flex duct hung from attic ceilings vs. laid on attic floors?
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  1. #1
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    Aug 2012
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    flex duct hung from attic ceilings vs. laid on attic floors?

    My home has the attic flex duct laying on the floor of the attic in North Carolina,

    My sisters home in Orlando, FL has it suspended from the ceiling.
    She's thinking of burying the flex duct under the blown fiber glass insulation as this article describes.
    http://www.etccreations.com/hvac/atticducts

    Which got me wondering why the flex duct is suspended off attic floor to begin with?

  2. #2
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    Quote Originally Posted by newstudent View Post
    My home has the attic flex duct laying on the floor of the attic in North Carolina,

    My sisters home in Orlando, FL has it suspended from the ceiling.
    She's thinking of burying the flex duct under the blown fiber glass insulation as this article describes.
    http://www.etccreations.com/hvac/atticducts

    Which got me wondering why the flex duct is suspended off attic floor to begin with?
    Has to be hung up in Houston per code. If it's laying on the floor, the bottom side of the flex can begin to condensate because of the dew points, also it most likely to be pulled loose by the cable guy, you know the bull in the china closet guys?
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  3. #3
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    May 2004
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    duct is suspended to allow better air flow. ducts that bend, twist & are crimped
    reduce delivered air flow. code is no more than 2" sag in 4' of flex.
    not many companies do this..but some do. it isn't rocket science, just time
    and effort to do it right.

    in humid climates burying ducts in insulation on attic floor creates condensation
    around the duct where it touches the insulation..or another duct.
    maybe it works in Penn. where article is from, but not in humid climates.

    a better investment is mastic seal of ducts and either strapping them
    up (no more than 2" sag in 4') or creating an unvented attic with foam
    insulation at the roofline.
    with either, mastic sealing of ducts is needed.
    existing homes have lots of duct loss, between unsealed or badly sealed
    ducts, returns, plenum connections and supply boxes 30% duct loss isn't
    uncommon.

    look for information based on the climate you live in. florida solar energy center
    would be a good unbiased site for her to visit.

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

  4. #4
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    He has a little problem with his math on that site also. He doesn't allow for the thickness of the flex insulation, and uses 5.5" of blown insulation in an area that would only be 1.5 to 2"

    Then he suggest blowing an extra 3" over top of the duct. but doesn't mention that the extra insulation will crush the flex ducts insulation lowering its R value. plus it can crush or oblong the flex reducing air flow.
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  5. #5
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    Quote Originally Posted by newstudent View Post
    She's thinking of burying the flex duct under the blown fiber glass insulation as this article describes.
    http://www.etccreations.com/hvac/atticducts
    Why?

    Does she have a problem she's hoping to cure? Or "hey, this seems like a great idea and I've got money to throw at stuff."?
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  6. #6
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    Why?
    Does she have a problem she's hoping to cure? Or "hey, this seems like a great idea and I've got money to throw at stuff."?


    Yes her problem is that her electrical bill is high due mostly to AC use and according to the article it helps reduce energy. She's going to get Radiant Barrier installed.

    Would buying roll fiberglass insulation withOUT a vapor barrier and wraping it around the hanging flex duct be better? condensation will obviously still have the ability to form between the humid air and foil flexduct skin, but unlikely to do damage since it is off the ceiling floor.

    3" of blown fiberglass insulation is very light, I would of never guessed it would crush the flex duct insulation significantly.

    I've read FSEC web site and they basically suggest adding a vapor barrier. They make no mentioin of increasing the flex duct R value by increasing the insulation around it.

    I realize that Central Florida is more humid than the Carolinas, but we get really humid here too and I have never noticed condensation in/around the attic flex tubes.

  7. #7
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    Make your expertise uniquely valuable.

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  8. #8
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    Also pay special attention to the comments below the article by David Butler and John Tooley.
    Make your expertise uniquely valuable.

    Make your influence uniquely far-reaching.

  9. #9
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    Quote Originally Posted by newstudent View Post
    Yes her problem is that her electrical bill is high due mostly to AC use and according to the article it helps reduce energy. She's going to get Radiant Barrier installed.

