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External duct insulation and factory-insulated flexible duct shall be legibly printed or identified at intervals not greater than 36 inches (915 mm) with the name of the manufacturer: the thermal resistance R-value at the specified installed thickness; and the flame spread and smoke-developed indexes of the composite materials. All duct insulation product R-values shall be based on insulation only, excluding air films, vapor retarders, or other duct components, and shall be based on tested C-values at 75°F (24°C) mean temperature at the installed thickness, in accordance with recognized industry procedures. The installed thickness of duct insulation used to determine its R-values shall be determined as follows:
1. For duct wrap, the installed thickness shall be assumed to be 75 percent (25 percent compression) of nominal thickness.
2. For factory-made flexible air ducts, the installed thickness shall be determined by dividing the difference between the actual outside diameter and nominal inside diameter by two.
Linings shall be interrupted at the area of operation of a fire damper and at a minimum of 6 inches (152 mm) upstream of and 6 inches (152 mm) downstream of electric-resistance and fuel-burning heaters in a duct system. Metal nosings or sleeves shall be installed over exposed duct liner edges that face opposite the direction of air flow.
If ducts used for cooling are externally insulated, the insulation shall be covered with a vapor retarder having a maximum permeance of 0.05 perms [2.87ng/(s- m
2
x Pa)] or less and is not required to be covered. All joints and seams shall be sealed to maintain the continuity of the vapor retarder.
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