Yeah, you'll likely only get half an R-value with that thin of pad. There have been some studies by the US Military (via Richard N) and humans seem to be really only notice a difference in temp between roughly .5 R. That is to say that you may or may not feel a difference and if you do it will be slight. Our bodies really just register a few types of temperatures. On the lower side that's roughly:
Cool, Cold, Bitter, and "There isn't a word because even the Eskimo's teeth are chattering too much to speak"
The numbers are great but our bodies just aren't that finely tuned to measure temperature =/