I wanted to ask what folks are using to insulate behind shower enclosures.  Is there reason for concern for BATT insulation behind a tile backer board screwed directly to the studs on an outside wall?  Or should I plan on using a thin piece of faced foam board attached to the studs under the tile backer board?  I know that sometimes these areas are left open  but I do not want to have any open voids in the exterior walls.  Please let me know what you all are seeing or doing in your homes.  Thanks.

Cement backer is not a vapor barrier. However- if you use Redguard- or Latacrete liquid applied membrane OR Schluter kerdi products on the wall over the Backer board- you are fine. You definitely want to insulate and airseal behind the shower and tub.

Right, air seal and insulate in the stud cavity, I get that. I was just wondering if there was something people were using to create the air barrier that was ok for behind backer board.

Easy,
 You could use an air barrier behind the cement board. But it does not gain you any advantage. As soon as you use one of the liquid applied membranes or Kerdi, you have made both an airtight seal and a waterproof layer. If air can get through the assembly- then bulk water can get through and you have bigger problems than air sealing.
 If you still think you need one- use intello plus or Certainteed Membrane.
Cheers,
Eric