Thinking of ditching WM Ultralite 20 for ZPacks 20 Bag. Good idea or not?

This is the only gripe I have with my Zpacks 20. My Katabatic Alsek has a much nicer footbox. I asked Joe about shaping the footbox, but he said that it would be difficult to change the pattern of the bag so recommended that I try a wide and maybe have it slimmed in the body area. I think I will try to live with the narrow footbox. I don't know how the old one was, but there is definitely not a lot of room in the footbox. When I lie on my back, my size 9 1/2 feet are definitely poking out. In other words, you can see my toes. Now since I have a LONG size, it's not as a bad of a problem. I tried a REGULAR length, and it was too short. And I'm only 5'7". I've thought about going with a wide instead or having the bottom of the bag made wider like Joe suggested, but (a) I don't know if I can afford to upgrade right now and (b) it would add weight.

Other than the footbox, I think the ZPacks bag is hard to beat. I owned a WM Summerlite and the loft was much less than the Zpacks 20. The Alsek has slightly more loft (and more oz of down) than the Zpacks 20. I would say that the Alsek and Zpacks 20 are just about the same in terms of warmth, since the Zpacks 20 is narrower and zips up on the bottom. The Alsek is simply a much nicer bag, but I save about 3.6 oz with the Zpacks 20 if I recall correctly. For me, thought, the Zpacks 20 is a 30 degree bag, not a true 20 degree bag. If I was going out in the fall/spring/winter with temps dipping down to around 20 degrees, I would get a Zpacks 10. I would say that the Zpacks 20 is an all-around summer bag (and winter bag depending on where you live).

As far as deciding between bags, you could do what I do/did:

1.Wait until the wintertime when it's as cold out as you would plan to use the bag.
2. Set up your tent in your backyard.
3. Purchase 3-4 bags and hope your wife doesn't find out.
4. Now try sleeping in each bag and see which one keeps you the warmest–or the most comfortable.
5. You can do this on successive nights if you like to play scientist like me to get repeated trials.

I have done this multiple times to test out different variables (clothing, hat, tent closed/open) to push myself to the extreme to see what I can tolerate. The results are quite useful, I think. Anyway, you have some data points to go off of in the backcountry.

OP – Be careful in thinking about warmth. I've used a friends 20* bag a few times and was always warm in it. It is a larger bag than I need because I am significantly shorter than he. I bought a 35* bag in the correct length by the same manufacturer and have been cold a few times and on the edge a few others as well. And from what I could tell based on temps recorded at a weather station near where I camped, the temps were very low 40s to high 30s at most, yet the 35* rating wasn't sufficient for me. I continue to use the setup, now with a 12 oz hooded down jacket, but am doing research now because I want to have a better comfort zone against the nights that drop below 40*. I think for me I need a 20* bag. It can get the job done on any trip I'll take it on, since I'm not camping in the shoulder season, much less the winter. Sleeping bags are like shoes, if you don't have a nice walk, you won't have a nice trip. Having to endure the cold all night is just the same.

I second that thought. The worst thing is shivering all night just because you wanted to save 3 oz on insulation. I wouldn't consider anything less warm than the Zpacks 20 for summer mountain camping (at least in Colorado, where I usually go). I might be a cold sleeper, but I want a 20 degree bag if temps are around freezing.

If I were to do a thru-hike with some serious elevation in the shoulder seasons, then I would go with a ZPacks 10 degree bag. But maybe that's just because I have a very low body fat percentage (around 8%).