In an effort to understand the process I created this spread sheet.
This spread sheet contains the formulas and data for Specific Heat Density and Volumetric Heat Density. It's is probably more than we need to know.
I would like to add Coefficient of Heat Transfer and then create a systematic approach to entering the required data in order to design a Solar Thermal Storage System.
Just for grins, I just did a measurement of an unheated room inside my shop. This room has no insulation in the floor and R19 in the walls and ceiling of this room which takes up about 1/4 of my shop. The outside temperature has ranged from 43F - 65F and inside the room has ranged from 46 to 56F. So what I learned from this is that the thermal mass inside this room levels out near the low end of our daily temperature swings.
Update June 15, 2012
It's now June 2012 and the temperatures are over 100F With the ad of an evaporative cooler the garden room stays at about 82F. In February with a little help from an electric water heater element in line with the pump the room stayed a comfortable 70F. The power to control the environment is probably costing me about $1.50 - $2.00 a day at 33 cents per Kilowatt Hour.Compared to previous green house attempts this is extraordinary. This is still my first year and I'm experimenting, but I think I used too much supplemental lighting during the winter months. Next winter I will cut that back and only use it to extend the hours rather than add brighter light.