On top of the pad, as I don't have it against bare skin. I've used a piece that was 4'x5' underneath both our pads, basically under our torso portion of the pad, and placed the individual pieces on top. We use thermarest neo-airs that are rated between R3 and R4. This took us below zero. The greatest gain to be had in r-value is to use the dead air space inherent in the pad sandwiched between the 2 layers of Reflectix. One layer will reflect back well, though. I've been using Reflectix commercially since it came on the market in the late 90's. I've used it for various insulating projects, but especially for radiant residential heating purposes. You can read many arguments on its effectiveness, I just know what it can and can't do from years of using it on the job and for myself in winter backpacking.