So here is the deal as far I understand it pertaining to:
[b]Waterproofing[/b]
Is measured in: millimeters of water, as a measure of hydrostatic pressure
If you have ever swam more than 10-15 feet below water, you can feel the pressure build up of water (caused by the difference between the pressure inside your body and that of the water outside). Well, jackets operate the same way, in which the lab test measures how much water pressure the jacket can withstand before water permeates the fabric. Added pressure is measured by the height of the column, but it doesn't necessarily mean there is a thin water column on top of the fabric exactly, since 45,000 mm would be 45 m, or about 140 feet! However, it is a calculation of pressure put on the jacket. This translates to how long water can sit on a fabric before it leaks through. So, the mm number is an approximate way of telling how well it will resist water permeation. It does have to do with the size of the pores in the fabric. For example, standard cotton has huge poors and hence why one's shirt get soaked easily and requires a long period of time to dry. However, as I will state below, eVent fabric has millions of very small pores that are too small for water droplets in liquid form to pass through. The reason it is so effect is because water vapor, or water in steam form, has significantly smaller molecule size and therefore can pass out from the jacket without letting the larger liquid water droplets in
[b]Specific waterproofing numbers[/b]
Hyvent: I've mostly seen that Hyvent is between 20k and 25k mm in waterproofing. It breathes quite well and will be sufficient for just about any outdoor adventure.
eVent: is comparable to Gore-Tex in waterproofing, but is comprised of a greater pore/surface area ratio, thus giving it higher breathabiltity
Gore-Tex: I have never heard of single layer Gore-tex, though I am sure it exists in some design, but double layer Gore-tex is 28,000, as Iggy said earlier, and 3-layer is 45,000 mm.
Dermizax, etc: Companies like Spyder, Orage, and Helly use this material and I believe it is generally 20,000 mm, though again like Gore-tex, a three layer material has improved waterproofing.
Lastly there are companies like Burton, Foursquare, Oakley, etc, that don't name their fabrics used and use a generic, waterproof fabric that is either from a lesser well-known company or made in house (vertical manufacturing as opposed to horizontal, which is buying all processed part from outside sources).
I hope that is somewhat helpful.
