Plastic fiber insulation is an insulation material made primarily 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.