I spent more time on determining what insulation to use than any other item. I thought about spraying in a ceramic based paint, foam, insulating blankets, I was going out of my mind. My problem is there are 3 different frame depths. I was more concerned with noise than insulating properties. I went with foam carpet padding. It is very dense for noise, easy to cut, and was just the right thickness. Oh, and Home Depot has tons of it. I used 3M's best spray clue to keep it in place until I put the fiber board up.