I got a pair of Montbell UL Down Inner Pants at the beginning of the Pacific Crest Trail because I assumed it would be so cold at night that I might need some extra insulation to bolster my sleeping bag. I went through the first 2100 miles of the PCT without even carrying them in my pack. It just wasn’t cold enough to bother with them. I felt pretty stupid for having spent that money.
That changed in a big way at the end.
In Washington I finally packed the down pants and began to wear them every night. Montbell’s sizing, which is on the snug side, fits me perfectly, so wearing them inside the sleeping bag didn’t feel constrictive at all. It just felt like a little extra puff in my sleeping bag. Outside the sleeping bag, it didn’t feel much different than a normal pair of pants.
I used the down pants again on Four Point at the end of February. I wore them inside my sleeping bag on really cold nights, and also under my soft shell pants when in camp for some extra warmth.
When you look at these down pants, they don’t look like much. They’re super thin and feathery light. They don’t look like they’d be very warm. I found that wearing them alone, they make a small difference. They’re warmer than normal pants, but it won’t blow your mind. However, if you wear them as supplemental insulation, they’re amazing.
Wearing them inside a sleeping bag made the sleep system noticeably warmer, and more comfortable on cold nights. Wearing them under my lightly insulated soft shell pants (which I sized a little on the baggy side for this purpose) is also fantastic as extra insulation. Of course, you wouldn’t want to hike in them, because that would make you sweat like crazy. I find legs don’t need quite as much insulation as the upper body, anyway, especially when moving.
I had resisted getting down pants for the longest time, but I’m glad I did it. I’d recommend them if you plan on doing serious cold weather backpacking, or if you need to extend a three-season sleeping bag into the late part of autumn. For day hikes and warmer parts of the year they’re probably overkill.
Besides the Montbell pants, you might also consider Feathered Friends (heavier, but much puffier and full zippers so you don’t need to take off your boots), Western Mountaineering (similar weight and fill, but different fit), or Nunatak (wicked puffy, and built to order). All good companies, all good products.