insulate knee wall with faced or unface
The short answer is that if you're going to randomly add insulation to a wall, adding unfaced insulation is the best answer without more information. The kraft paper facing on insulation works as a vapor barrier and you usually want to avoid trapping moisture in a wall cavity.

If this is in unconditioned space, then definitely use unfaced.
If this is in conditioned space, then post a picture of the wall (or describe the wall assembly) and tell us where you live and we can give you more specific recommendations.