If there is room in your outer/shell layer, I’ve found that polypropylene Polar Fleece Thermal Sweatpants can add a lot of warmth. They seem to breathe well and they add a lot of insulation. They do add some bulk but for $20 they’re a real value. When it’s super cold or it’s pretty cold and I’m planning on doing a bunch of sitting or glassing, I’ll put these fleece sweatpants on over my thermals and under my shell layer.
