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Ductwrap For Attic AC?

Hello Everyone,
Quick question here regarding ductwrap for a standard cooling only attic system with sheetmetal supply plenum and flex runs in upstate NY.

At the shop I install for we have always wrapped our air handler plenums with SOFT-R fiberglass duct wrap. I have seen the "Reflectix" or "bubble wrap" insulation and this looks to be a much more aesthetically appealing and looks to be easier to work with. However I have heard different things regarding the "Reflectix" insulation such as the R value is lower than SOFT-R duct wrap and you need a barrier of layers to use "Reflectix". I have also heard that "Reflectix" does not insulate well enough and the plenum would sweat if Reflectix only is used to insulate duct work.

Another alternative I have considered is having the sheet metal shop we use to create our plenums "pre insulate" the inside of the plenum. However my question is will the ATD (takeoff) sweat when it is cut into the plenum?

Thank you anyone who has info regarding my questions any help would be greatly appreciated. Just looking for better way come this summer!

Comments

  • Jamie Hall
    Jamie Hall Member Posts: 17,092
    edited March 2020
    The bubble stuff, if it has no reflective layer, is only as good as any other insulation of the same thickness. Which is to say, not very. The reflective stuff is OK if you have a fairly high temperature source which you are trying to insulate from a low temperature sink, as it only affects radiant heat energy. For duct wrap for air conditioning? No...

    On sweating. It's all about dew point and surface temperature. If the surface temperature of the metal is below the dew point, it will sweat. Insulation on the outside -- your duct wrap -- will prevent humid air from reaching the cold metal. Insulation on the inside will allow the metal to be as warm as outside. Take your pick. If the takeoffs are uninsulated, chances are they will sweat.
    Br. Jamie, osb
    Building superintendent/caretaker, 7200 sq. ft. historic house museum with dependencies in New England
  • pecmsg
    pecmsg Member Posts: 2,335
    You need to study the PSYCHROMETRICS CHART to understand what's going on.
  • HVACNUT
    HVACNUT Member Posts: 4,196
    edited March 2020
    Mask all joints.
    Mask/air seal all openings between boots, plenum and sheetrock or finished ceilings, walls, and floors. Especially returns. No attic air should be pulled into the system.
    R8 fiberglass scrim insulation.
    Never use an internal liner. Its nothing but a disease breeding ground.
  • ChrisJ
    ChrisJ Member Posts: 11,975
    edited March 2020
    > @Jamie Hall said:
    > The bubble stuff, if it has no reflective layer, is only as good as any other insulation of the same thickness. Which is to say, not very. The reflective stuff is OK if you have a fairly high temperature source which you are trying to insulate from a low temperature sink, as it only affects radiant heat energy. For duct wrap for air conditioning? No...
    >
    > On sweating. It's all about dew point and surface temperature. If the surface temperature of the metal is below the dew point, it will sweat. Insulation on the outside -- your duct wrap -- will prevent humid air from reaching the cold metal. Insulation on the inside will allow the metal to be as warm as outside. Take your pick. If the takeoffs are uninsulated, chances are they will sweat.

    I've had a section of my return that I still need to insulate sweat in the attic. Guess the dew point got to 68-70 up there. I know because of the water stains under it on the flooring.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • ExistingAmount
    ExistingAmount Member Posts: 6

    The bubble stuff, if it has no reflective layer, is only as good as any other insulation of the same thickness. Which is to say, not very. The reflective stuff is OK if you have a fairly high temperature source which you are trying to insulate from a low temperature sink, as it only affects radiant heat energy. For duct wrap for air conditioning? No...

    On sweating. It's all about dew point and surface temperature. If the surface temperature of the metal is below the dew point, it will sweat. Insulation on the outside -- your duct wrap -- will prevent humid air from reaching the cold metal. Insulation on the inside will allow the metal to be as warm as outside. Take your pick. If the takeoffs are uninsulated, chances are they will sweat.

    HVACNUT said:

    Mask all joints.

    Mask/air seal all openings between boots, plenum and sheetrock or finished ceilings, walls, and floors. Especially returns. No attic air should be pulled into the system.

    R8 fiberglass scrim insulation.

    Never use an internal liner. Its nothing but a disease breeding ground.

