This is exactly right - there are calculators that assess the heat/cooling load of your home (https://www.borstengineeringconstruction.com/Heat_Loss_Analysis_Ca

This is exactly right - there are calculators that assess the heat/cooling load of your home (https://www.borstengineeringconstruction.com/Heat_Loss_Analysis_Calculator.html https://www.borstengineeringconstruction.com/Heat_Loss_Analy... for a basic one) and with sensible insulation + leaky buildings that were common in the past, the heat loss to air infiltration can approach the total load you'd otherwise need. Using that calc - with typical R-Values for an old building of 1,000sq ft + ACH50 of 10 (converted to 0.5 for this since ACH50 values are at pressure) with a 0º outdoor temp and a 70º inside temp, the heating load would be 50k BTU. Doubling the insulation on the walls, doubling the R-Value of the windows, and doubling the roof insulation has the same impact as bringing the leakiness to an ACH50 of 3. One of those can be done with a few tubes of caulk, a few rolls of weather stripping, and some cans of spray foam. The other involves removing siding/drywall/roof material. Everyone in building science will tell you that the "best dollars" on saving heating/cooling costs are spent in making your home tighter.