Gore Tex has done an excellent job in not telling the whole story about it's membrane. I hesitate to comment here, but feel a slight need based on some inaccurate information...

Gore Tex, after realizing their first generation was failing because of oil and dirt contamination, did something that would greatly effect it's breathablility. They added an additional layer of material, polyurethane (PU), to combat the oils put out by the body. This layer is oeliophobic or "oil-hating" and is necessary to keep the Gore Tex membrane (ePTFE) from leaking (yes, Gore Tex can leak!). In reality, Gore Tex is 80% space. Without the surface energy created by the hydrophobic ePTFE membrane, Gore Tex would leak which is exactly what happened with the first generation of Gore. The oils broke down this surface energy. It is interesting to note that Gore Tex has only changed slightly since the second generation was introduced (Gore XCR and the newer Pro Shell still use the PU layer).

The problem with this layer is that it is a hydrophilic (water loving) monolithic, or solid, layer. In other words, water has to "diffuse (traveling from a greater concentration to a lesser concentration)" through it in order to reach the microporous Gore Tex layer. Thus Gore Tex's breathability is totally reliant upon this PU membrane for it's breathability. Because of this, Gore Tex is at its highest breathability when there is a 100% humidity level inside the jacket, tent, etc. That is why one needs to build up vapor and heat pressure inside the garment, tent, or whatever to "push" the condensation out. Gore Tex does not work at low humidity levels because of this PU membrane which may account for frost or liquid buildup inside a tent. This is also why it is nearly impossible to find a Gore Tex tent.

On the other hand, eVent doesn't have a PU membrane and is therefore very effective at low humidity levels. It is the premiere membrane for single wall tents, but unfortunately doesn't pass outdated fire retardancy tests. eVent is also an ePTFE membrane, but uses instead a hydrophobic (water hating) polymer infused by a super-critical gas treatment to encapsulate the pours of the Teflon membrane to achieve it's oil-hating (oeliophobic) characteristics. Thus it is truly a microporous membrane. This equates to it being 30% (at high humidity levels) to 200% (at low humidity levels) more breathable than Gore Tex!