Condensation on duckwork
Recently noticed condensation on my duckwork located in my vented crawel space. Did not notice problem last year. I did seal all joints with mastic during fall so this is first year using after sealing. Live in Kentucky and has been very humid this year and have several crawel space vents. Should I close off vents as was suggested by my Hvac contractor or what should I do to stop this problem?
The duct should be insulated and have a vapor barrier. Being exposed like that, I would install 3 inch fiberglass with the foil faced vapor barrier thats used in the trade.
Condensate under the house
Definitely need vapor barrier but make sure all your house vents are closed off. This keeps hot,humid, air from drafting to the cool part under the house. You may need to think about a home dehumidifier that can be installed under the house. That can be a little costly but it will save the wood under the house in the long run. I have had that same issue under my home as well.
bigdog... seal it and condition it.... fixxed !!! heres stuff from another thread
COLUMBIA, S.C. — Many building codes in the southeastern United States call for increased attic and crawl space ventilation to fight moisture and related IAQ problems. Craig DeWitt, Ph.D., P.E., challenged that notion at the South Carolina Association of Heating and Air Conditioning Contractors’ (SCAHACC’s) 2003 Indoor Comfort Science Conference.DeWitt, principal of RLC Engineering, LLC, Clemson, S.C., pointed out that in more humid climates, venting the attic and crawl space could spell disaster. “Code says to vent the heck out of them,” De Witt said. “If that doesn’t work, vent some more. However, once we get condensation in ductwork, it is self-propagating.”
In a paper he authored, DeWitt re-counted, “At an Affordable Comfort meeting that I recently attended, a speaker from Canada said that venting crawl spaces in the southeastern U.S. was lunacy. I have to agree.”
His argument is based on psychrometrics.
“Mold is a symptom of a moisture issue,” DeWitt explained. “It takes moisture to have fungus.” Symptoms of moisture include fungi (mold, decay), condensation, and wood changes.
“Mold is everywhere,” he pointed out. “We can’t make spaces sterile environments. The spores are already there.” By addressing condensation moisture, however, HVAC contractors address “the one [element] we can control.” There’s no need to become overly concerned, say, if the only evidence of moisture is a little condensation on the windows. “I don’t see any reason to call in a Haz Mat team.”
In order to understand how ventilation can lead to moisture problems, DeWitt said contractors should understand a few rules:
Water changes phase at room temperature, from liquid to vapor.“Psychrometrics are the thermodynamics of just plain old air,” he explained. Relative humidity (rh) equals the amount of water over the capacity of water the air can hold. Mathematically,
rh = amount of H2O/H2O capacity
The dewpoint is the temperature at which the amount of H2O = H2O capacity (capacity of the air to hold water). Below this temperature, condensation forms.
According to DeWitt, “From a psychrometric standpoint, venting a crawl space to remove moisture works when the outside air is drier than the crawl space air. ‘Drier’ does not mean a lower relative humidity, but rather a lower absolute humidity. Relative humidity is a ratio of the amount of moisture in the air relative to the amount the air can hold at that temperature. Absolute humidity is the amount of moisture in an amount of air.
“Air at 85 degrees F and 60 percent rh has the same absolute humidity as air at 70 degrees and 100 percent rh. So venting a 70 degrees/100 percent rh crawl space with 85 degrees/60 percent rh air will not remove moisture.”
DeWitt explained that the dewpoint temperature is the temperature at which condensation forms as air is cooled. “At the dewpoint temperature, the air is saturated and any further cooling will result in condensation. In the above example, both the 70 degrees/100 percent rh crawl space air and the 85 degrees/60 percent rh outside air have the same dewpoint temperature: 70 degrees. If we vent a crawl space with air that has a higher dewpoint temperature than the crawl space air, we will actually be adding moisture to the crawl space rather than removing it.”
To have condensation, you need moist air and a cold surface. Keep in mind that “moist,” “cool,” and “cold” are all relative.
So, let’s say the supply air is 55 degrees, there is some condensation on the coils, and the ducts are uninsulated. Attic conditions are 80 percent rh, 80 degrees on a sunny day; the dewpoint temperature is 75 degrees. Any bare duct will get condensation on it.
“If we vent a crawl space with air that has a higher dewpoint temperature than the crawl space air, we will actually be adding moisture to the crawl space rather than removing it,” is the message Craig DeWitt gave to SCAHACC contractors.
In addition to understanding the fundamentals of psychrometrics, DeWitt pointed out that HVAC contractors need to know something about the moisture and air dynamics (MAD) of wood, especially in crawl spaces and attics. Certain wood conditions can indicate moisture problems, even though moisture might not be present when the contractor is inspecting the system.Sweating ducts in the crawl space, for example, can create a cupping problem in wood floors directly above. Bowing can occur during the heating season.
Moreover, fungi “like to eat biological things” such as wood, he said. Mold and mildew need 80 percent rh or higher on a surface, where they grow. Decay fungi, erroneously called dry rot, needs liquid water. “There is no such thing as dry rot,” De Witt said. “It’s the result of a wet decay.” By the time the damage is discovered, the fungi have used up whatever moisture there was.
A sweating duct in contact with wood beams and thermal insulation can lead to these problems. The damage can still be visible even though the duct may not be sweating when the contractor sees it.
To get condensation on a surface, DeWitt reiterated, the surface temperature has to be lower than the dewpoint temperature of the air.
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Let’s say you’re in a crawl space with a 78 degrees ambient temperature, 90 percent rh; you need about a 3 degree temperature drop to go to 100 percent rh. “If we start seeing 90 percent rh any place, we are very close to having a disastrous situation,” DeWitt said. With insulation, the duct surface temperature is 74.2 degrees (60 degree air inside the duct). The dewpoint temperature is 74.5 degrees; that duct will not sweat.Now let’s say the air being moved is 50 degrees and the duct surface temperature is 74 degrees. That duct will sweat. You might think that higher temperatures in the attic would prevent duct sweating; the ambient summer temperature there is 130 degrees during the day, with a dewpoint of 74 degrees. At first glance it looks pretty good, but “there is a flipside,” said DeWitt. “It does get cool at night.” Temperatures in the attic can drop to 75 degrees at night, and the surface duct temperature drops to 72 degrees. With the 74 degree dewpoint temperature, the result is condensation.
At 90 percent to 100 percent rh, if the temperature is dropped about 3 degrees, you add 16 grains of moisture per pound of air, he said. Therefore:
A 1,500-foot crawl space that’s 3 feet tall = 4,500 cubic feet __________________
If Doctors Wrote Our Laws as Lawyers/Politician's do, we would all be "Dead"!
Keeny Mac is absolutely correct. Everything his thread refers to is Enthalpy. If outside Enthalpy is lower than under the house we would not have many problems. North Carolina is no exception to the humidity problem.
I foregot about that word..... heheheh
Originally Posted by Gopirates
Thanks, I forgot about that $.50 word... and forgot what it means as well..
I am going to look it up and use it... if you don't mind... heheheheh