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  1. #1
    Join Date
    Feb 2008
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    78

    Should I insulate my sweating supply plenum?

    My crawl space is averaging 72% humidity right now (Raleigh, NC). I don't see any mold or mildew on the joists, the floor insulation is dry, and there is no condensation anywhere - EXCEPT the supply plenum (covered in condensation, soaked on bottom) and to lesser extent the return plenum. This isa recently installed system - performing well, indoor humidity steady at 45%). My tech says it's normal in a vented crawl. I am aware of dew points, etc , so I'm tend to agree with him. I plan to eventually close the vents, get a dehumidifier, upgrade the vapor barrier (doesn't current extend to the walls), etc., but until then:

    - Will insulating the outside of the plenums help? (Tech says they are insulated inside already)
    - Will it do any harm? (trap moisture between plenum and insulation, etc)?
    - What type of insulation should I use? Where can get it?

    TIA

  2. #2
    Join Date
    Jan 2004
    Location
    Lancaster PA
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    68,074
    It may be insulated inside already, but may not be a high insulation valvue.
    If your plenum is sweating, but not the supplies tapped off of it. It indicates either low insulation value in teh plenum, or air leaks.
    The plenum has to be dried before wrapping it.

    You should check to see if its leaking air before you just wrap it.
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  3. #3
    Join Date
    Aug 2003
    Location
    Fort Worth, TX
    Posts
    11,338
    Agree with beenthere. Check for air leaks...sealing them helps your system more than just the condensation problem. Plenum should be dry before external insulation is applied. Duct wrap fiberglass insulation...all seams need to be sealed to keep moisture out, otherwise sweating may occur between wrap and sheet metal, causing insulation to be damaged.

  4. #4
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
    Posts
    2,965
    Time and money better spent just sealing the crawl and adding a dehumidifier.

    I installed my own system here in raleigh and have lined duct also wrapped in my crawl and the wrapped insulation was soaked in a few days.

    Close the crawl vents, add a good vapor barrier, and throw a dehum under there and pipe it outside.

    No sweating and insects don't like a dry crawl either!
    How tall are you Private???!!!!

  5. #5
    Join Date
    Feb 2008
    Posts
    78
    Wow - quick replies - thanks!

    It was leaking from the pressure regulating duct (zoned system) where it connects to the supply plenum, but that was fixed. There MAY be a slight leak or two where the zone ducts connect to the plenum, but that may be me just imagining things - it's not obvious, and it may be just the temperature of the plenum I feel and no airflow. Anyway, I'll check again, and slap on some mastic if I think I feel anything.

    The ducts themselves are dry, the little metal box that controls the damper, however, is also sweating, and that is a good foot or two up from the supply box - would that point to humidity and not air leaks?

    Speaking of mastic - isn't that stuff supposed to dry in like 4 hours? the stuff they installed with the new system is still soft - is that due to the high humidity? will it harden eventually?

    Gunnery Sergeant Hartman - does your vapor barrier go up the walls and attach there, or does it stop at or before the walls? My current vapor barrier stops about a foot from the walls, so there is some exposed earth. I'm trying to decide if it's worth re-doing or if just adding a dehum. will be enough to allow me to close the vents.

    Also, any recommendations for a decent, inexpensive dehumidifier for about 1700 square feet, average 2 feet high crawl space? I've seen the Sante Fe models for almost $1000 - wondering if there's anything around $500 or so.

  6. #6
    Join Date
    Jan 2004
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    Lancaster PA
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    The zone dampers themselves are not insulated. So thats why you see it there.

    Now that you tell us its zoned. That is why both your plenums are cold enough to sweat even though they are insulated.

    I've used incense sticks to check for air leaks aready, works better then your hand.

    Good chance your supplies are R6 or better, and your plenums are R4.
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  7. #7
    Join Date
    Feb 2008
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    78
    Quote Originally Posted by beenthere View Post
    Now that you tell us its zoned. That is why both your plenums are cold enough to sweat even though they are insulated.

    I've used incense sticks to check for air leaks aready, works better then your hand.

    Good chance your supplies are R6 or better, and your plenums are R4.
    The interesting thing is that one of the dampers is defective (stuck open), so really the system is one big zone right now, meaning the plenums will be even colder when they fix the damper and the zones are working.

    Good ideal on the incense sticks - I'll try it.

    Is there a way to tell what insulation is inside the plenum?

  8. #8
    Join Date
    Jul 2002
    Location
    Zelienople, Pa
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    Quote Originally Posted by richmod View Post
    Wow - quick replies - thanks!

    It was leaking from the pressure regulating duct (zoned system) where it connects to the supply plenum, but that was fixed. There MAY be a slight leak or two where the zone ducts connect to the plenum, but that may be me just imagining things - it's not obvious, and it may be just the temperature of the plenum I feel and no airflow. Anyway, I'll check again, and slap on some mastic if I think I feel anything.

    The ducts themselves are dry, the little metal box that controls the damper, however, is also sweating, and that is a good foot or two up from the supply box - would that point to humidity and not air leaks?

    Speaking of mastic - isn't that stuff supposed to dry in like 4 hours? the stuff they installed with the new system is still soft - is that due to the high humidity? will it harden eventually?

    Gunnery Sergeant Hartman - does your vapor barrier go up the walls and attach there, or does it stop at or before the walls? My current vapor barrier stops about a foot from the walls, so there is some exposed earth. I'm trying to decide if it's worth re-doing or if just adding a dehum. will be enough to allow me to close the vents.

    Also, any recommendations for a decent, inexpensive dehumidifier for about 1700 square feet, average 2 feet high crawl space? I've seen the Sante Fe models for almost $1000 - wondering if there's anything around $500 or so.

    I ran my barrier to the walls and not up it.
    I didn't have the time or energy to do it like the pros do.
    I cut blocks out of 2" DOW board and shoved them in my crawl vents.
    A cheap dehum from Lowes or HD will work just fine. Make sure to pipe it outside.
    How tall are you Private???!!!!

  9. #9
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    Jan 2004
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    Lancaster PA
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    Quote Originally Posted by richmod View Post

    Is there a way to tell what insulation is inside the plenum?
    Not without opening it up, and looking.

    If its a 2 zone system. And one zone is open 24/7 because of a defective motor, then the bypass shouldn't be open. Unless your short of return or supply, or both.

    Or, the bypass wasn't set up right.
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  10. #10
    Join Date
    Feb 2008
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    I believe damper is stuck partially open, since it doesn't blow quite as hard at the registers in that zone. Bypass only opens very slightly right now - barely moves. Tech said that's normal.

  11. #11
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    He also thinks its normal to just let the plenum sweat.
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  12. #12
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    Feb 2008
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    So are you saying that the bypass damper shouldn't move at all with both zones open? It moves literally about 5 degrees max (fully open being 90 degrees) - just a slight movement. Should I expect absolutely no movement? What does it indicate is the problem if it does move?

  13. #13
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    With the one zone only being 1/2 open and the other full opened. movement is more then ok.
    When both zones are full open. The bypass opening is indication of duct problems.
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