A recently developed 3-dimensional earth contact simulation program that operates as a subroutine of the EnergyPlus whole building energy simulation program was used to evaluate the energy performance and cost-effectiveness of retrofit slab-ongrade (SOG) foundation insulation. An optimized retrofit insulation design utilizing hydro-vacuum excavation was developed that generated 8.4 % larger metered (or site) energy savings at a $428 lower cost than the IECC 2012 requirement in a Minneapolis, MN climate for a 400 ft 2 test building. The energy performance and cost effectiveness of single and multi-family buildings was assessed for climate Zones 4 – 7. With reference to the Building America B10 benchmark, the highest site energy savings of 5 % was realized for a single family home in Duluth, MN, and the lowest savings of 1.4 % for a 4-unit townhouse in Richmond, VA. SOG foundation insulation retrofit simple paybacks ranged from 18 to 47 years. Thus it is likely that larger energy savings of 10% or more with concomitantly reduced simple paybacks can only be realized in well-insulated buildings.