You can certainly put batts in if you wish, but it serves no purpose other than insulation. Be sure to slice the batts so as to avoid a mold sandwich if you're using Redgard.

I recommend trying to do a better job.

If you're going to insulate, why not insulate properly? Or at least with a little more effort towards good worksmanship? FG batts are compromised enough when people try to install them correctly, never mind people installing them in a haphazard manner.

A "weak" vapor barrier will allow air to whistle through your stud bays and through your FG batts. Air movement through the batts can drastically reduce their effective R-value, and if that air is carrying warm moisture, in cooler weather the dew point can be reached within the thickness of the batt, resulting in water condensing out within the framing bay and saturating the batt.

It's a bad recipe.

I recommend you use unfaced batts. Let the insulation be just that: insulation.

For the vapor barrier, I recommend 6-mil polyethylene sheet plastic. It's horribly inexpensive. And by being a single continuous unseamed barrier from left to right and from top plate to sole plate, you're creating an effective vapor barrier.

Then install your cement board over the poly'd studs.

Then use a waterproof membrane over the cement board that is not a vapor barrier. Use something like Hydroban (not a VB) instead of RedGard (which is a VB).

If you want to use RedGard and faced FG batts, then do just as StoneTech mentioned. Install your faced (or unfaced) batts. If you install faced batts, slice up the kraft paper with a utility knife.

Then cement board all surfaces of the shower. Detail the seams and corners if needed.

Then RedGard all of the cement board. You should now have a nice water- and vapor-proof shower space. Then tile.

"Is there anything regarding the type of grout or adhesive that I should be aware of since it will be going on over the redgard? "

For tiling over any topical membrane (like RedGard or Hydroban), it's tile as usual. Use a modified thinset. RedGard is a "Custom Brands" product, their modified thinset line would be Versabond which is lightly modified, or Flexbond which is highly modified.

Use a modified grout. Custom's "PolyBlend" grouts are already modified.

Use a dry powdered thinset and a dry powdered grout (which the above mentioned are), each which get water mixed with them to make them usable. Don't use any "premixed" thinsets or premixed grouts that come ready-to-use in a resealable plastic tub. Those plastic tub materials can actually be water soluble. You probably understand using a water soluble thinset in a shower might not be the best idea. ; )

Best of luck with your project.

Is this an exterior wall you're insulating or an interior wall? I ask because I have an interior wall that I have considered insulating with a product called "ROXUL SafeâÂÂnâÂÂSound" to provide sound insulation from the new shower that will share a wall with the living room.