House built in the 1940's, Pacific Northwest, generally snowless climate, now with several generations of additions, such as a dormer, all in all at least twice as much living space compared to 1940s.

How do I carry out a survey of heat loss to identify and prioritized weak spots for limited dollars for improvements? 

Is it as simple a matter as getting an thermometer gun from a box store like Home Depot or Lowes or Harbor Freight, then mapping out the doors and skylight glass and window glass and various walls and ceilings? 

How do those examples of conductive/convective heat loss compare to mass losses such as faulty sealing around doors and sashes?

Lastly, are there any online estimator calculators or tables for major projects? for example adding thickness to a 250 sqft 2x6 cathedraled shed dormer to make it more like 2x8 or 2x10? or more simple projects like replacing an old single-pane exterior door with a thermalpane door?