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My concern is heat loss in the chilly crawlspace. No AC here, only forced air gas furnace.

The existing metal ducting in the crawl space is over 50 years but is intact. There is a very thin layer of insulating fiberglass surrounding the tubes, maiinly falling off. Does it make sense to replace with modern flexible insulated ducting, or would it be better to just insulate the existing metal ducts?

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    Both ideas sounds fine. Both will have their own difficulty curve especially inside of a crawlspace. What is your budget? What is your skill level? What is your patience level? Have the existing ducts ever been cleaned of dust? Even more important than insulation is sealing. Are the ducts properly sealed and not wasting conditioned air in your crawl space?
    – MonkeyZeus
    Commented Jul 28, 2021 at 16:44
  • Compare the rated R-value of the new flexible ductwork with the R-value of a layer of fiberglass placed around the existing ductwork. Do some magical calculation to decide if you're going to save enough on energy (in the hot air) to make doing anything at all pay off in a reasonable number of years. Can you make measurements now of the temperature difference between the two ends of this run (during cold season)?
    – Carl Witthoft
    Commented Jul 28, 2021 at 17:06
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    Flex duct reduces overall efficiency by introducing resistance. Don't use it unless you need the flexibility it provides.
    – isherwood
    Commented Jul 28, 2021 at 18:05
  • Questions about being "worth it" are usually either too vague (subject to complex measurement or hypothetical calculation) or a matter of prerogative (budget, motivation, skill set). This one is both. Voting to close.
    – isherwood
    Commented Jul 28, 2021 at 18:09
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    I'd like to suggest an edit to the question. The use of "modern" suggests that rigid ducting is obsolete. It isn't. Your choices are: Rewrap, replace with new rigid, replace with new flexible. I think the confirmation that there is nothing so wrong with your ducts that would warrant replacing them with new identical ones, would put the idea of replacing with new flexible ones in better context. What is truly "modern" is that the modern installer has a choice that is probably driven by the budget and the layout of the installation, not its age.
    – jay613
    Commented Jul 28, 2021 at 18:49

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I see no reasons for replacing the existing ductwork with flexible insulated ducting unless the existing ducting has rusted out.

I would just re-insulate with 3” fiberglass wrap if the existing metal ductwork is still in good condition.

Some of the reasons for not replacing with modern insulated flexible ducting.

  1. Cost of replacing existing ducting and installation New flexible metal ducting can be several times more expensive than just re-insulating.

  2. Flexible ducting has no advantage since you already have ducting installed

  3. Existing metal ducting if properly sealed has minimum health risk exposure risk to deteriorate of the insulation.

  4. Flexible ducting will have a slight reduction of airflow.

  5. Flexible ducting are harder to clean

I have the same setup as you described. 50-year-old ductwork in the crawl space covered with 1” fiberglass.

I will be re-wrapping the ductwork with 3” wrapping. Before wrapping the ducts, I will seal All the seams with mastic and the proper duct tape prior to wrapping.

See How to Insulate Ductwork (video) for all the details.

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  • If the 50 year old ducting is still in good condition (aside from its jacket) it will probably still be in good condition in another 50 years. Good luck getting new flexible ducting to last 50 years. You'll tear holes in it while you're installing it! IMO each one of the 5 above points is alone enough reason to not switch.
    – jay613
    Commented Jul 28, 2021 at 18:39
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Rats and possums can chew right through flex duct.

I was once tasked by the caregiver of an elderly person to find the source of an "off odor" in the house in which her client was "aging in place". The duct work was constructed of "duct board". I found a dead and decaying possum in the main duct, and I removed it through one register.

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