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  1. #1

    Confused

    I have received lots of good info from this forum regarding my sweating duct problems with my two new Lennox high efficiency var spd air handler & 2 spd compressor heat pumps.

    I am considering replacing the existing duct insulation and want to know if anyone has a link to software that can tell me, based on ambient attic temp and relative humidity, how much fiberglass duct wrap would be needed to eliminate condensation? Seems to me there must be a calculator out there somewhere.

    Thanks.

  2. #2
    Fiberglass???
    You want to use fiberglass???

    There are other alternatives to insulating that don't itch!

  3. #3
    Join Date
    Sep 2002
    Location
    South Dakota
    Posts
    6,579


    You tell me the current duct surface temperature now without insulation and tell me the dewpoint temperature of the air you want protection against and I will tell you the R-value of insulation you need to add to the duct.

    I don't use an insulation calculator and am not aware of one. I just do the math myself.

  4. #4
    I dont use a calculator, I do the math myself. NORMCHRIS WHAT DO YOU WANT A COOKIE !

  5. #5
    Join Date
    Jan 2004
    Posts
    144
    prodiyer I dont use a calculator, I do the math myself. NORMCHRIS WHAT DO YOU WANT A COOKIE !
    --------------------------------------------------------------
    Give me a break, it's Sunday the 3rd of July, and I'm drinking BLUE colored tequila & fresca. Just use 2" wrap (R-6.5) on the boots, pile the insulation around the drywall at the base of the boots, caulk between the boot and the drywall, and have a good life free from worry.
    Licensing laws are tough, and it's about time.

  6. #6
    Join Date
    Apr 2002
    Posts
    11,808
    The VS system can run quite a bit colder.

    You put ductwork in the attic and hope that it will not do what it naturally wants to do, SWEAT.

    So it sounds like you want to re-insulate metal ducts. Make sure these ducts are well sealed before you re-insulate. Leaking supply air short circuits the effect of the insulation and causes condensation.

    It also wastes cooling power, as it is like a refrigerated exhaust. You blow cold air out of your ducts, it depressurizes the home, and hot humid air is drawn into the house.

    A properly applied two-inch duct wrap should be fine, the R-level mentioned at 6.5 should work. Wrapping the insulation too tight compresses the fiber glass and reduces R value.



    [Edited by Carnak on 07-03-2005 at 08:42 PM]
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  7. #7
    NormChris & Prodiyer:

    I'm at the coast in SC... super hot & muggy. Attic can get up to 105 (or maybe more, but I have vented soffits and power ventilator), so I would like to use these numbers as the extreme:

    temp: 105
    rel. hum: 95
    ducts are round galv.
    duct temp 55
    dewpoint 103

    I sure would like to have that formula.... if you post it in an answer I will do an excel spread sheet for us all so anyone can just plug in the numbers.

    also, if the thickness of the fiberglass comes out too high, I will probably have to fiddle around with some other numbers... I just want to be through with these sweating ducts.

    Thanks to the other folks who replied. Was the cookie comment an inside joke?

  8. #8
    Join Date
    Jul 2005
    Posts
    40
    How is sweating ducts bad?

  9. #9
    Join Date
    Sep 2004
    Posts
    3,157
    ablib
    New Member

    Registered: Jul 2005
    Posts: 10


    How is sweating ducts bad?

    think of them as sweating armpits

  10. #10
    Join Date
    Jul 2001
    Location
    Gold Coast of Connecticut
    Posts
    4,570
    Most likely you have air leakage not an insulation problem.
    Aire Serv of SW Connecticut- Gas heat, dual fuel and central a/c systems installed and serviced

  11. #11
    Join Date
    Apr 2002
    Posts
    11,808
    Originally posted by beachwalker
    NormChris & Prodiyer:

    I'm at the coast in SC... super hot & muggy. Attic can get up to 105 (or maybe more, but I have vented soffits and power ventilator), so I would like to use these numbers as the extreme:

    temp: 105
    rel. hum: 95
    ducts are round galv.
    duct temp 55
    dewpoint 103

    I sure would like to have that formula.... if you post it in an answer I will do an excel spread sheet for us all so anyone can just plug in the numbers.

    also, if the thickness of the fiberglass comes out too high, I will probably have to fiddle around with some other numbers... I just want to be through with these sweating ducts.

    Thanks to the other folks who replied. Was the cookie comment an inside joke?
    thats quite a factor of safety as 105F at 95% or 103 dewpoint will never happen
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

  12. #12
    Yes, I know it is the extreme. These numbers are probably more realistic, but I was waiting for the formula to calculate the answer myself. I didn't want to ask someone to do two calculations.

    This IS an attic, so the temps and humidity are quite high and the duct temp (with humidity control on and cfms about 350 per ton) is around 55 degrees I think.....

    So maybe this is a tad more realistic....

    temp: 100
    rel. hum: 85
    ducts are round galv.
    duct temp 55
    dewpoint 95

    If anyone could post the forumula to determine R value of insulation needed to prevent condensation, I would appreciate it.

    And on the subject of appreciation:
    While we are enjoying our 4th of July, let's take a moment to give thanks to our men and women in the service of our country fighting around the world for us all.

  13. #13
    Join Date
    Apr 2002
    Posts
    11,808
    Probably 86F would be the extreme dewpoint in the civilized world, You could get those kind of conditions inside of some factory.
    The way we build has a greater impact on our comfort, energy consumption and IAQ than any HVAC system we install.

    http://www.ductstrap.com/

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