+1 to what aimless said.

Also, even though you're a cold sleeper, you still sweat (or water evaporates from you or whatever, the smart people can correct me) when you sleep, just not enough to notice. With the SOL bivy on the outside, you were trapping moisture in your down layer, effecting its insulating properties (especially over consecutive nights). Bivy on the inside traps that moisture so it doesn't get into your down, and if you regulate properly you'll actually stay pretty dry.