A sleeping bag rated at 0* (for men and warm sleepers) should be good for 10* for women and those who sleep cold. What kind of bag do you have? I suspect the rating is way off!

As mentioned, I use the dry bags rather than a pack liner. However, I've been told to look for brand name (not supermarket brand) trash compactor bags (like Hefty) at hardware stores, such as Ace rather than at supermarkets.
May your trails be crooked, winding, lonesome, dangerous, leading to the most amazing view--E. Abbey