The z-liner is used by OR in the GT Liners. A misunderstood characteristic of Gore-Tex fabric is its ability to pass water vapor through its structure. Most people associate the word breathable with respiration, as in taking a breath, and this is most definitely not what is taking place here. The main bit of physics in play here is the difference in vapor pressure between the inside and outside of the garment. We normally have a warm body generating vapor and an outer, cooler environment with a lower vapor pressure.
Vapor is encouraged to move from higher to lower pressure, i.e., to the outside of the garment.
Obviously, there are conditions where the inner and outer environments are similar, and therefore not conducive to vapor transfer. Warm, rainy, tropical forests are an example of an environment providing a diminished driving force. This is where design must take over from fabric technology. The ability to vent a Gore-Tex garment or bivy sack is crucial to its overall performance. The Gore-Tex membrane is a great advantage, but cannot bear the full responsibility for vapor transfer in any product.