We had our contractor tile in the window in our shower. It started cracking almost immediately, and he's filled in the cracks a couple times. It keeps cracking. I assume due to the temperature differences between the shower and the window, and the grout isn't elastic. Can I use a tile caulk to file the crack? Or should I just keep filling the cracks with caulk?
That should have never been grout, joints between walls and other objects should be caulked because movement in those joints will cause the cracking that you now see, and the cracking can lead to water getting behind the tile. This also goes for inside corners in a shower, and the joint between bottom of the wall and the floor or tub..
If you want to re-try grouting, try with a latex additive in it. Sometimes you need to add it yourself when mixing the mortar. The latex is supposed to add flex to the mortar so changes in space, like between wall, cement board, window etc. due to slight movements from temperature or moisture changes will not cause it to crack.
Please note, I only say this because I am currently re-doing my bathroom and consuming as much information as I can about all of this. I cannot promise using modified grout will be the solution. I just know I was advised to use a latex additive for a similar location in my bathroom where the wall and a ledge meet.
I had this problem the other day. What you could do to keep it looking nice is fill the cracks in with more grout so that it looks perfect and then put clear caulk over top or dig out all that grout and caulk it with white caulk. There is special caulk that is mildew resistant made specifically for the bathroom.