Steve...

I agree with your comments... We have developed a couple of free
design tools that might be of
interest: www.aud.ucla.edu/energy-design-tools HEED (Home Energy
Efficient Design) will show graphically the time lagged heat flow
through each of the different building elements, including slab edge
loss. The Thermal Mass screen lets you explicitly define the mass
inside the insulated envelope. To see the plot of heat flow into and
out of this mass you can run a 12-day analysis from the Climate
screen. One of our other programs called OPAQUE will calculate the
required time lag and decrement as well as U-factor of any wall section.

Murray Milne