By The Numbers – How Much Wind Penetrates Your Air Permeable Outer Layer at Hiking and Running Speeds?

UPDATED January 11, 2023: In October, I began a project to improve the accuracy of high air permeability measurements produced by my air permeability tester. The original configuration was developed to measure up to 400 CFM/Ft2. The instrument was originally calibrated using calibration plates whose performance was measured by Frazier Precision Instrument Company, the manufacturer of the Frazier Air Permeability Tester. A year later, I increased the range of the instrument by adding an additional rotameter. This was supplemented by several statistical techniques to measure the performance of fabrics such as 60 gsm Alpha Direct. Recently, I completed the present project to improve the accuracy of the tester at high air permeability rates. As a result of this latest project, numerous changes to the instrument were implemented. The range of the instrument was extended to beyond 1000 CFM/Ft2. I hired Diversified Testing Labs to measure the air permeability of a 60 gsm Alpha Direct fabric sample. The newly configured testing instrument produced results within 2% of the lab test results. I retested the air permeability of all garments whose original air permeability exceeded 300 CFM/ft2. This resulted in the following changes to the article: the introduction table and Table 2 were revised. Figure 9 was revised. Several minor editorial changes were made to the text to reflect the new air permeability measurements. The findings and conclusions of the article are unchanged. – Stephen Seeber (Author)

Introduction

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.