Polarguard , Quallofil and Hollofil are all excellent fillers. In use, you won't be able to tell them apart. Yet like any synthetic, it retains its loft when wet, making it suitable for conditions in which down would collapse. All EMS synthetic bag shells and linings are of 1.5 oz. high-count taffeta fabric. Mummy bags are the warmest, simply because there is less space for your body to heat.
Semirectangular bags are a good compromise between warmth and extra sleeping room.
Some other features also determine the efficiency of a sleeping bag. Most mummy bags have a boxed foot section, which helps keep your feet warm by keeping the insulation in place and lofting freely. Almost all sleeping bags have a draft tube behind the zipper to prevent air leakage. Winter bags often have a filled collar to snug around your shoulders to trap warmth. These tubes are sewn separately to the liner and shell to prevent thin, cold spots.