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  1. #14
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    Nov 2010
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    Quote Originally Posted by shophound View Post
    What then would be the dynamics necessary to make a difference between a duct with two vapor barriers vs. one? My guess at this point is that with only an outer vapor barrier, there is enough duct leakage to keep enough air moving through the fiberglass to prevent condensation.
    Interesting thoughts. However with that line of reasoning, that would mean that a properly sealed duct would have a condensation problem with FSK faced insulation since the vapor is trapped.

    Still see no reason why OP's proposed idea would result in any extra condensation/mold. Field observations with a layer of insulation going over a dirty old insulation (mold food) probably don't apply to a scenario with a clean bubble wrap followed by a clean FSK faced layer.

  2. #15
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    Aug 2003
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    Fort Worth, TX
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    Air is dirty. So if there's any leakage it will carry with it the food mold needs to grow.

    The OP can do as he or she pleases. I just think there are more effective ways to reduce heat transfer through attic ducts than create a potential for condensation. You can't guarantee that installation will stay pristine forever, just as my dripping duct in the subbasement here at work is no longer pristine. Attics get stupid hot in summer and that puts a strain on any sealing job on the exterior of any surface exposed to the attic. It also vastly increases the moisture and temperature delta into the insulation if a route is available for that to happen.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  3. #16
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    Jun 2004
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    4H: Hot, Humid Houston H.O.
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    Would you think it a factor, that the radiant barrier properties of that foil bubble wrap will be nullified by the absence of an air gap? The principle I am thinking of, is that every radiant barrier needs an air gap of an inch or so, to work.

    You are left with the insulating qualities of 1/8 inch of air bubble, which is not much.

    Hope this helps -- Pstu

  4. #17
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    Aug 2003
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    Fort Worth, TX
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    Quote Originally Posted by pstu View Post
    Would you think it a factor, that the radiant barrier properties of that foil bubble wrap will be nullified by the absence of an air gap? The principle I am thinking of, is that every radiant barrier needs an air gap of an inch or so, to work.

    You are left with the insulating qualities of 1/8 inch of air bubble, which is not much.

    Hope this helps -- Pstu
    You are correct, Pstu. Radiant barrier requires an air gap. While the bubble wrap may provide an air gap on the duct side (provided it's not crushed by the weight and pressure of added layers of insulation and outer duct wrap), there's no air gap against the insulation itself.
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  5. #18
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    May 2004
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    south louisiana
    Posts
    3,231
    the op stated that they were not going to the trouble of maintaining the air gap
    but using the foil bubble foil (or fsk..what does that stand for?) directly next to the
    duct and wrap over. not those exact words..but close.
    there is a way with the proper materials to achieve an air gap..not a 3/4" air gap
    but somewhat of an air gap. just really difficult to install spacers that provide the
    air gap and fit foil bubble foil on top.
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  6. #19
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    Jul 2004
    Location
    Massachusetts
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    Don't matter what's up against the duct. What matters is what the temperature of the outer layer is when the hot, humid, attic air hits it. If the surface of the insulation is at or below the dew point of the ATTIC AIR, then you'll get condensation. If the surface temperature of the insulation is higher than the dew point, you'll not have condensation. It's not the temperature of the duct nor moisture leaking from the duct. In fact, it's the amount of total insulation between the cold duct and the hot, humid attic. It's not cooled duct air that's going to condense, it's the hot attic air that will condense. So I say again, you need to have sufficient insulation to allow the conductive temperature through the insulating layer to rise above the dew point of the attic air to prevent condensation.

