Quote Originally Posted by JLMCDANIEL View Post
Face it with cement board. excellent for mounting tiles.

Thanks for your response.

Do I need an underlayment under the cement board like a plywood or OSB or can I mount the cement board directly into the studs of the knee wall? There's never going to be any load to speak of on this, but I just want to make sure I'm providing enough rigidity along the plane of the knee wall to prevent cracking grout down the line.