If you're looking for a synthetic sleeping bag for sub-zero temperatures we like the Mountain Hardwear Lamina Sleeping Bag. Not as fussy as down the synthetic Temperlite fill features bonded fibers that create heat-trapping bubbles for better thermal efficiency. The Lamina design also features bi-directional insulation to eliminate gaps and cold spots as well as body-mapping to concentrate warmth where we need it most. The bag's mummy shape limits bulk while holding in more body heat while top and bottom draft collars and a fully cinchable hood protect our head and prevent heat from escaping up top. Thermally efficient--and almost a pound lighter than its previous iteration--the Lamina -20F is a synthetic solution for our winter camping and mountaineering expeditions.