Man thanks for all the info! I have a DECENT pad, not great. R value of like 4.5-5. My bag is trash. Just some old giant synthetic bag. Looking to get a packable lightweight down bag or quilt. Looking at the Zpaks 10 degree quilt as well.I have the S2S Reactor Thermolite liner and never use it any more. For the weight and bulk it adds, I don't personally feel it does much. The listing claims it adds "up to 14 degrees" to a bag. Maybe if the bag was a pillowcase....
I would personally max out my budget on the sleeping pad before worrying about the bag. With even the most entry-level down bag, you'll lose a lot more heat through the pad into the ground than into the air. That's why so many folks are switching to quilts to save weight - the sleeping bag under you that gets compressed is basically useless. It's all about the pad - I'd rather have a 32F bag and great pad than a 20F bag and crappy pad, no matter what the temp is outside.
I just upgraded to a Thermarest NeoAir XTherm NXT pad and wow what a game changer. They claim an R-value of 7.3 but you know how those ratings get abused in this industry. Whatever it really is, it was the best purchase I made. I'm hot blooded during the day but a cold sleeper, so if I can't stay warm I don't sleep at all. That's why I got the Thermolite liner - I had hoped the extra "14 degrees" would add some range to the bag I was using at the time (a 20F survival / 32F comfort). It did not. I would be uncomfortable in that combo even at 32F. Even with the liner.
Using the same 20 bag and this new pad I just did an overnight here in CO at 8700', moderate winds (20 gusting 30), 22F overnight. It might have gotten lower than that - that's just what it was at 7am when I woke up. I slept so well I overslept! I was very comfortable all night and didn't wake up cold once.
Two more tricks you can use to get a lot more range out of a bag for minimal extra weight:
1. Down booties. They look goofy but I will NEVER be without these again. For 2oz and $25 they add extra insulation at a spot most likely to get cold - your feet. And you can even throw a pair of toe warmers in there and they hold them in place. I would rather have a 20F bag and these than a 0F bag without them - they're that good.
2. Down hood. I got the ZPacks Goose Hood, 1.3oz and $70. This is a personal choice and took 5-6 trips to adjust to. I still don't LOVE it so I only use it when I need it. But man if it's super cold out, say <15F or so, this is a life saver. If you can get it adjusted so it stays in a place that doesn't bother you (that took some fiddling) it's better than a mummy bag hood in my opinion. I'm a side sleeper and I toss and turn a lot. Quilt/bag hoods never seem to stay in place for me in a way that I don't end up smothered halfway through the night.
YMMV. But if I had to do it all over again knowing this, I'd skip the liner, buy the best pad you can afford. Then I'd add the booties and hood. THEN I'd get the best quilt or bag that my remaining budget allowed. Just my opinion, worth what you paid for it.
