A 21st century improvement on Monticello’s construction, the Insulated Concrete Form (ICF) house includes both thermal mass and thermal resistance. Polystyrene foam sheets about 3″ thick are used as forms when the concrete is poured, but unlike conventional plywood or metal forms, are left in place permanently. Conventional wall finishes are used over the Styrofoam both outside and inside. This sandwich configuration yields both energy storage and thermal resistance in one package. Slightly under 2% of new homes built in the U.S. in 1998 used ICF construction.