The actual Inertia X frame pad is just an empty airspace, so it has an R-value of 1.0. According to the literature, any air space from 0.5" up to about 4" has an R-value of 1.0 (scroll to bottom of link).
Making things a bit more confusing is the argument that a sleeping bag will loft into the cut out pockets and boost the R-value in these areas. This may be true, but I don't believe that the effect is very significant because no cut outs exists under the sleepers most critical areas (ie. shoulders and hips) so you definitely only have R-1.0 under these key areas. For me, the hips are the area where I want the most R-value. Sleeping bag users may get a higher R-value under their legs or lower back, but I doubt the overall result is much warmer than R 1.0.