i don't know, i think having sil only on the outer layer would cause internal moisture problems. humidity or not, water vapor from your body may pass into the quilt, but not be able to escape to the outside due to the sil.

i'm thinking have one shell piece sil, and the other breathable, and then a sil layer on top of the breathable sewn on 3 sides so it could be flipped inside out to expose the breathable layer for wash/dry.