From what I'm seeing, moving from terrible insulation to decent insulation is an excellent idea. Moving from decent insulation to excellent insulation... maybe

From what I'm seeing, moving from terrible insulation to decent insulation is an excellent idea. Moving from decent insulation to excellent insulation... maybe not so much. There is a point of diminishing returns where more R values in your insulation does not gain you much. According to this fact sheet from the North Carolina energy office: https://files.nc.gov/ncdeq/Environmental%20Assistance%20and%20Customer%20Service/IAS%20Energy%20Efficiency/Opportunities/Insulation_Guidelines.pdf https://files.nc.gov/ncdeq/Environmental%20Assistance%20and%... "Insulation installation which brings a building up to standard can result in a savings of 1—5% of HVAC costs depending on the conditions before the improvements are implemented." However, if you combine insulation with inspecting ductwork / windows / doors / attics and plugging as many air leaks as practically possible, the estimated savings rate rises considerably. Energystar's site (https://www.energystar.gov/index.cfm?c=home_sealing.hm_improvement_methodology https://www.energystar.gov/index.cfm?c=home_sealing.hm_impro...) for instance estimates 11% of utility bill savings with a combination of methods.