Blow in, not spray on, insulation is the easiest, cheapest, bang for your buck home improvement I ever made. It took 3 hours and ~$1000 of insulation from Home Depot.
The house maintains its temperature so much better. Outside noise is reduced. My bills are smaller. And I can keep my AC at 72 now - or whatever temperature I feel like really.
I life in Maine. We get freezing winter every damn year. My apartment was built in like the 80s, there is no insulation (three of my walls are 3 inches of concrete and then the outside), windows and doors are the utter cheapest shit that doesn't even seal, heat is electric resistive hot water, electricity has always been expensive in Maine, but that's okay they make me pay my own.
Result, building owner got to cheap out in a hundred ways, pass the externalities on to me, I have no way to fix it, and my electric bill in the winter for one person in 600 sq ft kept to 70 degrees costs $250.
Where's Greta when you need her. I feel like I should have standing to sue the property owner on the planet's behalf.