Fiberglass is a nuisance. It has to be covered correctly or glass fibers get into the air and some of us, like me, get itchy just looking at the stuff. Acoustic foam is every bit as effective down to 250 Hz and eons easier to use and in the end cheaper. It is also easier to specify for a certain situation in terms of thickness and profile.
Now unless you are lucky and able to have a room in your house that does not have parallel walls the single best way to deal with parallel walls floors and ceilings is to have speakers that do not send sound in their direction. Then if you are really clever you make your room without a rear wall and you don't have to worry about that either. You can leave your side walls and ceiling live for ambience and if you have speakers like mine or any other dipole just a little absorption behind the speaker using foam or fiberglass, if it does not make you itch completes the picture.
Now unless you are lucky and able to have a room in your house that does not have parallel walls the single best way to deal with parallel walls floors and ceilings is to have speakers that do not send sound in their direction. Then if you are really clever you make your room without a rear wall and you don't have to worry about that either. You can leave your side walls and ceiling live for ambience and if you have speakers like mine or any other dipole just a little absorption behind the speaker using foam or fiberglass, if it does not make you itch completes the picture.