The exterior doors look like wood and they thunk like wood when
knocked on, but they are, in fact, wood-grained and stained glass fiber-reinforced polyester sheet-molding compound insulated with polyurethane foam. Modern Plastics, a trade journal, says the doors were created by Therma-Tru Corp. of Toledo, Ohio, which developed a proprietary process from glass fiber furnished by Owens-Corning Fiberglas.
The polyurethane foam provides far more effective insulation than does wood, the company claims, and it also helps make the doors sound like solid wood. The plastic doors also resist warping, splitting and cracking, weigh about half the real thing and cost as much as two-thirds less than solid oak-paneled doors.