Is fiberglass soundproof

yes it is , you need enough of it and to cover certain spots . it reduces sound waves not completely entirely black all sound waves. like it reduces a level of noise up to 60% depending on how much fiberglass you use. But if you choose to use fiberglass as soundproof material to make a soundproof room i say go for it i use it for my garage since i have a drum kit and it works like a charm. Use fiberglass , or foam like acoustic foams it does just about the same work