Lower body insulation down to 20 degrees F

I did fine with RAB Vapour-rise pants down to the low teens on WT3. I wore only those for trekking, hanging out in camp, and sleeping.

If your lowest temp is 20 degrees, I'd use a thin wool baselayer, powerstretch over that, and then a good softshell pant (Mammut Champ for me). The breathability lets me wear this while moving and it is warm enough for about 30 minutes in camp- then I'd want to get in my bag. I would not carry anything extra for my legs unless it was going to be below zero. Instead, get a good pair of down booties…

Intrestingly, it was 20 degree or so here a few nights ago when my friend talked me into a midnight climb- I wore the above setup and was warm while sitting on very cold rock at the first belay for 30 minutes. Then I got chilly… and told my friend to climb faster.

If you are looking to drop down below that Patagonia has their Micro Puff pants on sale. Side zip and pretty warm for sitting around.

I use Patagonia R1 pants, Mont-Bell Thermawrap pants, and Marmot Precips as my lower body insulation down from the mid twenties to single digits.

If you happen to be an XL or XXL Patagonia has both their R1 pants and Micro Puff Pants (warm, but a little heavy) on sale CHEAP.