[TimJFowler]

I used Heating Degree Days* in the calculations so that I could directly compare the two bills.

Heating Bill Comparison
Before Attic Insulation:
Jan 14 - Feb 13, 2008 (31 days)
Natural Gas used - 121 Therms @ $141.72
Heating Degree Days - 1103
Therms per Heating Day - 0.1097

After Attic Insulation:
Jan 11 - Feb 10, 2009 (31 days)
Natural Gas used - 57 Therms @ $60.19
Heating Degree Days - 852
Therms per Heating Day - 0.0669

Natural Gas Use Difference .0669 / .1097 = 60.98%
~ 39% reduction in Natural Gas Heating Bill

* Heating Degree Days - the number of heating degrees in a day is defined as the difference between a reference value of 65°F (18°C) and the average outside temperature for that day.