Join Date: Jun 2003
Location: Vancouver, BC, Canada.
Interest: Backpacking, scrambling, skiing, mountain biking, skim boarding, running
A synthetic overbag will improve the performance of your down bag both by adding insulation (same as a liner) but also by moving the dew point out to the synthetic insulation so your down stays drier. Another alternative to add versatility to your system is to use a bivy bag. Not as warm as an overbag but it definitely is good for a few degrees extension to the temperature range.