What Montana-eer said about -20 F. bags.
As a former Norcic (XC) Ski Patroller and Alpine Ski Patroller (same training as EMT) and an Army ROTC winter survival instructor I can say that if the temps COULD be -10 F. you do need that -20 F. bag - and that winter mattress with at least an R 5 rating.
And you do need mittens in sub-zero weather even if you also carry gloves. Some winter backcountry skiers & hunters just get an XL size mitten shell to go over their gloves. That works too if your gloves are warm enough. Also I wear knit glove liners to sleep, as well as a light fleece balaclava.
But VBL (Vapor Barrier Liner) socks in your insulated boots are often either overlooked or plain unknown to hunters. As mentioned in other posts on LRH I prefer US Divers brand of 3 mm thick closed cell neoprene VBLs over thin Poly liner socks. They have shaped Left and Right feet and are factory seam sealed. They totally keep sweat out of your boot insulation, period.
No other socks needed if your boot insulation is good enough. Without VBLs your boots get progressively colder as the hours pass and more and more of your sweat compromises the insulation.
EricB.