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Sheet metal ductwork is often lined inside with a black fiberglass insulation material. What purpose does the lining serve?

Insulation is one answer, but ducts can be insulated on the external surfaces. I expect external insulation would have the same thermal effect without increasing the air friction losses or adding internal surface area for dust, dirt, or mold.

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  • 1
    Is insulation not a good, valid reason?
    – JPhi1618
    Feb 28 '18 at 18:40
  • I'm wondering if there's an underlying question here...
    – Hari Ganti
    Feb 28 '18 at 19:25
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I assume JPhi1618 is referring to thermal insulation with his/her comment, which is correct. Especially in the case of long runs, much heat can be lost or gained en route due to conduction through the duct wall. Obviously this reduces efficiency and hampers environmental temperature control.

In other cases it's about sound reduction. Fan hum and howl, along with the noise of airflow itself, can be problematic.

Drawbacks to internal insulation are dust and odor. Obviously they're more likely to accumulate on rough, porous surfaces, and cleaning is less effective as well.

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  • The asker called it insulation, so I was trying to see if they had a more specific question? This is the answer I had in mind tho.
    – JPhi1618
    Feb 28 '18 at 19:39
  • I called it lining, not insulation. I added clarification to my question. Thanks!
    – ndemarco
    Mar 2 '18 at 18:11
  • So trade offs appear to be: insulate inside and get sound reduction as a plus, but friction as a negative.
    – ndemarco
    Mar 2 '18 at 18:13

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