I just did a major renovation, and while the walls were open I decided to go big and did spray foam.
It's very expensive, but the benefits are that its totally airtight and waterproof. Very efficient. I don't have actual numbers, since I haven't moved in yet (soon I hope). And then, I can't A-B-A it :/
One thing that I've heard about AC systems is that you should run your ductwork in the conditioned space. Less heat gain from hot attics and if there's a leak, it'll just blow cold air into the room.
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