Should the internals of a register where connections are made have gray duct sealant?

looking in up at a ceiling register

  • where is the duct sealant in the picture?
    – jsotola
    Jul 25, 2022 at 1:47
  • I’m sorry, I’m asking whether it should have it. There isn’t any in the photo. Jul 25, 2022 at 2:12
  • are you asking about duct tape?
    – jsotola
    Jul 25, 2022 at 3:30
  • Are you asking if code requires it? If so, then you'll have to tell us where you live and when it was built. Are you asking if it's a good idea to apply it? Then please clarify that. In either case, please edit the info into your question
    – FreeMan
    Jul 25, 2022 at 12:12
  • Is there duct sealant on other connections in the system? Jul 25, 2022 at 12:52

2 Answers 2


When we built in 1994 the sealant on metal duct connections was part of a series of steps to make new construction more energy efficient. This program was promoted by the power company as "best practice" and if you did all the steps, they gave you a cash credit on your bill.

Using duct sealant was not a requirement by code at that time in our area.

So, your question could read two different ways:

Should my ducts have sealant (as in, would they be more efficient)? My power company says yes.

Should my ducts have sealant (as in, was somebody negligent in complying with the code)? Maybe, depending what the code said when your house was built.

  • And there are other ways to seal ducts to prevent air exfiltration than with a mastic type material.
    – SteveSh
    Jul 25, 2022 at 14:37

Should the internals of a register where connections are made have gray duct sealant?

Not typically, no.

Duct sealant, or mastic, is applied to the exterior of the joints. You don't want it on the inside because its texture would have a tendency to trap dust and introduces turbulence which reduces airflow.

If you must apply mastic to the inside due to accessibility issues then read the label and make sure it's non-toxic. It will smell for a few days until it fully cures.

  • My power company recommended that I take a bucket of mastic and a cheap work glove and smear it around the inside of the duct/register junction. I guess there are different recommendations from different folks.
    – RetiredATC
    Jul 25, 2022 at 17:06
  • @RetiredATC Why did they say this? Please see my updated answer.
    – MonkeyZeus
    Jul 25, 2022 at 17:49
  • I guess you would have to ask them. Perhaps the techniques have changed. I did post that it was in 1994.
    – RetiredATC
    Jul 25, 2022 at 22:12
  • @RetiredATC Sure, what's their number? Also, you wouldn't happen to have an operational 1982 DeLorean kicking around, would you?
    – MonkeyZeus
    Jul 26, 2022 at 20:02

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