The Arc'teryx Cerium series is composed of streamlined down insulators, and the SL moniker stands for super light, so you can probably figure out for yourself what the Cerium SL Men's Vest is all about. With a face fabric made of wind blocking Arato 7 polyester and coated with a water repellent DWR finish, the Cerium SL is as functional a fall outer layer as it is a winter midlayer and is filled with a strategic combination of 850 fill goose down and Coreloft synthetic insulation to keep you warm, reduce weight, and provide all weather functionality. The down is concentrated around your core to provide increased warmth, and the Coreloft is located in areas that are more prone to getting wet, so you don't lose insulating power if you get caught in a quick storm. Coreloft isn't as light or packable as down, but the Cerium SL still weighs a miraculous 4. 1oz and stuffs easily into the integrated stuff sack, so it will never slow you down or feel bulky. An elasticized rear hem helps keep snow and drafts from invading your inner layers, and the low profile collar is finished with a soft chin guard to protect your skin when you burrow down against the cold.