Questions: Air permeability vs. Breathability. Are they the same? Are we talking about the ability of water vapor to escape, vs. liquid water?
Unfortunately, these terms are often used interchangeable. “Breathability” relates to the action where water build-up or pressure is needed inside the shell to push liquid out after it condensates. This technique is inefficient and often times the downside is chilling when the user stops moving. Hence vent zippers are needed to dump out the moisture.
With Polartec NeoShell or eVent fabrics their functional microporous membrane allows for direct venting and true air-permeability where no phase change (vapour changes to water) is needed. Moisture (not liquid) passes through the membrane efficiently as long as there is driver such as heat and/or body contact on the interior.
Questions: How can a 3 layer fabric be waterproof if the membrane is porous?
The size of a water molecules is larger than vapor. The pores in the membrane are therefore engineered to be large enough for vapor to pass through but small enough to shut out water.
Questions: Why do you add vent zippers if the Air permeable fabric is efficiently letting out moisture?
The are 2 reasons why some Westcomb pieces have vent / pit zippers. First, many consumers are accustomed to having this feature on whether they use it or not. Second, not everyone sweats at the same rate or under the same conditions. The membrane is still a barrier and it is possible to sweat at a faster/heavier rate that exhausts the membrane’s efficiency.