I have a 1.5 story house with knee walls in the finished attic that has faced r-13 insulation in the 2x4 frames
My plan is to keep the current insulation in place and then use r-19 unfaced insulation and install it horizontally attaching it to the wall frames. Since I use the attic for storage, I will wrap everything in tyvec (or other house wrap material).
Sounds like it would work fine. Make sure you have a plan to insulate the sloped ceiling as well as the floor framing beneath the kneewall. If you only insulate the kneewall, the overall effectiveness will be limited.
I think the house wrap would compress the new insulation, losing some of it's R-value, especially when it sags. As stated by mpepin, insulate the floor 12"under the knee wall, add a cardboard baffle (under the wall in each ceiling joist bay) to stop any conditioned air movement to the attic space there. http://www.homeenergy.org/archive/he...95/950309.html
Be safe, Gary