Especially if it is a rental you could have mold back there damaged insulation. 

The salesman at Coburn's told me my water heater went out just in time because in about 1-2 weeks now new federal regulations go in effect that require manufacturers to add a minimum of 1" insulation built into the shell of all water heaters. 

I have added insulation in the attic, replaced and repaired storm window's, storm doors, etc., but as all homes have to breathe a little (They have to or it would be dangerous to live there) wondering what limits there are.

Even while national housing trends weaken, growing markets like Chattanooga and Nashville will likely be insulated from the harshest adjustments coming.

I set up LLC's for each property so I will be insulated, whether my investor decides to do so or not is their call.

Cathy is right, insulation (and window replacement) is most likely the easiest way to save monthly heating costs. 

insulation, maybe switch out bulbs to energy efficient bulbs, new toilets can cut down on water. do they pay electric but you pay gas?

My plan was to seal off the intake void under the furnace insulate it heavily.

Is the house well insulated (which will do a LOT to reduce cooling bills in the summer)? 

The insulation isn't designed to last 50 years on it and if you insulate it acts as a conductor and will catch fire.