I'm in the process of finishing off my bass traps finally. I read somewhere in this forum that it might be a good idea to place something like craft paper in front of the insulation to block out the high frequencies. Does anyone know if this is true? Has anyone tried this? Any feedback would be great.
Depends. If you leave off the paper, the trap will absorb mid- and high-frequencies as well as bass. If you put it on, it will reflect the higher frequencies. The choice depends on whether your room's acoustical situation needs HF absorption or whether it has enough. You want it neither too live nor too dead.
"Music in the Round"
Senior Contributing Editor, Stereophile
6mil plastic membrane can be applied to the outer (room facing) boundaries of the LF porous absorbers, and will reflect ~500hz+ specular energies back into the room. you can also apply any other diffusive or scattering treatments (reflection phase grating diffuser, binary amplitude diffuser (or MLS/slat/etc) as well to provide more mixed returns vs simple specular reflections via the plastic membrane.
Bpape often specs kraft paper for his rear wall treatments. One such build is Bacon Race in my signature. I've built a few Dennis Erskine designs where he specified 3mil plastic (Self adhesive carpet runner) over portions of the side walls (Back of room) and the back wall so that the room didn't become too dead. So to answer your question yes it is true.
If you have your traps in all four corners you should consider it for the ones in the rear.