Running my model (~4000SF, 1 story home, Climate Zone 5A , ~6000HDD) through BEOpt, using 16" dense pack celluolose celing, insulated slab with perimeter insulation, I get higher energy use if I add continuous R-6 Polyiso exterior insulation to my 2x6 framed walls. Is this a bug? Could this be due to reduced energy gain in the winter? Is there a possiblity of continuous rigid exterior insulation INCREASING energy use?