Which is a better conductor of thermal energy a plastic foam cup or window glass?

Window glass is a much better conductor of thermal energy than a plastic foam (styrofoam) cup. The exact "thermal conductivity" rating of window glass depends on its mineral composition. The thermal conductivity of styrofoam is very low, because it traps air in small pockets, and air has a very low thermal conductivity. That is why styrofoam is widely used as insulation.