    ChrisJ said:

    > @Jamie Hall said:

    > The bubble stuff, if it has no reflective layer, is only as good as any other insulation of the same thickness. Which is to say, not very. The reflective stuff is OK if you have a fairly high temperature source which you are trying to insulate from a low temperature sink, as it only affects radiant heat energy. For duct wrap for air conditioning? No...

    >

    > On sweating. It's all about dew point and surface temperature. If the surface temperature of the metal is below the dew point, it will sweat. Insulation on the outside -- your duct wrap -- will prevent humid air from reaching the cold metal. Insulation on the inside will allow the metal to be as warm as outside. Take your pick. If the takeoffs are uninsulated, chances are they will sweat.



    I've had a section of my return that I still need to insulate sweat in the attic. Guess the dew point got to 68-70 up there. I know because of the water stains under it on the flooring.

    Thank you guys for providing actual useful and real world applicable responses (not just a psychometrics chart). For now I will continue to use R8 SOFTR for attic AC installations. Thanks again to everyone for a great community where I can continue to educate myself in the trade of Heating/AC
  • pecmsg
    pecmsg Member Posts: 2,335
    R8 May be fine also it may not. That's the reason I posted the chart. You need to calculate the Dew Point of the surface of the insulation to determine if its thick enough to prevent condensation from forming.

    Everyone uses 3/8" wall thickness on the suction lines . How many times have you felt the insulation and its wet? Not thick enough to prevent condensation from forming. 1/2' or more may be needed.

  • ChrisJ
    ChrisJ Member Posts: 11,975
    pecmsg said:

    R8 May be fine also it may not. That's the reason I posted the chart. You need to calculate the Dew Point of the surface of the insulation to determine if its thick enough to prevent condensation from forming.

    Everyone uses 3/8" wall thickness on the suction lines . How many times have you felt the insulation and its wet? Not thick enough to prevent condensation from forming. 1/2' or more may be needed.

    R8 was the thickest I could find anywhere for ductwork.
    I used 1/2" on my liquid line and 3/4" on the suction line but it sure did make for a huge lineset. I think it's almost 2.5" in diameter for the suction line.
    Single pipe quasi-vapor system. Typical operating pressure 0.14 - 0.43 oz. EcoSteam ES-20 Advanced Control for Residential Steam boilers. Rectorseal Steamaster water treatment
  • ExistingAmount
    ExistingAmount Member Posts: 6
    ChrisJ said:

    pecmsg said:

    R8 May be fine also it may not. That's the reason I posted the chart. You need to calculate the Dew Point of the surface of the insulation to determine if its thick enough to prevent condensation from forming.

    Everyone uses 3/8" wall thickness on the suction lines . How many times have you felt the insulation and its wet? Not thick enough to prevent condensation from forming. 1/2' or more may be needed.

    R8 was the thickest I could find anywhere for ductwork.
    I used 1/2" on my liquid line and 3/4" on the suction line but it sure did make for a huge lineset. I think it's almost 2.5" in diameter for the suction line.
    Thanks again. Agree R8 is the highest I have seen also.
  • clammy
    clammy Member Posts: 2,668
    edited April 2020
    Remember that when wrapping duct to add the thickness of the insulation to your duct dimension to ensure that you are not compressing the insulation compressing the insulation lessen the rates r value , for closure I was taught to cut the insulation off of the wrap to have a proper over lap for agog vapor seal and enough to staple it closed . It should be stapled w the proper stainless staples and proper staple gun outward turning. It should be then have seams tapped w at least 4 inch wide scrim coil tape ,not done yet it should then be bail wired w the wire not being pulled tight. I prefer to hang duct so I add foam tape to my 18 ga strap . If stuck running on attic ceiling rafter I ll get some 2 insulation blue board and sit the duct on that less compression over all . Any time I go to the sheet metal supply house I see that bubble wrap flying out the door it’s garbage , I don’t even know if it goes much . I ve been using 1/2 wall on residential for ac for 20 years to me again any less is garbage the difference in price is a non issue bigger system in commercial spaces and different application are another story . Peace and good luck clammy
    R.A. Calmbacher L.L.C. HVAC
    NJ Master HVAC Lic.
    Mahwah, NJ
    Specializing in steam and hydronic heating