The Blue Springs series sleeping bags are made with MicroX+ insulation. MicroX+ Insulation consists of multiple denier staple-length fibers that have a siliconized finish for maximum insulation loft and compactness. It is an ultra lightweight insulation that has a tremendous weight-to-warmth ratio. The Blue Springs uses a 2-layer offset construction sometimes called a "bag within a bag." The contoured hood and mummy shape helps seal up your warmth and keep you warmer. If you choose the 35� option you will get a second zipper that allows you to zip the bag "half open" on each side to allow venting or sitting up which is a neat feature. The Blue Springs is a great backpacking sleeping bag to take along on your hikes that won't take up all the space in your pack or weigh you down.