First, indeed glad you aren't trying to go on a budget to minus-15° F
Sounds like you are doing what I pack for: hope for heated tents, bring enough for cold camping. I have been to slightly minus zero in a good cold camp tent (tent+fly, NOT a one layer tipi! That trip literally almost killed me) with just: layers.
Double up on what lots of folks said: add more stuff over getting a super-duper cold wx bag. Add:
- A bivy sack. Shell outside will add a lot, and not just for warmth directly as a layer but wind and moisture so keeps the bag more effective. Even a heated tent can have breezes if not set and sealed right, the ground can be wet, etc. Bivy helps.
- More ground insulation. Below about freezing I put folding foam mat down below my whole rig, which normally is just an inflatable (REI, but think Therm-a-Rest). Surplus USGI foam mats are cheap, I'd get the Z-fold one as it packs small, etc. The gold standard to me is the German folding mat if you can find one though.
- Liner. Thin, often slightly stretchy layer. Also keeps the bag from getting dirty or wet from you, so works better on longer term trips as both those kill the effectiveness of the insulating layer.
- Clothes. I have a separate compartment in my clothing bag of sleep-only clothes. A loose fleece sleep shirt, a watch cap, waffle pants.
- Camp booties. Feet are far from the core when sleeping, so get cold. I have gone all the way to having (when cold) dedicated in-bag camp booties. Don't walk around in these, and get the bag dirty. I sleep real well with these, can go down a lot colder with warm feet because that makes warm legs, etc.
Each layer adds a few degrees. a 20° bag with all the above will be hot at 20°, get you down to 0° easily. Etc. Some depends how cold you sleep also, of course.
Also try it out and fix issues. I can't sleep head inside (which is good to keep moisture down anyway) so can have a gap between my face/neck and the bag. I seal that with a "scarf." I wrap my puffy coat around my neck at night, serves as a sort of pillow, a place for my hands to keep warm when they do their own thing in the night, and keeps down system leaks. But generally: practice and find what else you need that you may already have, and utilize it.
I'll also say: don't necessarily fear down. I did for decades, but of late they are doing clever things with baffling, better bag materials, and even down coatings, so the issues of compressing, shifting, and becoming ineffective due to moisture are rapidly disappearing vs synthetics.
My tangential solution to the original question is: quilt instead of bag. See, you get no value from the part of a bag you lay on so why carry it around? Quilts are not quite what they say they are, most have zipped up toe boxes, and straps to tie to the foam pad so: it is like climbing into a sleeping bag, and rolling around won't let the quilt fall off you. I move a lot, sleep on the side, had a quilt setup for about 4 years now. Slept it comfortably in, among other places, Iceland.
Example only (I do not have this one) of a totally legit brand (Sierra Designs) quilt rated to 20°F for under $200
Amazon product ASIN B07P2JRBS8
www.snipershide.com
varuste.net
armynavyoutdoors.com
