High Performance Slab-on-Grade Foundation Insulation Retrofits

L. Goldberg, G. Mosiman

Research output: NRELSubcontract Report


A more accurate assessment of SOG foundation insulation energy savings than traditionally possible is now feasible. This has been enabled by advances in whole building energy simulation with 3-dimensional foundation modelling integration at each time step together with an experimental measurement of the site energy savings of SOG foundation insulation. Ten SOG insulation strategies were evaluated on a test building to identify an optimum retrofit insulation strategy in a zone 6 climate (Minneapolis, MN). The optimum insulation strategy in terms of energy savings and cost effectiveness consisted of two components: (a) R-20 XPS insulation above grade, and, (b) R-20 insulation at grade (comprising an outer layer of R-10 insulation and an interior layer of R-12 poured polyurethane insulation) tapering to R-10 XPS insulation at half the below-grade wall height (the lower half of the stem wall was uninsulated). The optimum insulation strategy was applied to single and multi-family residential buildings in climate zone 4 - 7. 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. There are other benefits of SOG foundation insulation resulting from the increase in the slab surface temperatures. These include increased occupant thermal comfort, and a decrease in slab surface condensation particularly around the slab perimeter.
Original languageAmerican English
Number of pages77
StatePublished - 2015

Bibliographical note

Work performed by NorthernSTAR, St. Paul, Minnesota

NREL Publication Number

  • NREL/SR-5500-64737

Other Report Number

  • DOE/GO-102015-4726


  • Building America
  • costs
  • energy
  • insulation
  • NorthernSTAR
  • residential
  • residential buildings
  • slab-on-grade


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