17 pieces of Thermo-Pan Panning. From their website:

THERMO-PAN® – This innovative high-quality panning, is the first alternative since sheet metal for constructing return air ducts between joists and stud cavities. Lightweight and pre-cut, it makes HVAC installations, repairs, and remodeling easy and cost effective by stapling into place on floor joists and stud cavities.

It consists of corrugated insulation faced with aluminum in various widths and sizes to fit almost any ductwork, from floor to ceiling. It is available in various widths, packed 25 to the box. It significantly reduces duct noises and labor and requires no shop fabrication. The result is a product that saves you labor costs and adds a higher level of quality to the job. Get Thermo-Pan and get The Thermo Advantage!