This Modular model has an intriguing structure: whether you sleep on your back or turn sides, the bag stays still, coupled with your Ultrasphere airpad. What would be the back panel in a regular bag, is free of insulation in this product. When using a regular bag, your bodyweight constantly renders a considerable part of the insulation useless, while quilts and this product allow the insulation to stay lofty in every part it's used in. Mythic Ultra series bags take advantage of both top notch down and a new Thermo Ionic Lining technology which reflects your body heat back at you. This technology is based on a super light layer of titanium incorporated in the fabric's fibers, offering extremely low weight and a great addition to warmth. Due to its unique structure, this bag is not standard tested for temperature ratings. Rab's estimate on its limit temperature is 0ºC when used with an airpad with an R value of more than 4. This would place the comfort temperature somewhere around +5ºC