Why does the coated face of duct liner need to face the air stream

Plain-English answer

While pre-assembling a short horizontal return drop on the ground, you slide the internal fiberglass acoustic liner insulation blanket into the sheet metal shell with the dark, coated neoprene face pressed against the metal wall, leaving the raw yellow fiberglass exposed to the active airstream.

The coated liner face belongs toward the air stream. If raw fiberglass faces the airflow, it can erode, shed, and fail inspection. The likely recovery is to check the condition, correct prep/setup if it is within your assignment, and bring the foreman clean information before the work creates rework overhead.

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Use this as training guidance. Foreman direction, approved drawings, project specs, manufacturer instructions, employer safety policy, and AHJ/code requirements always control the final answer.

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