Newly constructed portions of a duct system (anything between the return register(s) and the supply registers); must not consist of any un-ducted building cavities, such as platform returns, floor joist bays, wall stud bays, under-cabinet spaces, or other building assemblies unless those assemblies are fully lined with approved ducting materials. Wood, drywall, plaster and other building materials are not suitable ducting materials. The only approved ducting materials are:

• Flex duct with a UL-181 listing,
• Sheet metal, or rigid duct constructed of sheet metal, and
• Duct board (similar to fiberglass insulation compressed until it becomes rigid with covered with a foil facing).

Mandatory duct system construction standards require that all joints and seams in the duct system are sealed with mastic, tape, aerosol sealant, or other duct-closure systems that meets the applicable requirements of UL-181, UL- 181A, UL-181B, or UL-723 as applicable. The applicable listing shall be printed on all tapes used to construct a duct system.

The Energy Commission has only approved three cloth-back duct tapes with special butyl synthetic adhesives rather than rubber adhesive to seal flex duct to fittings. These tapes include:

• Polyken 558CA, Nashua 558CA, manufactured by Berry Plastics Tapes and Coatings Division and
• Shurtape PC-858CA, manufactured by Shurtape Technologies, Inc.