If your talking Air Conditioning/Heating then yes it is approved and is the standard.
1" Duct Board has an R-Value of 4.3
Bellaire Texas Air Duct Cleaning
No, you should not use a humidifier in a main trunk line that is made out of duct board. This will cause the duct board to rot from the moist.
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469-530-0587
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rubber cement, tacky glue, duct tape
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The center vent, or defrost duct.