Sorry if this has come up before but I am driving myself nuts here and haven't found anything conclusive online.
I have a 30* sleeping bag and am trying to figure out if I can get away with using it for shoulder seasons (e.g. spring ski touring) as well and then just buy a 0* bag for winter use; or if I really need a dedicated 15* shoulder season bag, as well as a 0* bag for true winter conditions.
On one hand, Andy Skurka carried only a 0* bag and a 30* bag on his 6mo Alaska Yukon Expedition. Although, he was able largely to make up for the big change in bag warmth by hiking at higher altitudes towards the end of spring with his 0* bag (which would also allow him to take advantage of his skis more), and then immediately switch to hiking around sea level when he switched to his 30* bag for summer and fall.
Weight considerations aside, is there any reason to have more than one bag at all? The worn insulating pieces should ideally provide enough warmth for you to be stationary in the late evening and early morning (for, say, 4am alpine starts), so really the bag must only account for the difference in metabolic heat produced when awake and asleep, which is a fixed amount, independent of how cold it is! Indeed, Skurka used a different combination of 6 synthetic and down worn insulating pieces to fine tune his total insulation carried for each season.
On the other hand, insulation placed in the bag is far more thermally efficient than the same amount of insulation placed in a jacket (not to mention lighter due to less fabric necessary), for the same reason mitts are warmer than gloves. So I guess anyone who owns bags colder than 30* is either carrying too much insulation (by not wearing the jacket to sleep) or does not have enough wearable insulation to be comfortable when stationary outside at night.
Seeing as how I spend the only time I am not active in my sleeping bag, and like to hike as light as possible, the logical conclusion is that I need to get a 20* bag. And in that case a a 10* and 0* bag too. Someone please tell me I am wrong! What is a sensible spacing of sleeping bags by temperature rating, and just how much does the thickness of your puffy layer influence this?