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

[Stephen Seeber]

Companion forum thread to: By The Numbers- How Much Wind Penetrates Your Air Permeable Outer Layer at Hiking and Running Speeds

Stephen Seeber helps us answer the question, “do air-permeable garments allow a noticeable amount of ventilation while hiking or running?”

[Keith T]

Stephen, thanks again for another outstanding article. Your research is the #1 reason I subscribe to BPL. My guess is that the lack of comments so far are because readers want time to thoroughly digest this complicated material, not because we lack any appreciation for your work. As the risk of revealing my ignorance, I’ll try to get the conversation rolling:

My takeaway from this article is that MVTR is a more useful spec than air permeability, so your research will concentrate on MVTR from now on. Air permeability is not a useful measurement because the Air Penetration Speed needed to effectively evacuate sweat/vapor cannot be achieved under low wind (i.e. normal backpacking) conditions because the boundary air layer next to the body stops/slows it.  The remainder of the article proves the foregoing assertions.

Some of these conclusions go against my intuitions. If the air boundary layer is so “strong”, why do I still feel so much colder in high wind situations over low wind situations? This feels true even when venting is not at play. Is it because the air boundary layer keeps getting replaced, which still prevents air penetration but allows convective heat transfer?

[Scott Emmens]

Another fantastic article, thanks Stephen

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