Insulation - Active
Active Insulation is a term used to describe clothing layers that keep you warm but allow moisture/sweat to escape. These are the ideal mid-layers to be worn when slogging (and sweating) your way uphill in cool weather and can usually be used as your outer layer too for long glacier approaches.
Polartec® Alpha® is the most commonly used fabric in this category and was originally developed for U.S. Special Forces who required a more advanced insulating material in their combat uniforms. This fabric is a new technology with active insulation that regulates core body temperatures during both dynamic and static activities. It helps eliminate the need to shed or add layers while on the move, something that can be difficult to do on steep terrain and which in any case spoils the momentum of your approach. Pit-zips etc have traditionally helped to balance airflow and warmth, but are not ideal as they do not work all over and they leave surface areas exposed to the elements in the process.
Patagonia have made their own active insulation under the name Nano-Air®. It works just like Polartec® Alpha® and Patagonia's slogan, "Put it on, leave it on" sums it up nicely.
At Needle Sports, we are very enthusiastic about this new technology and virtually all of us own and use items featuring Polartec® Alpha® or Patagonia Nano-Air®, or both, and thoroughly recommend it.