...if you already have an adequate summer down bag, you can also go the synthetic bag overbag route. this has the benefit of moving the dewpoint out of the down bag and into the synth, which makes for much better moisture management inside your sleep system. Read up about it, it is largely a Canadian 'discovery' i guess you'd call it. certainely the system of choice for well prepared expeditioneers.

read up about dew point and sleep bag insulations. its fascinating.

a relatively inexpensive synthetic overbag should give anyone with a summer down bag a very good winter sleep system.