Correlation between loft height, fill power and temperature rating for sleeping bags

Western Mountaineering extreme light bags have the following correlation between temperature rating and loft:
10 degrees – 6 inches
20 degrees – 5 inches
30 degrees – 4 inches
40 degrees – 3 inches
WM are a very reputable company. According to many first hand accounts here their temperature ratings are accurate for men, most women probably sleep about 10 degrees colder.

High or low fill power will insulate equally well given the same loft. With high fill power it just takes less down to get to the that loft, hence a lighter sleeping bag.

Sorry, can't answer the CLO question.

I own a WM bag, and consider the temp rating conservative. However, I believe the loft quotes above refers to total loft (ie, two layers). In practice, only a single layer will be fully lofted (the top).

I wouldn't be surprised if there was a difference (in principle) between insulation value of different fill powers at the same loft. First, lower quality down (lower FP) has more feathers, and second, the structure of an insulation material (and the size of the dead air pockets it creates) is an important part of the insulating value. It seems reasonable that 3" of 400FP would insulate less well than 3" of 850+ FP, as well as being heavier and less compressible. Comparing higher FPs (700 and 850, for example) prob. this doesn't matter.

Finally, a member here (Richard Nisely) has demonstrated that down can be compressed to some extent without losing insulation value (which, IIRC, he measures in CLO). So inches of loft isn't the whole story.

I thought he had an example that an over stuffed down baffle would increase the clo value. So if a lower fill was used that would mean the down clusters would be closer together kind of like the way an over stuffed high fill power, essentially making it warmer. So in fact a lower fill at the same loft would be a bit warmer but heavier.
Some please correct me if I'm wrong. That's just what I remember reading a while back.

Hey Thanks for the input guys! I managed to find the clo values for down in an older t
thread

800+ fill power is 1.68 clo/oz or 0.059 clo/gram

I think the reason why slightly overstuffed baffles are more effective is because the extra pressure on the down forces it into the corners and other "natural holes" and therefore limits coldspots. Try holding up a sleepingbag or quilt towardsa a strong lightsource and you'll see that the down is far from even spread out inside the baffles.

But I could be wrong, anybody have a link to Richard Niselys post? I can't find it.

Try these two:
http://www.backpackinglight.com/cgi-bin/backpackinglight/xdpy/forum_thread/12505/index.html?skip_to_post=94791#94791
http://www.backpackinglight.com/cgi-bin/backpackinglight/forums/thread_display.html?forum_thread_id=9637&startat=60