What I am wondering is what R-value do I need for say temps down to 35 or temps down to 15? I have an uninsulated pad and a very high end 35 degree bag but I am not very comfortable to 35 in it. I need an insulated pad. In the interest of weight savings what R-value would I get by with.
I know people are different but I would think there is a range most people find good for temps in the 30's vs temps in the teens.
If you are sleeping in a 35 degree bag at those temps you will get probably get cold, you could try something like a Sea To Summit Reactor bag liner for added warmth.
Or try a closed cell foam pad in conjunction and see if you still get cold. There are a ton of factors to getting cold in a bag such as food, sweat , were you cold getting in and finally bag and pad. I almost always evaluate every bag at home , warm and full . In the field results can vary, based on a lot of what you do. I prefer to have 15 to 20 degrees of leeway in the field if possible . I find I sleep pretty close to rating on most good bags given warm , fed and dry . Take those away and it varies
IMO you have to have a good insulated pad. I have froze in the middle of summer with a good bag and an uninsulated pad. I use an Xtherm now for everything.
Also, keep in mind that the R value on the pad will only affect your warmth on the bottom of your bag. If it is only a 35* bag and you are using it in 20* weather your ground pad isn't your biggest problem.