Recessed Shower Shelf

I am going to tile my bathroom. I took out a small square of insulation and 2x4 to make a shelf behind. I made a tray reccessed an exterior wall. Does the board hardibacker keep the outside wall coming into ruins? It goes outside wall, black felt, the board hardibacker, thinset, tile.

It depends on where you live! If you live in a cold climate, it would not be a good idea to make a nest in an exterior wall !!!!! I've been a tile installer in the North-East for 29 years. What we found is that if you remove an interior wall of an exterior wall to a niche that you get condensation on the tiled wall. The reason is, if you're in the middle of winter and it's very cold outside, you have a warm environment! Even with your tar paper, Hardi backer board, thinset and tile on the wall still get very cold at creating condensation in and behind this niche. Remember that you are only talking a few inches between the interior of the recess and the exterior wall, including shingles! I would either eliminate the niche or move it to an inside wall. But my "safe than sorry" opinion. Good luck!

You're OK unless you're in a cold state. Worry of freezing and address if necessary with insulation

You should be fine as long as you tape and thinset all the joints. I put a couple of layers of the Red Guard on the cement and duct tape for my recesses. Also be sure to caulk all the corners using 100% silicone.