Do any of you know the reasoning behind why the 2006 edition of the International Energy Conservation Code (IECC) removed the requirement for insulation for unheated slabs for some climate zones? I have done a few energy models recently with slab insulation as a potential energy saving measure. The numbers were very small, which was expected, but what surprised me was that the savings were negative. Adding insulation to the slab actually increased annual energy usage (very slightly). Cooling was unaffected, but fan energy was slightly higher during the heating season. My quick search for research reports or other insights to explain this came up empty. Do any of you have any thoughts? Are the results so insignificant that the differences I am seeing in the calculations could just be considered "noise"?

Keith Swartz, P.E., LEED AP