A much-neglected aspect of building design is the foundation. A building with an uninsulated, suspended floor can experience great thermal losses. These occur uniformly across the surface, in proportion to the temperature difference between the subfloor space and the conditioned space. These losses also happen in a floor above an unheated crawl space and floors projecting beyond the building below.
Buildings with a slab foundation also have thermal losses from the floor. The center of the floor is well insulated by the earth, so losses there are minimal. However, much energy can be lost at the perimeter of the slab. These losses occur by conduction since the slab is in direct contact with the ground, but also by convection from any portion exposed above grade.
Fortunately, foundation losses can be easily remedied. The perimeter of slabs can be insulated to a depth of at least 2 ft below grade, in the form of rigid board or foam. There should be an asphalt bed at the base of the added insulation, flashing at the top, and other such waterproofing measures as required to keep the insulation dry and performing as designed. This insulation is most effective in cold climates. It also protects the foundation from freeze damage.
Suspended floors can have batt or board insulation, or spray insulation similar to that used for roofs. The added insulation should include a vapor retarder and appropriate protection from weather damage or other activity that can damage the insulation.