I'm right in the middle of this at my studio and here is what I'll offer for advice. First I figured out which frequencies are the most problematic. If your room is a normal box shape, John Brandt has a great room mode calculator that will generate a lot of great information to start with.
acoustic tools Also consider using a tool like REW (Room EQ Wizard) with a cheap omni measuring mic to actually get data on what your room is doing.
In dealing with room modes, it is important to understand how many of them are related. For instance, my room is 12.5ft wide which is right the exact distance of the 90hz wavelength but also a fundamental of 180hz and 360hz and so forth. In my room the Schroeder frequency between pressure and flow modes is 118hz. Understanding how to deal with troublesome pressure modes is often a challenge where absorptive treatments often comes up short. The good news is that taking care of the 90hz problem helps with the other fundamentals.
I ended up with 2ft deep of absorptive traps along the entire rear wall and 2ft square in the front corners. It is a lot of cubic ft of absorption yet I'm still working out that 90hz issue from the side which will likely take limp membrane traps to resolve.
So all that said, a few inches of rigid fiberglass, foam or whatever will not do much below 200hz despite what anyone tells you. It becomes a matter of ratio of absorptive material to existing space and that ratio is usually too small to be effective.
Let me know if you'd like some help sorting this all out if the room mode calculator I linked is confusing.