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  1. #1

    Should flexduct be suspended?

    I'm about to do my own blown in cellulose insulation. I'm wondering if my flex duct should be suspended from the roof before I do this. It seems like burying the duct in insulation would make it more efficient, but I've read some information otherwise, can someone clear this up for me? I was planning on having my AC replaced this fall anyway, I just dont' want to get side-swiped by a contractor who wants to make an extra buck when the work isn't necessary.

  2. #2
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    If the flexduct is well insulated there should be no problem. I think condensation is the main concern - wet insulation is no insulation!

  3. #3
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    Office and warehouse in both Crystal River & New Port Richey ,FL
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    In a humid climate,flex buried in insulation can condense moisture or the outside jacket. Code in Florida requires it to be suspended.

  4. #4
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    Aug 2003
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    Fort Worth, TX
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    11,288
    If the flex is buried, and the cable guy or some other person someday must go in the attic to do something, what are the chances he will see the flex duct before either crushing it or tripping over it?

    Not only should flex be suspended, it should sag minimally between suspension points, and should not be crushed where the straps hold it in suspension.
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  5. #5
    Join Date
    Nov 2007
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    NC
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    Code in NC requires flex duct be supported with strap every 5 to 6 feet.

  6. #6
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    Jan 2004
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    IMO. get the AC and any other work done before you put in the insulation. That should be the last thing done in a attic...

  7. #7
    Join Date
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    Quote Originally Posted by dash View Post
    In a humid climate,flex buried in insulation can condense moisture or the outside jacket. Code in Florida requires it to be suspended.
    In Calif. they give you credit for buried flex ( title 24). Extra credit for deeply buried flex.
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  8. #8
    Join Date
    May 2007
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    Dry as a bone Tucson
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    buried flex

    Quote Originally Posted by bigburb View Post
    I'm about to do my own blown in cellulose insulation. I'm wondering if my flex duct should be suspended from the roof before I do this. It seems like burying the duct in insulation would make it more efficient, but I've read some information otherwise, can someone clear this up for me? I was planning on having my AC replaced this fall anyway, I just dont' want to get side-swiped by a contractor who wants to make an extra buck when the work isn't necessary.
    buried flex has a considerable less load on it than suspended flex, use spiral pipe instead and still bury it.
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  9. #9
    Join Date
    Nov 2008
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    Chapel Hill, NC
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    Quote Originally Posted by jimj View Post
    In Calif. they give you credit for buried flex ( title 24). Extra credit for deeply buried flex.
    It's not humid in California so condensation is not a problem and they can conserve energy by burying the flex in isulation (blown or otherwise).

    Having said that, I've never seen condensation on the outside of my R6 flexduct (in the crawlspace or attic). However, I had a whole sh_tload of condensation on the internally insulated metal trunk lines in both locations!

  10. #10
    Join Date
    Nov 2008
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    Chapel Hill, NC
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    Quote Originally Posted by jimj View Post
    In Calif. they give you credit for buried flex ( title 24). Extra credit for deeply buried flex.
    It's not humid in California so condensation is not a problem and they can conserve energy by burying the flex in isulation (blown or otherwise).

    Having said that, I've never seen condensation on the outside of my R6 flexduct (in the crawlspace or attic). However, the internally insulated metal trunk lines in both locations was covered in condensation every summer.

  11. #11
    Join Date
    May 2004
    Location
    south louisiana
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    3,137
    I learned here about burying ducts in insulation in Calif. prolly in other areas with little to no humidity.
    Knowing your location would allow specific to your climate OP.

    if in fact your ductwork does need to be suspended have hvac company use a 3" strapping as smaller sizes can restrict air flow.
    I think it is no more than a 1" sag between 4" strapping.

    and blowing in insulation should be done after all holes, gaps and
    penetrations from attic into conditioned spaces are sealed, after
    duct strapping and sealing..
    cellulose insulation has very fine particles that find their way into
    your conditioned space thru openings in recessed lights, around bath fan cuts in the sheetrock and at gaps between ac supply boxes and many many other tiny holes.

    best of luck
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