Yeah you can use fiberglass for the knee walls, Do not spray foam directly against the roof decking, Install durovents or likewise product from the peak to the soffit. Then spray foam against the durovent. This will allow airflow behind the insulation and allow the roof decking to breath.I'm finishing an unfinished second floor. It'll be a lot like most attics. I'll have a 4' knee wall, then a 5' slanted section and 6' wide ceiling. My rafters are 5'6" deep, so I'm assuming I need to spray foam that area. In an effort to keep cost down. Is it possible/advisable to staple in fiberglass insulation in the knee walls and ceiling where I have plenty of room. And only spray foam the slanted ceiling areas?
I guess if your looking to save money, you could possibly run the durovent from the peak down to the knee wall and foam it all in and fill in the knee wall with fiberglass and leave behind the knee wall un insulated.