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.