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

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?

great article, thanks

your conclusion about “opening cuffs, collars, front zips, and pit zips” matches my experience

I’m surprised how far the boundary layer extends.  A cruder way to measure is when it’s cold, use instant read cooking temperature sensor and measure against the outside fabric, and measure a distance away.  I haven’t measured any difference beyond 1 inch, and if the wind is greater than 2 MPH I don’t measure any difference.  But that’s hard to compare.

My Neoshell jacket is about the only fabric that I won’t sweat inside of.  I believe it has high MVTR.  It has excellent DWR – must have PFAs.  Kind of expensive $45 per yard from Discovery Fabrics.

Another fantastic article, thanks Stephen

Yep. +1

The R-value of this [boundary] layer can be around .6, so it is significant. As the wind blows, the still air is replaced by air swirling around your outer layer. So, the rate of convective heat loss will increase.

That may help to explain something that I have been struggling with. When I wear a highly air-permeable single layer (shirt), a gentle puff of wind (or even sometimes just movement from walking) can feel momentarily cool.

The same shirt, however, provides a noticeable amount of wind resistance, as you point out.

It feels as though there is some evaporative cooling in addition to the convection. As you say, probably difficult or impossible to measure. It is clear from your research that this stuff is fantastically complex.

Historically, rain and wind shells typically had loose fits for ventilation and layering. I find it disappointing that style-consciousness has tempted manufacturers away from designs that we know to be successful (and toward slim/athletic fits which are not well suited to shells).

At my running speed no heat is lost ! Ha ha

Love this article!

Let me make sure I have a few things correct.

If you disregard the sun, the Bryne is one of the best base layer for dumping heat and getting rid of moisture.

For the HH Lifa, the only ones I see that are polypro are the striped ones, is that correct?  Also, do you know the SPF/UPF?

Where I live there is usually a breeze of 5mph+.  With the numbers on the Lifa it looks there would be enough air movement to wear that as a base layer also and get some better protection from the sun more than the Bryne.

When it is cold, if you want to be able to dump heat and moisture but also be able to protect again cold winds, get an outer layer that has a high MTVR, low cfm.  Open up the layer by using the front zipper, pit zips and other means to dump the heat but then button it up if the winds are cooling you down too much.

Thank you.

My LIFA is 30 years old and still going strong as a winter hike base layer and shoulder season sleep layer.  I certainly prefer its moisture management over 200g smartwool, though its not as warm.

HH advertizes 90g for the top but on my scale it was 155g, same as my old one so I returned it.

Stephen, I had ordered the 48300 in black, large, claimed weight 90g, actual 155g, similar to 150g claimed weight for the 48800.

My scale is pretty accurate & L sizing shouldn’t add 72% to the weight of even an XS, so their 48300 weight spec is bogus.  Standard practice is to quote weight of Med.

I haven’t had issues with polypro’s rep for getting stinky even after decades of steady use, but I’m a heretic and use deodorant, even on trail.   I cut off and carry a 0.8 oz chunk, Arm & Hammer Essentials Unscented so to not attract critters large or small.

Looking forward to what you hear back.

Stephen, thanks for explaining your base layer system. Is this something you will be expanding on in future articles? Have you tried the lighter weight Byrnje – Super Micro? Only available in SS or vests unfortunately. I think it is more comfortable in milder temps. Thanks also for the model numbers of the HH garments, wading through their website was tedious!

Looking forward to hearing if they get back to you.

Cheers, Scott