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Thread: Duct Insulation

  1. #1
    Join Date
    Jun 2008
    Posts
    6

    Duct Insulation

    Last November, I upgraded my HVAC to a Trane XL19i. I was satisfied with it all winter long but am having a slight problem this summer that I would like some advice on.

    I live in Dallas, and the unit is in the attic. Supply ducts, except one, are in the crawlspace with ducted returns in the attic. All ducts are steel, wrapped with 40 year old duct wrap. With the old unit, the supply in the attic was typically the most forceful and the coldest (as it is the shortest run from the unit - approx 30'). However, with the new Trane, this vent is the hottest vent in the house, typically delivering AC at temperatures warmer than the ambient room temperature.

    The installing contractor reviewed this problem, took measurments and found that after the system was 'up to speed' there was a 20 degree drop at all the other vents and only a 2 degree difference at this one vent - after 10 minutes of dumping 100 degree air. Their suggestion is to add 3" of insulation to this duct (two 1.5" layers) and hope this corrects the problem. Since I did not have problems with heat gain in this duct with the previous HVAC system, I am concerned with the success of this approach. I also have concerns over triple wrapping a duct and condensation.

    Any suggestions/recommendations?

  2. #2
    Join Date
    Sep 2004
    Location
    Fort Worth, Texas
    Posts
    586
    I am assuming since the duct wrap is 40 years old then the connections on all that metal duct is also. I would have those duct reinsulated, and while the old insulation is off look at getting all the joints sealed and have mastic applied.

    I wouldn't dare to venture any other suggestions without looking at the layout. There are alot of details that can be missed by trying to diagnose problems over the internet or the phone.
    Installs, Changeouts, & Heat Stroke.....not necessarily in that order

  3. #3
    Join Date
    Jun 2008
    Posts
    6
    Thanks. Although the contractor was going to place 2 more layers over the original layer of insulation, I was going to have him remove the original and reseal prior to installing the new insulation.

    Anything special regarding the insulation? Is there the possibility of getting condensation between the two layers?

  4. #4
    Join Date
    Dec 2008
    Location
    North Carolina
    Posts
    2,846
    While there are some HVAC contractors who have Insulation crews (meaning whole house approach, not just duct work), insulation is usually an HVAC contractors act to meet (and sometimes exceed) local code requirements. Put another way, insulation is not usually an area of expertise.

    But you are right and and you also did not provide enough information to say one way or the other.

    You cannot double, triple or quadruple wrap insulation that has a vapor barrier.

    If really want to learn all you can possibly learn about insulation without being in the field and seeing the theory v practice, check out this website the Oak Ridge National Laboratory Website, the lessons are worth a HO's interests: http://www.ornl.gov/

  5. #5
    Join Date
    May 2009
    Location
    New Caney, Texas
    Posts
    360
    Just to be clear here, AIR LEAKS are the problem in 40 yr old system. To rewrap the system using either 1 1/2" or 2" duct wrap is a great idea but wholly inadequate without as previously stated a whole house duct seal with mastic.
    Seal all connections where air leaks could be present.
    This solution fixes the system as if it was a new install. Actually the metal ducting with the proper insulation and properly sealed is for my money the best air ducting system by far.

  6. #6
    Join Date
    Jul 2008
    Location
    Alabama
    Posts
    658
    Quote Originally Posted by DGIO-Not View Post
    While there are some HVAC contractors who have Insulation crews (meaning whole house approach, not just duct work), insulation is usually an HVAC contractors act to meet (and sometimes exceed) local code requirements. Put another way, insulation is not usually an area of expertise.

    You cannot double, triple or quadruple wrap insulation that has a vapor barrier.

    Most if not all HVAC contractors have an INSTALLATION crew...not insulation.

    and yes we do know some things about insulation......state and local codes rerquire that there be a minimum r-value of insulation on the duct work.

    And where did this vapor barrier come from?? There is no vapor barrier on duct work..

    Solid metal pipe has to be sealed at the joint to prevent air leaks....( tape, screws, and/or mastic).....and then insulated, stapled, and taped again.

    Air leaks is what causes the condensation on the pipe....in most cases.

    To the Orignal poster don't just let them tear off the existing insul. and redo it. Have them rip out all of the duct and put in new...hell you just put in one of the most efficient units out there and you had them leave the old probably undersized ducts.......not a smart move dude.
    You're only as good as your customer will allow you to be.........If they want junk, sell them junk, but make your junk look neat!!!

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