The house was built in 1975. There's about 3 inches of fiberglass batting in attic. One of the first things I'll do after closing is to blow in 12+" of insulation.
If you can pay for an alternative I’d strongly consider it. Blown insulation makes a mess of the attic. Makes it really hard to work up there if you need to get access, potentially blocks your soffits and decreases airflow, and makes it more difficult and dangerous to walk without risking falling through the drywall.
Blow what? Don't do foam. It'll massacre the timber.
If you want something else, look at hempcrete, wool, foam glass aggregate.