I've been using the same 32*F down sleeping bag for several years, and I generally used it with just a few pieces of generic blue foam, typically about three squares of 1/4" or 3/8". That is comfortable enough for me until the temperature dips below about 30-32*F. So, the rating of the sleeping bag seemed reasonable, and the foam padding was adequate.
Lately I purchased a new down sleeping bag rated for 30*F. However, it is no official rating, only the estimate by the manufacturer, so I don't know how much to trust it. In order for that temperature rating to be fair, it requires a thermally-neutral sleeping pad, and I am thinking that my old pad arrangement may not meet that. Of course, that might make the sleeping bag seem cold.
The question is: How much R-value do I need in a foam sleeping pad?
This is for summer use, but the temperature might drop below 30*F. I have down garments that I can use to get through the cold night, but I am trying to focus just on the pad R-value. I'm thinking maybe R=2.