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Question
The builder put gravel underneath the house and I have plastic on top of it to stop moisture problems. I may have nothing to worry about and I live in North Alabama so I understand ducts will sweat but there is a tremendous amount of condensation on my ducts and air handler. They are insulated and the air handler is insulated under the bottom and there is so much moisture that the insulation under the air handler is bogging down from retaining so much. Is this normal?

Answer
I would not consider this normal at all. Close or cover crawlspace vents. Install a dehumidifier in the crawlspace and run the drain outside, piped as far away from the house as practical. If you installed a sump pump in the crawlspace, drain it into the sump pit.

Make sure the condensate drain line from the air conditioner indoor unit doesn’t drain into the crawlspace and that the drain line is kept clear. If the condensate drains into the crawlspace, a sump or condensate pump should be installed.

In some cases, a new condensate drain line can be run with proper downward pitch all the way to the outside. The drain should terminate above grade outside so the line doesn’t clog.
Make sure the dryer vent doesn’t terminate in the crawlspace. Drying a load of clothes can produce 4 to 6 pints of water vapor. If the house uses a gas dryer, the products of combustion will add even more moisture to the space.

Plumbing leaks can be a major source of crawlspace moisture. They should be fixed as soon as possible because they can cause the wood to soften, making it easier for mold and rot to destroy. Make sure the relief valve and drain pan drain line from the water heater are piped outside or into the sump pit.
In humid areas, ventilating the crawlspace is only effective in the spring and fall when the absolute humidity of the air outside the crawlspace is lower than the absolute humidity of the air inside it. In coastal areas, that almost never happens in the summer.

As a general rule, if it’s above 75F outside, there’s more moisture in the outside air than in the 70 to 75F crawlspace air; therefore, if you ventilate the crawlspace, you’re bringing more moisture into the crawlspace than you’re removing. Anytime you drop the air temperature by 20 degrees, the relative humidity approximately doubles. When it gets to 95F outside and that air is brought into the 75F crawlspace, the relative humidity doubles. If it’s 60% RH outside, the relative humidity in the crawlspace is at the dew point. At dew point conditions, water vapor turns back to liquid.

If exhaust fans are running in the crawlspace, you can also suck conditioned air out of the house into the crawlspace through cracks, making it colder in the crawlspace. This can increase condensation in the crawlspace, making the problem even worse.

Try to avoid the use of toe space grilles in kitchens and bathrooms above the crawl space. When cold air is blown into the space between the floor and the cabinet base, the temperature of all the flooring under the cabinet drops to about the supply air temperature. I’ve been in a number of crawlspaces where the plywood under the cabinets was black and mushy from rot, and I’ve seen hardwood floors buckle next to toe space grilles.

Repairs to hardwood floors can get very expensive, especially if they need to be done every year. Icynene insulation can be sprayed on wood under toe space grilles in the crawlspace to stop condensation and seal the wood. Be sure to seal around the duct penetrating the floor to reduce unwanted air movement.

Add gutters and downspouts to your house to carry water away from the house. Divert the water from the downspouts as far away from your house as is practical. Putting oversized splash blocks at the bottom of the downspouts helps, but piping them at least 10 feet away is preferable.

Insulate your floor to reduce condensation on the wood. If you have trusses instead of floor joists, make sure the insulation is up against the floor docking. Using Icynene foam insulation is better than using fiberglass batts. If the insulation is at the bottom of the floor truss, gaps in the insulation allow moisture-laden air to get above the insulation to condense on the underside of the floor and drip on the insulation. Wet insulation rarely does any good.

The exception is expanded foam insulation. Icynene foam insulation can be sprayed on the underside of the floor facing the crawlspace, providing both insulation and a vapor barrier. It can also seal cracks between the house and the crawlspace, reducing unwanted airflow between the two areas.

The ground should slope away from the foundation of the house so water does not puddle near the foundation when it rains. If the water table is high, you may want to install a French drain, provided there is lower ground nearby where you can pipe away the water that was collected.

Lay plastic on top of the gravel and pour lightweight concrete over the plastic. This will protect the plastic, which can be disturbed or damaged by people crawling over it during maintenance.

Feedback
I live in a small town of just east of Huntsville which is a pretty populated area, even talked with techs over there and they say the same thing, vents need to be open. There are about 6 different a/c techs in my neck of the woods and all say the same thing an no telling how many in Huntsville which has close to 400,000 people in the metro area and you ask me if I want to be a leader? Wow what an opportunity!. To be the first out of 400,000 people to close my vents, maybe I should run for mayor, lol. I guess since I am no expert and I am trying to seek advice from someone on this board a little closer to my home and all the techs are saying vents should be open and ducts are suppose to sweat that I am a little hesitant to tell them how to do their occupation.

Answer
I may be in Massachusetts but the laws of physics aren't any different where you are. For one thing, you're suffering from herd mentality. Just do what your neighbors do and you too can be 'cool', right? Well others have already told you about sealing the crawl space. This is new technology on the grand scheme of things, but do you want to be a leader or a follower? If the former then get the crawlspace sealed. Others have already told you that warm/hot, moisture-laden air is moving through your crawlspace. When it contacts the cold AC system components, the air is cooled to its dew point and the moisture precipitates out as liquid. Voila, droplets of water form. If all is perfect with your AC system, as you seem to think it is, then you must stop the exchange of air through the crawlspace. If you're unwilling or unable to do that, you will continue to have moisture problems. It's that simple.

Answer
Look, I've said it before in this thread, you don't have to go crazy and spend a lot of money to solve this problem. Seal the vents, and install a cheap dehumidifier.

I put in a 30 pint per day GE from Wal-Mart that cost me $148.00 and yesterday I crawled back to the air handler for a look-see. Now our dew points here locally have been ranging from 70 to 78 degrees, no kidding. Everything outside at night is sopping wet and you can hardly breathe during the day it's that miserable. But under my house it's 76 degrees and the RH is 48%. No moisture anywhere.

I pulled the cover off my air handler to look for condensation inside the unit and it's nice and dry. For a few hundred dollars and a little effort, this crawl space moisture problem is a problem no more.

Mold & Mildew