Plastic fiber insulation material is primarily made from recycled plastic milk bottles (polyethylene terephthalate or PET). The fibers are then formed into batt insulation similar to high-density fiberglass.
The insulation is treated with a fire retardant so it doesn't readily burn; however, it does melt when exposed to flame.
The R-values of plastic fiber insulation vary with batt density: R-3.8 per inch at 1.0 lb./ft3 density to R-4.3 per inch at 3.0 lb/ft3 density.
Plastic fiber insulation is relatively non-irritating to work with, but the batts reportedly can be difficult to handle and cut with standard tools.
In many areas of the United States, plastic fiber insulation might not be readily available.
Article source: The U.S. Department of Energy’s Office of Energy Efficiency and Renewable Energy (EERE). For the most up-to-date information please visit the EERE website.