New answers tagged ducts
May I introduce you to smurf tube? It's actually designed for the job, and meets your flexibility requirement. (Technically ENT (Electrical Nonmetallic Tubing), and it comes in non-smurfy orange as well as smurf blue.)
It looks like water damage to me. Likely either a roof leak or a condensation problem with one of the ducts (or something else entirely that you don't know is there?). It's also possible for the leak to be elsewhere above the ducts and this is just where the water found its way down to the ceiling. Do you have access to the area above(attic)? If so, ...
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