A 30 degree rating means that you can feel comfortable down to that level of temperature outside the bag. A mummy style bag conserves more of your body's natural warmth than a traditional style sleeping bag. The feeling as if being in a cocoon comes from closing it around yourself. This is my review of a mummy style bag.
ThermalC° filler provides superior insulation and warmth
Voyager's semi-sculpted hood surrounds the head with warmth
Self-healing nylon zippers with EZ Glider sliders
HiViz reflective trim for high night visibility
Draft tubes minimize heat loss at the zipper
Offset quilting to minimize cold spots
Compression stuff sack with EZ Pak makes repacking your bag a breeze
Dimensions: 30 x 90”
Weight: 3.1 lbs.
Color: Amazon Green with Blue Trim
Shell: Polyester 185T/210 Polyester honeycomb ripstop
Fill Weight: 1 lb. ThermalCº
Temp F: 30º
High Sierra Voyager 30º Mummy Sleeping Bag
Compression Stuff Sack with EZ Pak