Bottom Line: Internal insulation exposed to the airstream needs a UL 181 durability basis. Exposed internal insulation that is not impermeable to water cannot be used from the cooling coil exit to the downstream end of the drain pan.
Why It Matters: This is inexpensive to fix on paper and expensive to fix after submittal or in the field. If the drawings show generic lined duct through the coil discharge section, the reviewer has to ask whether the product is allowed, where the drain pan limit ends, and where the lined-to-unlined transition occurs.
5-Minute QC Sequence: 1. Locate the cooling coil discharge and downstream end of the drain pan. 2. Trace every exposed internal liner in that segment. 3. Verify product impermeability and the UL 181 durability basis. 4. Show the lined-to-unlined transition on plans or details. 5. Confirm equal thermal performance where liner is interrupted.
Where To Show It: M-001 should state the UL 181 durability basis and coil-discharge restriction. M-101 should identify lined duct and the transition. M-501 should show the drain-pan limit and liner detail. M-601 should list liner type, durability basis, and project-specific coil-discharge remarks.
Red Flags: lined duct shown at coil discharge with no water-impermeability statement; downstream drain-pan limit not identified; M-601 lists internal liner generally while the coil detail is silent; duct detail shows liner with no UL 181 basis; lined-to-unlined transition is not dimensioned or located.
Paste-Ready Note: Materials used as internal insulation and exposed to the airstream shall have a UL 181 durability basis. Exposed internal insulation that is not impermeable to water shall not be used from the exit of a cooling coil to the downstream end of the drain pan.
Field Tip: If a superintendent can stand at the coil discharge section and still ask where the drain-pan limit ends or whether the exposed liner is water-impermeable, the drawings are not clear enough yet.