A nice solution to preventing vapor moisture condensation inside the outer shell of down bags is using a synthetic insulation quilt as made by Mountain Laurel Designs (MLD).
They have 3 different weights of insulation using Climashield which is about the best synthetic insulation for retaining loft after repeated stuffings. I think I’ll get the lightest version.
The body vapor condenses in the colder MLD “over bag” quilt and, being all black, it dries faster when hung out to dry.
**You can also choose an insulation thickness that will help extend the temp range of a 3 season down mummy (or quilt) and kill two birds with one stone.

