Polymer compositions with reduce oxygen permeability are described. The polymer compositions may include one or more polyolefin polymers, and a nanoparticle filler. The nanoparticle filler represents about 2 wt. % to 5 wt. % of the weight of the polymer composition and reduce the oxygen permeability of the composition by about 20% or more. Also described are roofing systems that may include an insulation layer and a thermoplastic polyolefin layer made using the polymer compositions to protect the insulation layer from weather degradation.