For concrete slabs, most of the heat loss occurs around the edges of
the slab. The method used in the heat loss calculator is based on the ASHRAE Fundamentals chapter on Heat Loss. It defines an effective R value that is based on the loss per foot of perimeter. The table below allows you to estimate the effective R value for some common construction situation.

Wall Construction Method  Insulation  effective R (hrftF/BTU) (R value per foot of perimeter) 
8 inch block wall, brick facing  Uninsulated  1.47 
R 5.4 from edge to footer  2  
4 inch block wall, brick facing  Uninsulated  1.19 
R 5.4 from edge to footer  2.04  
Metal stud wall, stucco  Uninsulated  0.83 
R 5.4 from edge to footer  1.89  
Poured concrete wall with duct near perimeter 
Uninsulated  0.47 
R 5.4 from edge to footer  1.39 
This table is based on the ASHRAE Fundamentals, Residential Cooling and Heating Load Calculations, 2005
I changed the last column from a U value to an effective R value.
Note that this is an effective R value based on the slab perimeter, not the slab area. As such it cannot be compared to R values that are defined in the usual way  based on area. The best way to do comparisons is to use the Heat Loss Calculator to define your whole house, and then see how the loss from the slab compares to other parts of the house.
Gary April 18, 2008