Help! Sweating HVAC ducts

[Dixie]

What needs to be done to stop my ducts from sweating copiously?

The A/C ducts for my house are in the crawl space. There's pools of water under the ducts, and the insulation around the ducts is wet. I need to get this resolved because mold is start to grow on the joists. But what needs to be done?

Let me provide a bit more info in case that helpful. The crawl space is about 2.5' high. The main duct runs lengthwise, right in the center. It covers at least 75% of the run. And in most places there isn't enough room to go over or under the duct. (There's a crawl spaces door at each end of the house.) The ground is covered 100% with a vapor barrier of 6 mil plastic. I should also mention there is a skirt vent ever 5' around the house. I also need to state the obvious. I live in S. Carolina where it's hot as hell and humid. "A screen door away from hell" describes summers here perfectly...

I did cut slits in the vapor barrier to drain the pools of water. I also removed the crawl space doors. But that's hardly a permanent solution. Do I need fans? New ducts? What?

[john70t]

Not a pro, but I would think:
1). Tape all duct connections and replace that insulation with something designed for ductwork.
2). Spray down joists with 50/50 bleach and water to kill mold.
3). Insulate underside of house where hot crawlspace meets cold floor, and use vapor barrier with tuck tape. This has to be a complete seal or water will accumulate inside and accelerate the growth. I don't know, mabye a partial vapor barrier like tyvek would be better(?)

[Danny_Ocean]

Condensation is a sign of poor air-flow. As an experiment, run a box or shop fan under the house for 24 hrs. to see if that changes anything...

[mca]

I've read that a bleach solution is a no no. Might want to research before spraying.

I can vouch for the SC summers. At least it is only 90 today in Charleston.

[URY914]

Do the bleach spray. We use it all the time. Just be careful of the fumes.

[cgarr]

If your insulation is loose fitting could be the problem, you may have to get new duct which is insulated between the metal inner and outer. Its all about dew point, when you take an ice cold beer outside what do you do to keep it from sweating? besides drink it fast?

[MarKoBrow]

Is this a new condition or have your ducts always shown signs of condensation during times of high humidity? I yes I would be concerned with the size and CFM of return Air. If no than I would first lean towards drying out insulation and cleaning evaporator and duct interiors. Considering it is a crawl space and not an attic the reasons for condensation can be entirely different so venting is not always the answer.

[Hugh R]

Check your condensate drain line, it may be plugged. I hooked a couple of garden hose bibs/valves up to it and attached a garden hose and blasted the gunk out of it. One in line valve (closed) to keep from pushing water back into the condensate pan, and a hose bib to attach to the garden hose to blast it down the drain line. Problem solved.

[billybek]

If you increase the amount of airflow in the ducts, the temperature of the supply air will go up too. This results in less sweating. Make sure all the diffusers in the house are open, air filter clean and check the evaporator to make sure it isn't partially plugged.
Otherwise, get better insulation installed and make sure it has a foil barrier that is foil taped at all the connections...

[williecoyote]

The current school of thought (which goes against everything I was taught) is to
seal all vents, foam any holes which penetrate the floor, put plastic on the ground and up the foundation walls, and install a crawl space dehumidifier with the drain line run away from the foundation.

Google crawl space moisture problem, or something similar. You will find much info.

[MarKoBrow]

Originally Posted by williecoyote

The current school of thought (which goes against everything I was taught) is to
seal all vents, foam any holes which penetrate the floor, put plastic on the ground and up the foundation walls, and install a crawl space dehumidifier with the drain line run away from the foundation.

Google crawl space moisture problem, or something similar. You will find much info.

The original post did not clarify if this was an existing problem or a new condition. If the system was designed correctly there would be no need for extra dehumidification systems. The google solution that is outlined in the post above is a recipe for future mold problems.

[Dixie]

Originally Posted by MarKoBrow

The original post did not clarify if this was an existing problem or a new condition...

This issue occurred once before. It was an=bout six years ago. I put down the vapor barrier and all was well. Obviously the solution was marginal. One summer of 100 degree temps at 85% humidity was too much.

The suspected culprits remain, poorly installed ducts, poor ventilation, both, or other. All I know is I don't want to deal with someone akin to the Travelers trying to "fix" the problem.

[turbo6bar]

Originally Posted by williecoyote

The current school of thought (which goes against everything I was taught) is to
seal all vents, foam any holes which penetrate the floor, put plastic on the ground and up the foundation walls, and install a crawl space dehumidifier with the drain line run away from the foundation.

With today's push for ever tighter and more efficient homes, sealing the crawlspace is definitely advocated by building scientists. A sealed vapor barrier on the floor and insulated foundation wall and rim joist are part of the non-vented crawlspace mantra. In addition, you must ensure the exterior of the house is designed to shed water away from the foundation.

I recently read an article about duct sealing in crawlspaces. I cannot recall where I read the article, but the author recommended sealing the joints with mastic and then covering the ductwork with insulation fastened with staples. He stated there are no sealing tapes suitable for long-term performance/durability in the crawlspace. Of course, if you seal the crawlspace and allow the humidity to approach the conditioned space, I don't see why tapes cannot perform adequately.

If your ductwork is flex, then you definitely have humidity problems or serious leaks to cause condensation.

[MarKoBrow]

Originally Posted by Capt. Carrera

This issue occurred once before. It was an=bout six years ago. I put down the vapor barrier and all was well. Obviously the solution was marginal. One summer of 100 degree temps at 85% humidity was too much.

The suspected culprits remain, poorly installed ducts, poor ventilation, both, or other. All I know is I don't want to deal with someone akin to the Travelers trying to "fix" the problem.

Based on the fact that the problem may exist more when the temps are humidity are high, I would direct my attention to the overall airflow of the system. It sounds like your system is under designed for the environment in which it operates. Does the system have enough CFM designed into the air handler? Is the evaporator undersized for any reason? Does the system rely heavily on flex duct? If so are they collapsed? Restrictive return grills and undersized filters will also contribute to this problem. I would fully recommend that you contact a contractor that specializes in heat pump design. The chances are that they may be from more of a system design school of thought opposed to throw products at a problem and see if it helps.