Thanx Chris, good to see I'm in the ballpark for the down calculations.
With regard to Climashield APEX, I originally built my quilt using just 5.0 (clo 4.1) assuming it would be good to around 30 degrees. Well, the first night I took it out last summer over 11k, the over-night temperatures dropped to the high 20s. (Last summer in the high Sierra never seemed to warm up.)
Suffice to say, I froze my ass off. It wasn't just sort of cold, I could feel the cold literally seeping through the quilt. Of course, I had all my clothes on, beanie, etc, but by 5am, I said screw it, and built a tiny fire to warm up a little bit. (For those that don't know, fires are prohibited over 10k.) Since I was up there specifically to fish, I was active enough by 6am to warm up and stay warm.
Now, granted, I had an open-ended tarp, so I didn't have any "tent effect" to help contain some warmth, but it was also a cloudless, windless night, so it was just still, cold air.
Anyway, when I got back home, I did some more research, and ended up re-reading many of Richard Nisely's threads, including these two:
According to RN, you need 3.88 clo to achieve an EN rating of 50 degrees. Btw, the first thread listed above got a little heated, with Roger mentioning 'dodgy claims'. After some more research, I came to the conclusion that I really did need a clo of around 6.2 to get to 30 degrees.
Since I had made my quilt in a duvet style, it was easy enough to open up one-end, flip it inside-out, and sew on another layer of 2.5 oz. Voila, 7.5oz of Climashield APEX, good enough for 30 degrees.
Being the eager beaver that I am, I got to test out the new & improved quilt the next week back up in the same general vicinity. As this was the summer that never warmed up, the first night @ the trailhead had people coming out who were describing sub-freezing temps, hail showers, etc.
Well, I got lucky as the storm cleared out, but each night the temps were definitely falling into the high 20s. Once again, I had only my tarp (no wind, dry conditions), all my clothes on, etc, so all things were equal. And the quilt? Worked like a champ - even though I was definitely feeling some cold by dawn, it was nothing one couldn't just curl up and go back to sleep.
So, based on my own (subjective) experience, combined with more formal supporting calculations, I feel that you really do need 7.5oz (1.8" loft, 6.2 clo) to achieve a 30 degree syn quilt/bag.
That being the case, at least for my own personal experience, I was wondering if an 11-12 oz difference between down & insulation is reasonable.