I made an UQ with the top layer (the one against the hammock) out of sil and the bottom layer made out of 1.1 w/ DWR. It works just as I hoped. I have heard that the sil would feel clammy, but I haven't noticed it at all. I do think it works well as a wind break. My only fear with both shells being sil is if it got wet i wouldn't be able to dry it in the field. I also imagine that if it was all sil it may prevent some loft since air will struggle to get in. How warm is summer in Cali anyhow and what are you using for insulation?