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Seal old ductboard from inside with duct mastic?

For the next few days, I have what appears to be an opportunity to easily and cheaply make a small improvement to the overall airtightness (or nearly complete lack thereof) of my house’s 40 year old ductboard ductwork… the plenum is off (pending the installation of a new centeral AC unit in a few days), and I can easily get a pole and paint roller into the first 10 feet of the trunk duct to apply a few coats of duct sealant mastic to the inside.

If it makes a difference, this is old-school 1970s-era ductboard with yellow (now nasty-gray) raw fiberglass on the inside, not the modern kind with black outer layer designed to hide the fact that the fiberglass below has turned just as nasty and black on the inside.

Before I do that, I want to do a sanity-check & make sure it’s actually a good idea… I don’t want to end up having the ductboard disintegrate because the mastic was water-based, or otherwise end up causing more harm than good.

I’m particularly concerned about the possibility of accidentally creating a double vapor barrier & trapping moisture inside the ductboard (between the sealant on the inside, and the mylar foil (in its present, aged and compromised state)… on the outside). I suspect my concern is more theoretical than anything since we’re only talking about the first ~12 feet of a trunk duct that extends another 30 feet beyond two 90-degree bends AND presently leaks like a screen door in a submarine anyway, so in theory any moisture trapped inside the coated portion could eventually wick its way over to an uncoated and leakier section… but still, I feel like I should at least consider it as a possility.

I know there are companies that commercially mist the duct from the inside and coat it. I’ll probably take advantage of them someday… but as mentioned, right now I have an open duct, a pail of duct sealant & mastic, and a roller & pole that I can use… if, and only if, it’s a good idea .

There’s no time to order anything that can’t be received within 2 days via Amazon Prime, so if duct mastic is the wrong product to use, and the RIGHT product isn’t something I can buy locally in the Miami area tomorrow, I’ll have to just skip this project and do nothing.


  1. IS it a good idea (sealing the first ~12 feet of a 40 year old ductboard trunk duct from the inside using sealant and a paint roller?

  2. Is water-based duct sealant the right product (or at least, a better-than-nothing product) to use, or is there something better that I can get my hands on within the next 2 days (either Amazon Prime, or a local store)?

Note that “doing it properly” (re-taping its joints and coating the outside of the duct with mastic) isn’t an option at this time. The duct runs inside a soffit that’s drywalled, veneer-plastered, inaccessible, and absolutely will not be torn down to gain access. Someday, I might cut a hole in the ceiling of a closet along the way, remove the old duct “arthroscopically” (reach up through a 2×2 foot hole, cut out a 1-foot section, pull it down, pull the rest of the duct forward, repeat until the whole duct has been removed in 1-foot chunks), and replace it with flex duct… but that’s a future project, not a project for next week. This is solely an opportunistic, “I can do this easily… can I and should I?” project.

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