I'm looking into a buying or building 20* UQ. I'm considering a non-breathable upper layer (towards hammock), either Cuben or the reflective stuff from DIYGS. I'm curious about moisture issues. It seems that since the moisture has no where to go it would collect and become bothersome, but I've seen a lot of folks lately using non-breathable stuff like Cuben or IX on top. So, I hoping some experienced folk can shed some light on this for me.