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I may have a chance to go to either Montana or North Dakota this winter. If so I'll have the opportunity to test my -20 F. LL Bean 750 fill goose down bag.
So... if I go up to the Great White North I want to sleep in -40 F. weather - in my -20 F. bag. Yeah, a 20 F. problem.
My avatar shows me in my Eddie Bauer down "sweater", which I could wear in my bag. I've tried it and there is no problem with copmressing either bag or jacket.
Now I'd like to add Montbell's lightest down pants as well. Haven't bought them yet but I'm close. I do have synthetic Thermolite Ultra insulating pants but naturally, they are much heavier and bulkier.
My clothing "sleep system" for -40 F. in my -20 F. bag:
1. Polar weight polyester base layer
2. polyester fleece balaclava
3. PSOLAR breathing mask (to warm inhaled air & protect my nose/chin areas)
4. heavy wool/acrylic sleeping socks over thin polypropylene liner socks
5. above mentioned down puffy layer
6. thin polyester liner gloves (yeah, it really helps)
-> I think this would be enough extra clothing since the bag has a great collar and hood, with elastic draw cords on each.
BTW, my experience in these cold temps, like the -22F. I've camped in, is that a pee bottle is an absolute necessity. I'm now looking for a wide mouth collapsible bottle with a VERY secure lid. (Anyone know of such a thing?)