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Discussion Starter · #1 ·
I am installing new ductwork in my home, currently there is no A/C and the heat is hot water radiators. I am using mastic and UL rated foil tape to seal all the joints and connections. My question is, can I apply the foil tape to the inside of the duct? In other words, since this is all new equipment, I have access to the inside of the ductwork. I found it easier to apply the mastic to the inside and outside of the connection, but the foil tape is much easier to apply to the inside. I get smoother seams and the connection is better. My concern is that in the future, if the foil tape fails, will I hear flapping?

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Well, the constant airflow by the tape may pull any exposed edges up and then off the inside. I can't envision how it's easier to apply the tape on the inside than the outside, without reaching up inside the ductwork (?)
 

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Discussion Starter · #3 ·
It's easier on the inside because I'm using rectangular duct. The duct comes in halves, so before I assemble the two halves to get my final rectangular trunk line, I measure, cut the openings for and install the boots for the takeoffs. That is what I'm taping.

I didn't mean to confuse you, after I have all the boots placed, I then assemble the halves of the rectangular duct and when I connect the other fittings, I tape the outside seams. It's just the initial takeoff boots that I am taping on the inside.
 
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