    Would buying roll fiberglass insulation withOUT a vapor barrier and wraping it around the hanging flex duct be better? condensation will obviously still have the ability to form between the humid air and foil flexduct skin, but unlikely to do damage since it is off the ceiling floor.

    3" of blown fiberglass insulation is very light, I would of never guessed it would crush the flex duct insulation significantly.

    I've read FSEC web site and they basically suggest adding a vapor barrier. They make no mentioin of increasing the flex duct R value by increasing the insulation around it.

    I realize that Central Florida is more humid than the Carolinas, but we get really humid here too and I have never noticed condensation in/around the attic flex tubes.
    This approach is called "the poke n hope". Typically "let's try this" provides little to no satisfaction, and an empty wallet, are the result. Sales pitches selling products that promise solutions, without accountability for results are a sucker trap. Don't fall for them. Solutions come for comprehensive understanding and customized design, not from magic products.

    She needs to take a comprehensive look at where her homes energy is going. Develop an informed and thoughtful plan of attack. Throwing money at crappy ideas expecting to get anything other than crappy results is delusionally optimistic.

    See the bottom line of my signature. You need to understand the illness before prescribing cures. Not the what, but all the Y's.
    Which makes more sense to you?
    CONSERVATION - turning your thermostat back and being uncomfortable. Maybe saving 5-10%
    ENERGY EFFICIENCY - leaving your thermostat where everyone is comfortable. Saving 30-70%

    DO THE NUMBERS! Step on a HOMESCALE.
    What is comfort? Well, it AIN'T just TEMPERATURE!

    Energy Obese? An audit is the next step - go to BPI.org, or RESNET, and find an auditor near you.

  10. #10
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    Quote Originally Posted by tedkidd View Post
    This approach is called "the poke n hope". Typically "let's try this" provides little to no satisfaction, and an empty wallet, are the result. Sales pitches selling products that promise solutions, without accountability for results are a sucker trap. Don't fall for them. Solutions come for comprehensive understanding and customized design, not from magic products.

    She needs to take a comprehensive look at where her homes energy is going. Develop an informed and thoughtful plan of attack. Throwing money at crappy ideas expecting to get anything other than crappy results is delusionally optimistic.

    See the bottom line of my signature. You need to understand the illness before prescribing cures. Not the what, but all the Y's.
    X2

  11. #11
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    Aug 2012
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    " Sales pitches selling products that promise solutions, without accountability for results are a sucker trap. Don't fall for them."

    Where did this come from?!? Sorry there aren't any insulation salesmen knocking down our door. Insulation is a proven product, not snake oil.

    Obviously increasing the insulation around ducts will cause less energy loss. Ducts used to be OK at R6, now R8 is the code in many places, for obvious reasons.

    All I"m asking is the best way to do it.
    #1. With vapor barrier ( on or off attic floor)
    #2 Without vapor barrier. ( on or off attic floor)

    I've personally tried #1 in the carolinas and never noticed any moisture issues. When I stick my hand underneath the added blanket of insulation the ducts feel very cool. They never felt that way before. Obviously it is lowering energy transfer and causing the AC to run less. How long will the return on investment be? I have no idea, but it was cheap to get installed.

  12. #12
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    Quote Originally Posted by newstudent View Post
    [I][B]
    All I"m asking is the best way to do it.
    #1. With vapor barrier ( on or off attic floor)
    #2 Without vapor barrier. ( on or off attic floor)

    #3 No insulation.

    "in humid climates burying ducts in insulation on attic floor creates condensation
    around the duct where it touches the insulation..or another duct.
    maybe it works in Penn. where article is from, but not in humid climates."

    "Then he suggest blowing an extra 3" over top of the duct. but doesn't mention that the extra insulation will crush the flex ducts insulation lowering its R value. plus it can crush or oblong the flex reducing air flow. "

    "She needs to take a comprehensive look at where her homes energy is going. Develop an informed and thoughtful plan of attack. Throwing money at crappy ideas expecting to get anything other than crappy results is delusionally optimistic"

    and I x4.

