Stack effect in airtight house – GreenBuildingAdvisor

I built a 2 story 2000 sqft house in southern Vermont with zip sheathing and double stud walls with 12 inches of dense pack cellulose and 22 inches of cellulose in the attic. This will be our first winter living in the house. So far it is performing well. Temps haven’t been very cold. 20’s at night, 30-40’s during the day. The 12000 btu cold climate heat pump has been keeping the house warm while using about 12-17 kwh per day (we have a woodstove for when the real cold hits.) The down stairs stays at 72 while the upstairs is 68. Am I right to assume that the airtight envelope is preventing a stack effect and the “heat rises” principle? Even if that is the case does a 4 degree temp difference seem high? With a thermal imaging camera I did find several stud bays upstairs that seem to be light on insulation at the top. Based on my rough calculations these cold spots are equivalent to having an extra 4×5 triple pane window on the second floor. Would this be a significant reason for the 4 degree temp difference? I did all of the prep work and rolling for the cellulose so these light spots are almost certainly my fault and not the installer. Would topping off the cellulose significantly reduce the temperature difference or is it not worth the effort?