Quote Originally Posted by WaffleBox View Post
I'm just not sure what the outer shell gets exposed to that the inner shell doesn't. My top quilt is only outside of its stuff sack while enclosed inside my bug net. Same with under quilt. The only way the UQ would touch the ground or anything else is if my suspension broke.
When I sleep in the winter, I've had condensation built up on my TQ around my face area multiple times. This is the one example I've had experience with where the added water resistance was welcome. I'm not disagreeing with you- membrane10 and other 10d fabrics can be used as outer shells.

I've also heard, and agree with, the added wind resistance argument, especially for under quilts. The outer shell takes the brunt of the wind in an under quilt - and occasionally a tq. Lighter, 10d fabrics are more breathable, allowing this wind to rob the quilt of built up heat. Again, this comes to personal preference, because a strong wind will take the heat out of any quilt, hence the use of UQ protectors. It'll just happen easier with 10d fabrics.

Do any of these things matter much? The differences may be negligible at best, or they may make a significant difference. I don't know. However, the people who I know have lots of experience in making quilts choose to make their quilts with a stronger outer shell. So it has become my personal preference as well.

Any down proof material can be used as a shell, it just depends on what matters to you most: lightweight, durability and cost.