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.