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  1. #1
    Join Date
    Jan 2019
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    Sweating and extra insulation...

    Hey guys, so i've been looking in to sweating ducts in humid climates and see a lot of ducts that are r8 insulated and still dripping. (in research but not on my particular job)I understand dew point and all that good stuff but still lack big picture vision.

    Essentially, the question is; Would placing say an r30 unfaced batt over already insulated duct in the attic take care of sweating, or create more issues? I understand that, yes, it will over insulate and decrease the temp beneath the fiberglass(but above the duct envelope), but standard fiberglass allows humidity to pass through , right? So is this something you would do?

  2. #2
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
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    Quote Originally Posted by AL59782 View Post
    Hey guys, so i've been looking in to sweating ducts in humid climates and see a lot of ducts that are r8 insulated and still dripping. (in research but not on my particular job)I understand dew point and all that good stuff but still lack big picture vision.

    Essentially, the question is; Would placing say an r30 unfaced batt over already insulated duct in the attic take care of sweating, or create more issues? I understand that, yes, it will over insulate and decrease the temp beneath the fiberglass(but above the duct envelope), but standard fiberglass allows humidity to pass through , right? So is this something you would do?
    No. In other climates they bury duct in blown fiberglass. In the Southeast if you did that the ducts would be raining.

    The problem with attic ducts sweating is usually from the duct touching something/anything really but the worst is touching another duct. The problems are the dew point is lowered at the point that it’s touching and the insulation is getting compressed causing a lower r value, generally there’s substantial duct leakage lowering the dew point of the vapor barrier surface as well. 98% of the time if the ducts are well hung and above the attic floor insulation with only air surrounding them and all joints are well sealed (with mastic) then you won’t have any sweating.

    Now crawl spaces are far harder to handle, usually the only fix is sealing up the crawl space (encapsulation) and conditioning it (dehumidifier).

  3. #3
    Join Date
    Feb 2004
    Location
    New Mexico
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    Rool duct insulation gives an R rating but only under it's install instructions. How tight it's pulled reduces the R rating.
    There's another possible problem with sweating ducts. Is the air flow ok. A plugged filter, restricted duct, closed supplies, all that stuff that might drop the temp too much.
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  4. #4
    Join Date
    Jan 2019
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    Thread Starter
    I started looking in to this issue when i stumbled up on new IIRc code regarding buried ducts which are now "legal".

    Cant post links if you punch in "energyvanguard buried-ducts-allowed-2018-building-code" it will come right up.

    My issues is with one of the pipes that is 3" from the drywall making it difficult not to be tempted to insulate above. Its enough space to get the required R8. Plus i am going over all the seams from scratch with mastic. So as far as leaks and restrictions , no issues. Roof slope is very small and in that spot raising the duct higher (above R38 req) would put it through the roof lol, literally. Its already too close to the sheathing so in the summer i'd hate to even guess how how it gets even with insulated duct.

    At this point i am considering close cell spray foaming it from the register up which will give it at minimum R10...but that's time and money...and i am over both of those.

  5. #5
    Join Date
    Oct 2010
    Location
    Anderson, South Carolina, United States
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    Quote Originally Posted by AL59782 View Post
    I started looking in to this issue when i stumbled up on new IIRc code regarding buried ducts which are now "legal".

    Cant post links if you punch in "energyvanguard buried-ducts-allowed-2018-building-code" it will come right up.

    My issues is with one of the pipes that is 3" from the drywall making it difficult not to be tempted to insulate above. Its enough space to get the required R8. Plus i am going over all the seams from scratch with mastic. So as far as leaks and restrictions , no issues. Roof slope is very small and in that spot raising the duct higher (above R38 req) would put it through the roof lol, literally. Its already too close to the sheathing so in the summer i'd hate to even guess how how it gets even with insulated duct.

    At this point i am considering close cell spray foaming it from the register up which will give it at minimum R10...but that's time and money...and i am over both of those.
    I would use insulated boots (store bought internally insulated) sealed well to the Sheetrock. Then you shouldn’t have any issue blowing fiberglass on top of it.


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  6. #6
    Join Date
    Apr 2012
    Location
    Millsboro, DE
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    181
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    Crawl spaces must be sealed and dehumidified to control ambient T/H. Homes must be reasonably well airsealed (a Blower Door Test will ID openings you can't see). System airflow must be within manufacturer spec to maximize discharge temperature. Instruct Owners to use normal temperature setting of 75+/- (ACCA Design), to avoid creating a "Moisture Magnet". Always check ambient T/H, caculate dewpoint, and duct surface temperature (many downloadable calculators out there).

    I try to resist the temptation to install fibrous materials downstream of the Air Cleaner, to limit my liability in case someone brings up however many states have banned the stuff.

  7. #7
    Join Date
    Feb 2019
    Location
    Stockholm
    Posts
    12
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    I had the exact same problem. I did not know what the problem is due to temperature and moisture. It turned out the filter was too dirty. After cleaning everything turned out!

  8. #8
    Join Date
    Apr 2012
    Location
    Millsboro, DE
    Posts
    181
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    Airflow, airflow, airflow!!!

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