So, this go round of weatherizing my home I decided ot buy a laser-thermometer (you point it at a spot, it reads the temperature of the spot) - it wasn't cheapo (35.00) - but well worth it! Now i can SEE where I need to insulate with foam (strips and the crack-filler puffer stuff).

Two areas where I was suprised to find leaks (other than the usual around the doors and windows) - but around air ducts when the heat is NOT running and around lightswitches.

They DO sell sheets of insullation that go underneath your lightswitch covers - they make a HUGE difference.

And I'm going to go under the house and install strips of thick "window-air conditioner unit insullation foam" around the air ducts to help seal those gaps as well.

And, apparently wood air ducts do a much better job than metal ones do, of course.