Unfortunately, if the outer sleeping bag shell isn't breathable, there will be condensation on the inside of it, especially in cold weather! This does mean unavoidable heat loss to avoid wetting out your insulation.

A vapor barrier between your base layer and the sleeping bag helps, but it appears that everyone reacts to this differently. For me, the point at which I'm comfortable in a VBL inside my sleeping bag is in the low 30's F. For others, it has to be a lot colder to avoid the VBL's becoming a sauna inside.