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Thread: Sweating ducts

  1. #1
    I live in Tn and am having a problem with sweating ducts under the house. An excessive amount of water is on them. I have layed plastic under the house. I have the temp. set at 72%. Is that the problem? Do I need a dehumidifier or is there some other problem. Everyone I ask has a different opinion.

    Thanks

  2. #2
    Join Date
    Jun 2003
    Location
    Madison, WI/Cape Coral, FL
    Posts
    6,270
    Originally posted by panama red
    I live in Tn and am having a problem with sweating ducts under the house. An excessive amount of water is on them. I have layed plastic under the house. I have the temp. set at 72%. Is that the problem? Do I need a dehumidifier or is there some other problem. Everyone I ask has a different opinion.

    Thanks
    Here is another opinion. Plastic on the earth and closed the outside vents to keep the outside damp air out of crawlspace. Enough dehumidification to maintain <55%RH. The ducts will stop sweating even without insulation. Wal-Mart has a great %RH meter with remote crawlspace senser for 29 dollars. Check out new high eff. dehu Santa Fe Advance. dehu TB

  3. #3
    Join Date
    Sep 2004
    Location
    Fort Worth, Texas
    Posts
    586
    I am assuming you live in a pier and beam. What kind of foundation ventilation do you have?
    Installs, Changeouts, & Heat Stroke.....not necessarily in that order

  4. #4

    sweating ducts

    I have several closable vents installed but that is it.

  5. #5
    Join Date
    May 2004
    Location
    Fayetteville TN
    Posts
    656
    most common cause of sweating ducts (or insulation on the ducts) is low air temperature in the duct due to low air volume. (dirty coil, closed grills, etc) Another possibility is that the insulation is not air tight (assuming duct wrap)
    If You don't have time to do it right, when will you have the time to do it over?

    "Perception is Reality" Look & Act like a Professional

  6. #6
    Join Date
    Jan 2003
    Posts
    168
    We have been receiving a number of calls from dealers and homeowners about the formation of condensation on ductwork in crawlspaces recently. A review of the current weather conditions and a psychometric chart can be used to explain what is occurring.

    For example, the outdoor air temperature is 97 F with a dewpoint of 78 F. This means that humidity in outdoor air used to ventilate a crawlspace will condense to any surface with a temperature equal to 78 F or less. High outdoor air dewpoint temperatures have been common for the past month. The night time air temperature has been dropping to the mid-seventies during this period, and the vented crawlspace temperature will be somewhere between the conditioned space temperature and the outdoor air temperature. The crawlspace could easily drop into the low-seventies at night if supply duct air leaks exist. Structural surfaces will become cool enough to condense humidity from air that has a dewpoint in the high-seventies. Outside air should only be used to ventilate a crawlspace when the dewpoint of the outdoor air is lower than the indoor air dry bulb temperature.

    The homeowners that I have talked to are leaving their crawlspace vents open to "let the area dry out." When we have ambient conditions as we now have, the opposite occurs - they are letting the water (humidity) into the crawlspace. The humidity in our current outdoor air will condense to crawlspace surfaces just like it does to a glass of cold water. Crawlspace vents should be closed during periods such as we are currently experiencing. Generally speaking, our summer dewpoints are too high to keep the crawlspace vents open, and they should be kept closed all summer.

    If supply air duct leaks exist in a crawlspace, the duct wrap and its vapor barrier will drop even lower in temperature. This will increase the amount of moisture that will condense to the ductwork. Duct joints must be sealed vapor tight, and the duct wrap must be properly installed with a sealed vapor barrier to prevent condensation from appearing.

    Closing the crawlspace vents, and properly sealing the ductwork and the insulation vapor barrier is necessary.


  7. #7
    Join Date
    May 2006
    Location
    east central indiana
    Posts
    1,117
    The judge don't know when Red's in town.

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