooler still are even more vulnerable. There may well be a problem with the ducts as energy_rater pointed out. Fixing that might put you in safer territory.

The fact that the condensation slows down when you merely close the crawlspace vents indicates that there is far more moisture available from the air circulating under your house then from the ground.

read thru it and then the articles at this site
dealing with the same issues.

its a real eyeopener for homeowners & trades people
http://www.energyvanguard.com/blog-building-science-HERS-BPI/bid/20490 /Vented-Crawl-Spaces-Some-Common-Problems

you'll find that there is no one size fits all.
air sealing, duct sealing, vapor barriers
all parts of the crawlspace need to work
together.
houses as a system. if we can understand
this, we can solve most issues.

best of luck.

Low cost electricity area in all of NC (that I know of).

The electricity spent on dehumidifying the crawl is not entirely wasted. A lot of that humidity is coming into the living area anyway so it lowers the load on the a/c system. The better the crawl is sealed, the lower humidity down there and the less electricity used.

I suspect that if you seal it well, there is essentially zero extra electricity used but that would depend on a lot of factors.

A large secondary benefit is reduced insect load.

Evidently our HVAC guy didn't go all the way to the other side of the house when he was under there. (He wasn't here to check on the condensation problem anyway--he was just the one to discover it and assumed everything was A-OK under there except for the vents not being closed.) The condensation reduced initially after closing the vents, dehumidifiers running 24/7 having to empty every 2 hrs. But still SOOOO wet and condensation on ductwork and system picked up again. DH went under and inspected all the ductwork, vents, etc and found that the ductwork connecting to the masterbath vent had come loose, was laying on the ground and basically working to cool the crawlspace!!! I guess that explains the slight breeze we were getting through the uncovered/unfilled plumbing holes under the tub deck! Reconnected the duct, condensation seems to have slowed tremendously if not stopped altogether, still having to empty the dehumidifiers every 2 hours, but making progress!

Thanks for all the help!

Get dehumidifiers that can drain through a hose.

If you cannot get outside the crawl space with gravity drainage, a condensate pump will do the job.