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Adding AC to existing gas furnace (Ductwork insulation question)

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Hello,
A little background before I go into detail: 3rd floor apartment (850 SQFT) that currently has a natural gas forced air system

Question: Currently I have a job coming up that requires adding Air Conditioning to an existing gas forced air system. The current system has ductwork correctly sized for conditioning however they never added AC to the system. The systems ductwork was never insulated since it incorporated heat only. The systems ductwork is boxed in by drywall and pressed up to the floor joists of the attic. Above the ductwork the is R-19 fiberglass insulation and a approximately 1/2" thick Styrofoam board.

My question is do I need to insulate the current ductwork?

With the system only ever having heat I have never seen the system perform with AC and I am unsure if the current ductwork would sweat if it came into contact with the hot attic air in the summer time even with the R-19 above it. I typically only do system changeouts so this is a new one to me. I have done numerous full attic system installs and I understand that everything must be insulated because it is located outside the conditioned space. However I cant tell if this ductwork would be considered as in the conditioned space or out of it.

Thank you everyone in advance i really appreciate any input. Attached are pictures which should help give a better visual as to what I am try to describe. Thanks again.



Comments

  • HVACNUT
    HVACNUT Member Posts: 5,865
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    Try to insulate anything accessible at least up to to ceiling joists. Anything that is boxed in and not accessible, the ducts probably have enough air leaks that the enclosure would be conditioned space and not sweat.
  • HVAC_P
    HVAC_P Member Posts: 20
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    HVACNUT said:

    Try to insulate anything accessible at least up to to ceiling joists. Anything that is boxed in and not accessible, the ducts probably have enough air leaks that the enclosure would be conditioned space and not sweat.

    Thank you for the informative reply.

    If I have the option to replace the ductwork and insulate it, Would you insulate on the exterior or the interior? Also have you used any of the bubble wrap (Reflectix) insulation? If so how did you like it? I typically use the R-8 fiberglass duct wrap and it has always served its purpose buts it is hard to make it look good.

    Thanks again and thanks in advance for any more information you might be able to provide.
  • HVACNUT
    HVACNUT Member Posts: 5,865
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    Always insulate outside. If it were mine, masking, tape, and fiberglass insulation. 
    If just hitting the existing spots now, I would probably use the bubble wrap. 
  • EBEBRATT-Ed
    EBEBRATT-Ed Member Posts: 15,629
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    With the insulation above the ductwork I would consider the ductwork in the conditioned space.

    It should be insulated though and at least sealed. Most heating ducts that are more than a few years old were never sealed

    Since the space is only 850 square feet (ton and 1/2)?? there probably isn't a ton of duct there.

    If you can get paid to seal and insulate why not??
  • HVAC_P
    HVAC_P Member Posts: 20
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    HVACNUT said:

    Always insulate outside. If it were mine, masking, tape, and fiberglass insulation. 
    If just hitting the existing spots now, I would probably use the bubble wrap. 

    Thank you again for the informative response. With the bubble wrap insulation. Do you have to use a spacer between the bubble wrap (Reflctix) and the surface of the duct? Thank you, I really appreciate your wealth of knowledge.
  • HVAC_P
    HVAC_P Member Posts: 20
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    With the insulation above the ductwork I would consider the ductwork in the conditioned space.

    It should be insulated though and at least sealed. Most heating ducts that are more than a few years old were never sealed

    Since the space is only 850 square feet (ton and 1/2)?? there probably isn't a ton of duct there.

    If you can get paid to seal and insulate why not??

    Yes 1-1/2 tons for the AC size. Plenty of return air and 6 - 6" supply runs.