One of the messiest (but strangely fun) jobs upstairs. We used cellulose insulation to fill in all the joist cavities prior to putting the subfloors down in the new closet areas.
It was definitely a two person job, as you really need someone to run the machine and keep jamming the chunks of insulation into the hopper. But once you get a good routing going, it really goes quickly. The only catch is that it blows so much dust that you can’t even see what you’re doing!