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  1. #1
    Join Date
    Sep 2013
    Location
    Lafayette, LA
    Posts
    30

    Insulating supply plenum made of steel

    I have read two methods of insulating metal supply plenum -- inside the plenum or outside the plenum.

    I'd like to hear the pros and cons for the two. It appears to me that putting insulation inside could lead to mold growth and blow the insulation particles in the house. The outside insulation, if not done well could have trapped air, and may lead to sweating.

    Sincerely,

    LafayetteLA

  2. #2
    Join Date
    Jul 2006
    Location
    Virginia
    Posts
    4,905
    have it wrapped on the outside you get much better airflow
    We really need change now

  3. #3
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
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    7,780
    Seal all air leakage points with mastic then wrap the outside. In not a fan of having insulation on the inside of air stream. It can come loose and cause airflow problems and distributes fiberglass particles through the house.

  4. #4
    Join Date
    Sep 2013
    Location
    Lafayette, LA
    Posts
    30
    Quote Originally Posted by jtrammel View Post
    Seal all air leakage points with mastic then wrap the outside. In not a fan of having insulation on the inside of air stream. It can come loose and cause airflow problems and distributes fiberglass particles through the house.
    jtrammel -- thanks for your reply. Outside insulation sure looks like the right way. What do you wrap the outside with?

    Also, I live in South Louisiana, and its pretty humid here. I am sure the exterior will sweat. But I don't see how the condensation will be drained. It seems to drain directly on top of the air handling unit.

    LafayetteLA

  5. #5
    Join Date
    Oct 2005
    Location
    Pacific Northwest
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    2,288
    Internal duct liner doesn't frey like ductboard. It has it's place. It makes for a much cleaner installation in the fading art of sheet metal fab.

  6. #6
    Join Date
    Dec 2008
    Location
    Buffalo NY
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    3,145
    They have foam insulation now for internal lining so it does not break down like fiberglass.


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  7. #7
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
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    7,780
    Quote Originally Posted by LafayetteLA View Post
    jtrammel -- thanks for your reply. Outside insulation sure looks like the right way. What do you wrap the outside with?

    Also, I live in South Louisiana, and its pretty humid here. I am sure the exterior will sweat. But I don't see how the condensation will be drained. It seems to drain directly on top of the air handling unit.

    LafayetteLA
    If the duct is wrapped correctly it won't sweat. Use r8 foil backed duct wrap

  8. #8
    Join Date
    May 2004
    Location
    south louisiana
    Posts
    3,332
    I have read two methods of insulating metal supply plenum -- inside the plenum or outside the plenum.

    I'd like to hear the pros and cons for the two. It appears to me that putting insulation inside could lead to mold growth and blow the insulation particles in the house. The outside insulation, if not done well could have trapped air, and may lead to sweating.

    Sincerely,

    LafayetteLA
    __________________________________________________ ___________________________________
    Hey neighbor..well nearby neighbor.

    sheetmetal plenums are made ready to install or special order to fit specs.
    usually the plenum comes with fiberglass insulation with a black covering that is
    glued to the inside of the plenum.

    ductboard can also be installed, usually on the job to replace or repair insulation
    failure. ductboard is a different type of fiberglass with a black anti microbial coating
    unlike older (20 years ago & longer) ductboard without this coating.

    if the cuts for the ductwork leaves the insulation on the inside raggedy instead of
    smooth with flanges of duct collars bent over the whole of the insulation instead of
    part of the way through the insulation, then you have more of a chance of particles
    entering the ducts & exiting into the house.
    the worst of it blows out farily soon...but if it has duct leakage around the duct take offs
    (duct collars) then condensation can get inside & cause more issues.

    externally insulating the plenum keeps all insulation out of the plenum & air being supplied
    from the plenum. I've done this in a few cases of chemically sensitive & asthma sufferers
    with plenum issues.

    in either case...
    mastic sealing all the seams, corners & duct takeoffs on the supply plenum come first.
    using a paint on mastic & paintbrush apply mastic 'nickel thick' to areas listed above.
    do this when plenum is dry...not in the afternoon when it has started to condensate.
    allow mastic to dry.

    I usually start early in morning & put central unit to run with fan only..or off totally.
    keeps condensation down so that you can work & let mastic dry.

    I sometimes use mastic tape at the duct collars to cut down on time. instead of waiting
    for mastic to dry, I'll use Hardcast brand #1402 mastic tape to seal ducts to plenum.
    depends on time etc. more material costs but less time...

    ductwrap ...I'd call Solar Supply, best prices (although prices are lower at Opelousas Solar), and not coburn's.
    material costs will be much less for all materials @ Solar. and no I don't work there...just buy my materials
    there...after being turned off by treatment by coburn's.

    R-8 ductwrap...not R-6 that codes still allow here in La. if not in stock..allow them to order & wait for it.
    it is worth the time for the xtra R-value. they may have it in stock now though.

    ductwrap comes in 4' width rolls...I forget how many feet per roll.
    100 ft for R-4 (old ductwrap not sold anymore) 75 ft for R-6 & 50' for R-8..
    is that right pros??

    supply plenums are difficult to wrap because of ducts take offs. you have to cut out
    the duct wrap for each duct collar/take off prior to installing. the duct layout on plenum would
    determine where to start insulating..& degree of difficulty.

    cut ductwrap so that the ends butt up insulation to insulation, there is a flap
    that covers these butt joints. you'll understand this better once you see the ductwrap.

    once installed, tightly enough to keep insulation in contact with plenum on all sides,
    duct stapler is used to attach the foil skrim kraft paper (fsk) to the fsk of the wrap
    at the overlap. fsk tape is used to cover overlapped seam. I like to add a double row
    of staples on both duct wrap & fsk tape.

    no insulation shows once plenum is wrapped. the foil facing on the ductwrap should
    be continous, and sealed to fsk or vapor barrier of ducts (depending on type of duct).


    are the ducts sheet metal or flex duct?
    is the plenum an upright plenum with ducts off sides?
    or long supply plenum?


    post more info about your supply plenum ducts & layout.

    do you plan on having ductsystem tested prior to duct sealing &
    after work is done to verify the work?

    best of luck.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  9. #9
    Join Date
    Oct 2005
    Location
    Pacific Northwest
    Posts
    2,288
    Rooftop ductwork doesn't leave much choice.

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