Eric, where did you actually find the wm en ratings? Thanks for the correction. So that makes my body, despite its age, still a reasonable judge of bag temp, since I thought 32F for the summerlite might be pushing it. But it's in the ballpark, maybe within 8 degrees, the bags I'm noting are bad are not in the ballpark, they are maybe 15, 20 degrees off in rated temp, and far less loft. Keep in mind the summerlite has continuous baffles and you can rearrange the down to be more on top if you want. Looks to me then like nf/lafuma are using lower mid ratings, and wm using upper mid, but because you can move the down around in wm, I suspect you can get fairly close with them if you do it right.
Let's see what bpl says re sleeping bag temperature ratings.
Consequently, the best we can do is to measure the loft and key performance features of the bag, comment on our subjective experience near the temperature rating of the bag, and ask you as an informed consumer to compare it to the loft and temperature ratings of bags from other manufacturers in order to make a wise decision. Nevertheless, we recognize that some guidance in correlating a bag's loft to its warmth is useful to consumers. Table 1, below, represents our best effort at such a correlation:
Temp (°F) Loft (in) Temp (°C) Loft (cm)
50 1.2 10 3.1
40 1.5 4.4 3.8
30 1.8 -1.1 4.6
20 2.2 -6.7 5.6
10 2.6 -12 6.6
0.0 3.0 -18 7.6
-10 3.5 -23 8.9
-20 4.0 -29 10
One key thing to understand about sleeping bags is that if they say it has, say, 3.5 inches loft, that counts top and bottom, and it's the top that keeps you warm, and the sides, so that actually means 1.75 inches, which is in the 40 degree range, as you see from the chart. My guess is that good bag companies that use good down and list the loft know that it's about the loft and down/fill quality. Compare similar quality down and loft, and you'll probably be in the same temp ballpark.
Keep in mind, if you aren't aware, that Elijah is 14, and trying to get into backpacking, so when I say, keep it basic to avoid confusion, that's really what we should be doing. And since loft is what bpl offers as a reasonable tool, just like every other authoritative source, like military specs, etc, that's what a beginning backpacker should use, and grown up ones, and I believe it is largely what they use, when you strip away the extra words etc.
We can debate the fine points of heat retention of different fill materials, humidity resistance, and construction methods to our heart's content in some more focused and dedicated threads in the future, and dig up older ones from the past.
By the way, this type of fine grained analysis is what brought me to bpl in the first place, and it was the absurd failure of some new lafuma and nf bags to even remotely approach their alleged ratings that made me start realizing that it was time to learn what's really going on here, coupled with a trip to Northern Norway, where very well intentioned but totally clueless relatives ended up freezing my butt off camping.
Elijah, you can learn a huge amount from these threads, if you read through them, but it's also important to realize that often what appear to be debates or disagreements are actually just people fine tuning and adjusting their understandings. This is how gear, and other, geeks, do things, and it results in amazing information, I have certainly brought my own level of understanding up by a very very large amount by reading these forums now for some time.