Note that there are multiple ratings in the EN system, even though those in the US who claim they use it list only one.
Good website. • The EN 13537 Upper Limit or Maximum Temperature is the highest temperature at which a ‘standard’ adult male is able to have a comfortable night’s sleep without excess sweating.
• The EN 13537 Comfort rating is based on a ‘standard’ woman having a comfortable night’s sleep.
• The EN 13537 Lower Limit is based on the lowest temperature at which as ‘standard’ adult male is deemed to be able to have a comfortable night’s sleep.
• The EN13537 Extreme rating is a survival only rating for a ‘standard’ adult woman. “In the risk range a strong sensation of cold has to be expected and there is a risk of health damage due to hypothermia.” This is an extreme survival rating only and it is not advisable for consumers to rely on this rating for general use. The best guideline temperatures for purchase decisions are the TComfort and TLimit ratings.
It appears that in the US, for the most part, the only one of these ratings used is the "Extreme" rating.
There is still a lot of subjectivity in all this, because each of us has an individual body "thermostat." However, at least with the EN 13537 ratings, there is some objective basis other than the wishful thinking of the manufacturer!
For me, I'd want the "Comfort" rating plus at least 5*F (I'm a really cold sleeper). Most men would want the "Lower Limit" or better Why would anyone would want a bag that would just barely keep you from dying from hypothermia (the "Extreme")? To quote "The King and I", "'Tis a puzzlement!"
Personally, I do fine with a 20* F (-7*C) Western Mountaineering Ultralight--assuming I have adequate insulation underneath on cold nights! I've had it on warm nights in the Cascades (after 90+F* days), and I haven't yet been too warm in the thing. It has a full-length zipper, so on warm nights I can use it as a quilt or even sleep on top. Generally in the West, certainly in the Cascades, nights get cool. I have started the night on top of the sleeping bag and ended up inside and closing most of or all of the zipper by 3 am. This was also true of the three nights I spent in Michigan's Upper Peninsula a few years ago. I suspect that if I were only camping in Ohio or southern Michigan (or points south), though, I'd want a bag that wasn't so warm!
If you are going to be backpacking into the "shoulder season" (September and October) when nights get really cold, 20* (assuming the bag is accurately rated) is the minimum. This past October I spent an 18* and a 20* F night in my WM Ultralight. I was wearing my insulating puffy jacket (Montbell UL Thermawrap), base layer, balaclava and, for a vapor barrier, my non-breathable rain jacket and pants. Unfortunately I did not have enough padding underneath. I therefore was in the odd situation of being too warm on top while my back muscles were shivering like crazy! If I'd had adequate padding underneath, I definitely wouldn't have needed my puffy jacket. With lots of insulation underneath and the puffy jacket, I'd undoubtedly have been fine at 10* F. Extra foot insulation helped, too--I had fleece sleeping socks over a dry pair of Smartwool PhDs, and my feet were resting on my nice warm dog!
I couldn't copy the diagram from the site I linked above, but if you look at it you'll find that the difference between a woman's and a man's being comfortable is 5 degrees C. At last, scientific recognition of what most married couples already know!
