We’re big fans of the trend towards base layer and insulating garments that are both loftier (more insulating) and more breathable (less dense knit). This extends their comfort ranges across a wider variety of temperatures. Worn alone, their open knit structure doesn’t result in overheating and allows moisture to pass, but when used as a base layer under a shell or other layers, the loft and air pockets trap body heat to keep you warm. The Patagonia Capilene Air Hoody works like this, and we recommend it as an all-purpose base layer for backpackers.