heat conductor

Plastic is an organic polymer and there are different kinds of plastics.

Take the most common type of plastic, polyacrylonitrile-butadiene-styrene, for example, its thermal conductivity is 0.17 W/m-K at 23 degree Celsius.

For air, its thermal conductivity increases with temperature. The value at 23 degree Celsius is about 0.0258 W/m-K

It is, therefore, clear that plastic conducts heat better than air at room temperature.