I had a Mt'n. Hardware Phantom 32 F. bag W/ a 1/2 zipper but took it back to REI W/O even camping with it. It had too little loft and I felt the 1/2 zipper restricted its versatility. I was correct.

I got a Western Mountaineering Megalite,30 F. bag with a full zip and great loft. It has been used to 22 F. and was OK as I had on light polyester long johns and a stocking cap.

Also I used it ZIPPED OPEN as a quilt on 75 F. nights and loved the comfort of the quilt function
with the foot hooked over the foot of my old full length UL Thermarest.

Therefore I'd strongly recommend a full length zipper and a bag with good loft, like my WM bag or a Feathered Friends.

Eric
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"There are no comfortable backpacks. Some are just less uncomfortable than others."