which month had the lowest gas usage, May or June?
subtract that number, since it closely indicates the gas used for cooking, water heating, etc, from your last months gas --
now divide that number by the DegreeDays
now divide that number by the sq foot ( just for info )
now track these numbers
do you have storm windows & doors? they add insulation & protect the main window, cheaply!
have trees to act as wind breaks?
wasn't the basement insulated on the outside? shame, shame -- how much of this wall is exposed? was that taken into account?
read at BUILDINGSCIENCE.com
are the ducts insulated? are the plenums insulated?
it seems that you had an excellent contractor, in- as- much- that the rooms are holding within 1F of each other!
harvest rainwater,make SHADE,R75/50/30= roof/wall/floor, use HVAC mastic,caulk all wall seams!