Bags have general ratings but every person and their sensitivity to cold is different...as are the conditions such as dry or wet cold for your choice in bag fill material. Most are rated for short periods at that rating but some things you can do to help the situation that makes a significant difference.
I personally like 0* bags for all around use...cold weather to 15*F and up they are very comfortable....over 50*F I sleep on top under an old poncho liner. I have a Mountain Hardware and love it...one of my best all around bags and I have used coleman, every military bag made in a 20 year career....and this MH with synthetic fill has been my best all around for warmth, durability, lightness and able to DRY QUICKLY if wet...big issue if in areas that have rain or if traveling on water by canoe. I have dried out my bag completely by fire one evening after a torrential downpour caught us unguarded with my bag airing over a line when out hunting and it was clear skies...except for one little cell that went over camp. Temps that night were 35* and 3 hours around campfire with bag over lap facing fire and rotating dried it completely.
Some things I do...based on experience and learned in survival schools I attended in military:
- I either sleep naked or in underwear as any clothing you have against your skin (except silk or pure synthetics) will hold moisture that comes from your body during night and chill you down.
- I wear wool socks on my feet...and a wool or synthetic beanie on my head while sleeping. Keeps extremities warm and vastly improves comfort....put on dry socks though before bed not ones you wore in boots all day.
- Carry a box of the small hand warmers you get from walmart in orange packs. An hour before bed throw a single one down in foot well of bag...you will be so toasty after you get in and zip up tight that you may have to vent the bag during night all the way down to the teens.
- If using an self inflating mattress pad like my Thermarest XL (extra long and wide)....just as you enter bed...blow the final air into it for firmness...the warmth of your breath will warm inside of pad and help keep cold from you better as well until you are settled.
- big thing...go piss before you go to bed...or keep a gatorade jar in tent...having to get out of bag and go visit the wood line away from camp will chill you and cause you to have to warm up again. If you do leave bag...be sure to close it up as it will keep your body heat in surprisingly well until you return.
I have been in -5* with a 0* bag and been toasty all night following these rules. Try them out and sure you will find similar results. Good Luck.
as far as bags...any of the big names known for expedition use will work...Wiggys, Big Bertha, Marmot, Mountain Hardware, North Face or Sierra Designs....get a copy of Campnoor, Backcountry.com ...or go to
MooseJaw Moosejaw.com and they have last years models on sale and the savings can be significant.
One last thing...get one of the right size. Get too short and restrictive and you will never be comfortable and get it too large and it takes longer to warm...much like tent sizes...smaller in colder weather to help hold heat nearer your body some.
I am 6'3 and 255 and make sure I get one large enough that I can have a bit of room at bottom of bag to put my jeans and shirt for next day to keep them warm....and enough shoulder room to rotate and turn on my side if I want while sleeping. Here is my bag...but is only one of many many options out there...but pay attention to the dimensions as well.
http://www.moosejaw.com/moosejaw/shop/product_Mountain-Hardwear-Lamina-0-Sleeping-Bag_10207862_10208_10000001_-1_