I'm actually shopping for a new sleeping bag and am a bit in doubt about a question that I do not find treated anywhere: By investing an arbitrarily huge amount of money I can buy a sleeping bag to sleep snuggly down to -30° C and even below. But what about the other side? Let's say, I buy a sleeping bag with a comfort rating of somewhere between 0 and -10° C, can I use it also in a warm summer night? Or is there some temperature where it will be just too warm to use it even with zipper fully opened and used more like a blanket?
I'm aware of the fact that I will carry more weight and volume than necessary for an insulation that I would not need. I'm just interested about the comfort at the warm end of the temperature scale. So if I buy a bag with some reserve for cold fall or winter nights, will it be unusable in summer or only not the best choice I could have had? If it is unusable above a certain temperature is there some rule of thumb that says something like "a bag that has a comfort rating of X won't be of use for temperatures roughly Y degrees above that X rating."