I do a lot of snowshoeing and winter camping in the Colorado Rockies. Why wouldn't a synthetic bag rated at -30 not be sufficient in negative temps? I used the Lamina last week in a snowcave and 4 season tent (both times in a bivy) and woke up freezing. The temps got down to -12 but I thought in a -15 bag I would have been ok. Is down the way to go? I was told down was the worst option for winter camping because it could get wet from snow and fail to retain warmth, I just want guidance on the best option for winter warmth in snowcave/tent as well as cut back on space/weight.

Synthetics for damp conditions, down for dry conditions. High altitude and cold climates are typically drier, so down is preferable. Down bags also last longer because synthetic fills (like thermic micro) degrade faster. As far as staying warm while sleeping in the Rockies: the sleeping bag is only one part of your sleep system. Your sleeping pad is every bit as important as the bag you're in. Closed cell foam pads are cheap, lightweight, waterproof, versatile (can be used as splints, seats, emergency sleds, etc.) warm, and indestructible but they are bulky and not very well padded (check out a ridgerest). Insulated air mattresses are more comforatble, more expensive, pack down smaller, can leak/rip which makes them useless, are typically heavier, and require more care (I use the downmat 9). Clothing is equally important. Companies rate their bags based on different assumptions about what the sleeper is wearing.

Colorado is generally a lot "drier" of snow than other places. Generally you need to manage your internal moisture more there than exterior moisture. Don't go to bed with too much clothing or you'll just sweat and wake up hours later moist and freezing. And if you don't need to don't curl up in your bag totally zipped up with the hood cinched tight. Did you build your snowcave correctly? It shouldn't be that cold inside, the sleeping area should be a bit above the entrance to get the full protection of one. If you are hiking a bit to camp I'd get a down bag. If you are camping by the road or in your backyard go with a cheaper and bulkier synthetic.

When looking at the temperature ratings for sleeping bags you need to keep in mind that there is no standard that defines what a 0 degree bag is vs a -10 degree bag vs -20, etc.... There are plenty of reasons for this, but one thing for sure is that different people need different amounts of insulation to sleep comfortably at different temperatures - there's lots of variables at play here. As for the difference between synthetic and down bags, I think the accurate statement is that if you have a down bag and a synthetic bag that both weigh the same amount, the down bag will be warmer and pack down smaller than the synthetic bag. It is certainly possible to make synthetic bags that are warm down to the same temperature rating as the warmest down bags, but they will be heavy and bulky. As you point out, down bags do have their drawbacks because, once wet, down loses its loft and no longer provides any warmth. Its necessary to take many precautions when using down sleeping bags (such as lining the stuff sack with a trash compactor bag to keep water out). Honestly, I've never slept in a snowcave, so I'm not sure how safe it is to use a down bag in this situation. There are down bags though that have waterproof shells - Mountain Hardwear makes a few; other companies use materials such as GoreWindstopper or Epic which are highly water resistant.