I often feel that if my outer layer had just a little more outside air passing through it, I could be cooler or dryer and, in general, more comfortable. Perhaps I’m confusing standing before a fan with the almost negligible breeze I create by hiking up or down a mountain. This article will examine how much outside air can penetrate an exterior layer while running, walking, or standing in a moderate breeze. The results of this article are based on extensive testing of a wide range of fabric air permeabilities at three windspeeds. I will not examine how air permeability impacts a fabric’s ability to remove vapor from within. Improved moisture vapor transmission from within would seem to be a likely outcome of increased air permeability and one I hope to investigate in a future article.
In my last article, I discussed, in general terms, what happens when moving air runs into a fixed object. If you are the fixed object, most of the air you encounter will move around you. The air that comes into contact with your exterior layer will have lost nearly all its forward speed, so it will have little pressure left to penetrate to your skin.
I do admit I have a bias on the issue of air permeability. The bias originated when I switched from a regular Houdini to a Houdini Air. The Houdini Air has far greater air permeability than a Houdini. (About 24 times greater, when new.) However, when I wore the Houdini Air during winter ascents below treeline (low wind), my base layer would get wet from sweat, just as it would when I wore my regular Houdini. The increased air permeability did not matter. However, once past the tree line and faced with high winds, I got cold as winter winds reached my wet base layer. This experience suggested that higher air permeability could increase ventilation in high winds. However, at a hiking pace, in low winds, the impact seemed negligible. This concept led me to create several articles evaluating the role of Moisture Vapor Transmission Rate (MVTR) and Air Permeability Rate (APR) in removing moisture vapor. Read the first article in this series if you missed it.