Last year I bought an LL Bean -20 F. goose down mummy bag. Excellent design and quality and it was on a huge one-day, double sale for $265. or so. Free shipping. It was fine at -10 F. but I have a chance to go to either Montana or North Dakota this winter and I'd like to test it to -40 F. (in my solo tent) if I get the chance. So here is my setup: 1. -20 F. LL Bean goose down bag 2. Thermarest TrailPro self-inflating mattress 3. Ridgerest closed cell mat (under the Trail Pro) 4. polar weight polyester long johns 5. fleece balaclava 6. heavy sleep socks over thin poly liner socks 7. Eddie Bauer light down jacket 8. Montbell light down pants 9. thin knit polyester liner gloves (they really help) 10. Psolar face mask (warms air W/ layered copper screening that's warmed by exhalations) This is now out of production. ** May keep my Norwegian wool sweater on under the puffy. ** I might try using a Vapor Barrier Liner of pants and shirt made from waterproof silnylon. I'll wear it over the long underwear for comfort. This keeps heat-robbing sweat vapor out of the down bag and clothing. On a multi-day trip a VBL suit in these temps is mandatory. So with those clothes I think I'll be able to take my -20 F. bag to -40 F. I've taken my +20 F. Western Mountaineering Megalite down bag to +5 F. with synthetic puffy clothes with good comfort. But,yeah, -40 F. may just be a different animal. Only been in -40 F. once in my life in 1979. I've slept in -22 F. in a snow cave but they are much warmer than a tent. Eric B.