I would suggest that folks interested in energy efficiency improvement take a spin with NREL's BEopt software.
It allows you to make a simple 3D model of your home, enter its current state of construction (insulation, HVAC system, roof, solar panels, etc.), and then uses the DOE's energy plus (e+) software to run energy analysis for different scenarios of modification to the building.
For example you can find the optimal balance between improvement to your insulation vs getting a more efficient HVAC system in terms of dollars or energy usage, depending on your goals for your home.