Yes, stacking sleeping pads will increase R-value. The R-value is a measure of how well a material resists heat flow. The higher the R-value, the better the material is at insulating. Sleeping pads are typically made from closed-cell foam, which has a very high R-value. When you stack multiple sleeping pads, you are essentially creating an air barrier between you and the ground. This air barrier will help to keep you warmer by trapping your body heat.