    On or off, whatever. Refer to local codes. To many variables depending on location specifics, especially the size/shape/build of the attic.
    "Better tell the sandman to stay away, because we're gonna be workin on this one all night."

    "Dude, you need more than 2 wires to a condenser to run a 2 stage heatpump."

    "Just get it done son."

    Dad adjusted

  13. #13
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    May 2004
    Location
    south louisiana
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    ok ok...lets slow this roll.

    personally I've wondered about wrapping unfaced batt insulation around existing ductwork.
    even the lowliest R-value (R-13) is twice what code here in La. mandates. (we are still
    at R-6..not R-8 whiny builders association members connected to good old boy politicians.)

    just never had a house I could do that kind of experiment on. I've seen old hard duct with
    R-2 kraft paper ductwrap..circa 1930's installed..get this..tied with string around the ductwork.
    why wouldn't it work with existing vapor barrier & R-whatever unfaced batts?
    I can't think of why it wouldn't. would I do it? not until I had a chance to verify it
    for myself. without the vapor barrier on added insultion..would condensation occur
    at 1st vapor barrier?? I don't know..don't think so..but can't test my theory on
    my clients. in my house..ducts are inside living space, so can't diy here..but I do
    wonder..

    so for your sister's house..do read the energy vanguard article & comments. they
    cover all the points about condensation & ducts.

    my climate is hot humid..maybe not as humid as your sisters..but it sure seems like
    @ 95%RH it should be raining..but it isn't always the case.
    burying the ducts just doesn't work for us. I've seen houses where you could
    trace the path of the ductwork inside the house by the water stains on
    the ceilings.

    the best option..although not a quick solution, is to seal house leakage.
    then seal duct leakage, strap ducts, rb or foam or conventional insulation.

    blower door testing measures amount of air leakage into the house,
    and used properly can show you exactly where leakage occurs.
    duct testing, like blower door testing measures total amount
    of duct leakage...and also can show you where it leaks.

    once leakage sites are sealed, using proper materials that last
    & don't shrink or dry out..then suspending ducts to get them off attic
    floor & insulation should be done.

    the reason people don't think that the ducts condensate when they rest
    on the insulation is because they check them at the wrong times of the
    day. even when ducts are attached too close together on the plenum
    they condensate. if the insulation jackets touch..they condensate.
    add in leakage at duct take off..and plenums can be very wet depending
    upon time of day. some stay wet all the time..every house varies to some degree.

    adding a radiant barrier helps to reflect heat out of the attic, but ducts are
    still exposed to attic temps.
    completely sealing attic with foam insulation creates an unvented semi conditioned
    attic. temps outside and inside attic will be close. not 5 degrees warmer than the house..
    semi conditioned. this helps out in that ducts are in cooler attic.
    as air barrier & thermal barrier (foam insulation) is now at the roofline
    rather than the attic floor..all the little leaks from inside the house
    into the attic don't suck as much as they do normally.

    the trade off is this..seal leakage at attic floor/ceiling
    recessed cans, oversized cuts at supply boxes, bath fans
    just to mention a few..or foam insulate.
    first is labor intensive with inexpensive materials
    the second is just expensive. done in one or two days
    for average sized house.
    still before foam..bath fans have to be vented out
    of attic, insulation has to be moved from eaves of house
    so that foam can make a complete roof to attic floor seal.
    so foam requires prep work also.
    in either case duct/return leakage should be sealed.

    usually these decisions are budget dependent. so having
    an energy rating helps to weigh the upgrades/costs &
    homeowners budget against costs & performance of upgrades.

    this is a good time of year to do attic work. not as extreme outside
    & in attics. for hvac & insulation companies we are getting into a slower
    than heat of summer/cold of winter time of year.

    research what you can..ask lots of questions & consiter an unbiased
    testing, improvement & verification rating/audit.
    hire an independent, or take utility free audits & know that you
    can save more than 10-15% of your utility costs.
    the things you learn from the testing will suprise you.

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

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