  7. #20
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    Nov 2010
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    156
    Quote Originally Posted by skippedover View Post
    Don't matter what's up against the duct. What matters is what the temperature of the outer layer is when the hot, humid, attic air hits it. If the surface of the insulation is at or below the dew point of the ATTIC AIR, then you'll get condensation. If the surface temperature of the insulation is higher than the dew point, you'll not have condensation. It's not the temperature of the duct nor moisture leaking from the duct. In fact, it's the amount of total insulation between the cold duct and the hot, humid attic. It's not cooled duct air that's going to condense, it's the hot attic air that will condense. So I say again, you need to have sufficient insulation to allow the conductive temperature through the insulating layer to rise above the dew point of the attic air to prevent condensation.
    Good explanation. Applying this to the OP's concern, most likely the hot attic air will remain above the dew point at the surface of the inner layer of foil bubble wrap.. therefore no worries.

  8. #21
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    Jun 2010
    Location
    Philadelphia PA
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    2,190

    FSK= foil+scrim+kraft

    The duct wrap vapor barrier,
    Anyone believe like I do that a reasonable amount of leakage, with that cool DRY air is reducing the opportunity of condensation for the duct?
    Of course larger amounts of leakage will give you cold surfaces where the sweat will appear.
    Isn't it interesting that moisture needs a surface to form on. Ice forming on aircraft at 30,000 Ft is an example or rain wouldn't happen without dust in the atmosphere, hence "seeding clouds" to make it rain
    You have got to learn from other people's mistakes! Because God knows you don't live long enough to make them all yourself !!!!!!!!

  9. #22
    Join Date
    May 2004
    Location
    south louisiana
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    air entering the coil
    has to reach 100% RH
    to dehumidify.
    air leaving the coil is 90% humidity
    and colder.
    where is the cool dry air?
    The cure of the part should not be attempted without the cure of the whole. ~Plato

  10. #23
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    Aug 2003
    Location
    Fort Worth, TX
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    11,358
    Quote Originally Posted by energy_rater_La View Post
    air entering the coil
    has to reach 100% RH
    to dehumidify.
    air leaving the coil is 90% humidity
    and colder.
    where is the cool dry air?

    Warm that same 90% air leaving the coil back up to room temperature and its RH will drop to + or - 50%. Warm it up to attic temp and it will go even lower. It's all about dew point. If the return air is running a dew point of around 55 and the supply air's dew point is 50 or less, the absolute moisture content of the air passing over the cooling coil was reduced. As genduct noted, moisture needs a surface at or below dew point for condensation to occur. This is called nucleation:

    nu·cle·a·tion definition

    Pronunciation: /ˌn(y){uuml}-klē-ˈā-shən/
    Function: n
    1 : the formation of nuclei
    2 : the action of a nucleus in starting a process (as condensation, crystallization, or precipitation)


    Merriam-Webster's Medical Dictionary
    • Electricity makes refrigeration happen.
    • Refrigeration makes the HVAC psychrometric process happen.
    • HVAC pyschrometrics is what makes indoor human comfort happen...IF the ducts AND the building envelope cooperate.


    A building is NOT beautiful unless it is also comfortable.

  11. #24
    Join Date
    Jul 2010
    Location
    North Carolina
    Posts
    83
    Tightly wrap the bubble wrap and Tape the seems well.

  12. #25
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    Jun 2004
    Location
    4H: Hot, Humid Houston H.O.
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    3,304
    Quote Originally Posted by JakRabbit View Post
    Tightly wrap the bubble wrap and Tape the seems well.
    If you do that well, it will be as if the bubble wrap were not even there. Nothing special about this material, it's not a radiant barrier in this case and not really even insulation. Is this not true?

    Regards -- Pstu

  13. #26
    Join Date
    Jul 2008
    Location
    Rochester NY
    Posts
    4,739
    2 moisture barriers = bad idea. (didn't read all the posts, so if this was mentioned here's my support)

    Unless you can be really certain humid air can not come in contact with surfaces inside your vapor barrier (spray foam is the only way i would be confident), I would be very uncomfortable with having my vapor barrier adjacent to my cold metal. If it gets wet it will never get dry. Also, if the outside gets cold and you havent air sealed your insulation layer, now you have wet fiberglass that can't dry due to your SECOND barrier.

    Plastic has a higher perm rating than I think id be comfortable using in any application other than over a window or on a basement floor.
    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.

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