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.
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Discussion
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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?
Another fantastic article, thanks Stephen
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.
Hi All:
I was away for a bit of a backpacking trip. I got a great opportunity to retest my homemade radiant barrier, which, so far, eliminates condensation in my tent. This will be the subject of an upcoming article.
Thank you all for your kind words. Now, some responses.
Keith T: I think your last sentence is getting at what happens. As described in the paper, the boundary layer actually increases heat loss. The increased rate of molecular collisions occurring in the boundary layer increases the heat transfer rate from you to the moving air. When the wind isn’t blowing and you are not moving, there is a still air layer next to your outer garment that provides extra insulation. The R-value of this 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.
Jerry: I never tried to replicate your cooking thermometer method. It’s interesting that you could detect a temperature difference. I imagine that when surrounded by moving air, the rate of heat dissipation will increase rapidly, so you would need a very sensitive and fast-acting measurement device, of which a kitchen thermometer is neither. Yes, Neoshell has excellent MVTR. Its performance is generally better and occasionally much better for some lightweight Neoshell fabrics than the now-discontinued Gore Shakedry.
Yep. +1
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).
” 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.”
another factor that would do that is the air layer between shirt an skin can be squeezed out, and then replaced with cold air. If you stand still and don’t let the fabric move you’ll be a little warmer
At my running speed no heat is lost ! Ha ha
Hi Stephen, thanks for all the hard work you put into these articles. I am experimenting with using Brynje fishnet baselayers in warmer conditions and am searching for the best highly air-permeable piece to layer on top. Is the Helly Hansen Lifa top you tested the version made of 100% Polypropylene or the “Lifa Active” version made of 60% Polyester, 40% Polypropylene? Do you find that there is a temperature where the Lifa + Brynje combo becomes too warm? Have you experimented with any other fishnet baselayer and highly air permeable top combo other than those mentioned?
Brenton:
In the summer or winter, I wear layers that promote sweat evaporation on the skin and movement of sweat vapor through my layers to the ambient. The Brynje fishnet is the base layer I wear in summer and winter. What I wear over that varies with the weather conditions.
I wear a Brynje beneath a white HH Lifa, 100% polypro in the summer. The Lifa and Brynje are both short-sleeved. The Lifa is not available in short sleeves, but a pair of scissors and some sewing took care of that problem. Why white? I spend a lot of time above tree line in very intense sunlight. A white shirt produces a cooler shirt surface. Somewhere on BPL, you can find a little article I wrote a few years ago that compares surface temperature differences between white and black t-shirts in the Colorado sun. It is definitely something one should consider.
Can this combination get too hot? Absolutely. When that happens, I remove the Lifa. I am sure it isn’t pretty, but there is no better way to dump heat than the slight breeze you create while walking on nearly bare skin. I try to limit this exposure to below the tree line. Surprisingly, I have not gotten sunburned while wearing the fishnet without an additional layer, but if you are going to make a habit of hiking in a fishnet, you might apply sunscreen on your torso.
Bear in mind that I live in Colorado. When it gets hot and humid where I live, I move up to high elevations, so in general, I am not exposed to the heat and humidity that I did not enjoy when I hiked in Maryland and Virginia.
I do not wear any polyester or wool clothing in warm temperatures. I don’t wear any wicking fibers that will absorb moisture and inevitably become saturated. In hot conditions, sweat can wet the fishnet via diffusion, but a wet fishnet is a whole lot better than a wet base layer shirt.
I have not tried any other fishnets. I have made plenty of use of the Finetrack over fishnet. It works well. The problem with the Finetrack elemental layer is that it is only available in black. Also, the Canadian distributor of Finetrack in North America appears to be gone. Now, you must buy from Japan with this link. There are probably other very high air permeability light base layers around, but I have not seen any that are hydrophobic like the ones I am using.
<p style=”text-align: left;”>with respect there IS a difference between men and women regarding temperature. So sorry! Guys usually have higher volume to mass ratio. They have more body “ stuff”…. esp when young . Gals get cold easier. Maybe hormones. Sorry. Biology can suck.</p>
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.
Worn as a base layer below a highly air-permeable layer, fishnet is an excellent base layer. It provides a greater opportunity for sweat to turn to vapor on the skin and prevent sweat from accumulating on fabrics in contact with your skin. When I am below tree line and am too warm in this combination, I remove the outer highly air-permeable layer. Above tree line on a sunny day, I will be more cautious about removing my outer layer. I don’t have much of an issue with insects where I hike, so fishnet by itself can work for me.
I have been using a black LIFA shirt for a couple of years. It is highly air permeable. This year I purchased a white LIFA shirt, cut the sleeves and then had sort sleeve summer shirt to wear over my fishnet. Over the weekend, I was doing some testing for my next article and near the end of the testing, the curve fit I was producing went well south. I started to retest my air permeability results to ensure they were accurate. I found that the white LIFA shirt had half the air permeability of the black LIFA shirt. I looked at them under the microscope and found that the fabrics were very different. Today I called Helly Hansen customer service to find out about this. They promised someone in product development would contact me to resolve this issue. So, at the moment, I would hold off on purchasing a Helly Hansen LIFA until I get some information from them. The White LIFA is pretty good but the characteristics of the Black LIFA are more suitable to hot and humid conditions.
The one that should work is style 48300. I just ordered another of these to confirm. The one that did not work is style 48800. I also ordered two short-sleeved white versions. I will test these when they arrive to see if they are suitable.
I will post back when I can clarify.
The two-layer approach allows sweat to evaporate from the skin rather than moving into the base layer, where it may or may not vaporize. I would not wear a fishnet by itself without regard to sun protection. The LIFA shirts are cheap. Purchase one (when I figure out the fabric issue) and try it by itself and in combination
Your last point is correct. Use ventilation to dump heat as needed, then zip up to maximize warmth. Of course, your layering choices must be correct as well. I select the warmth of the layer over the fishnet according to expected temperatures and then make additional layering changes as needed.
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.
David D: They claim 90g for one of the LIFA models and 150 for the other similar looking style with different fabric. My black LIFA is likely the one claimed to be 90g. It weighs 144 grams. Weight will vary with garment size and we don’t know which size they weigh for their specification. If they respond to my inquiry, we might find these things out. Without a doubt, the fabric used for my white LIFA will be heavier than the fabric used for my Black Lifa.
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.
I have been reading your and other’s articles for awhile and this is what I am using. I think it would be great if you could do some experimentation and find some of the more air permeable sun shirts that are in the SPF/UPF 40-50 range and also bug shirts
Layering in XXL (I had a ton of OR credit). The last OR Wind and motive are 3300+ MVTR and the Kor is pretty good also.
Sun OR ActiveIce Spectrum Sun Long Sleeve Tee
Bugs Patagonia Sun Stretch
Wind Strong OR Helium Wind Hoodie
Light MH Kor Preshell
Rain OR Motive
I am going to look at the Lira once it gets sorted. It would be great to know if it has any sun protection.
I ordered a Ferrosi Hoodie and will give it a look.
I am trying to get myself to order the Bryne.
Thank you!
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
Hi Scott: I did try a Super Micro. It uses a much smaller mesh size. That means there is more fabric and less air in contact with your skin. It actually reduces the rate of evaporation on the skin. It is interesting that you find it more comfortable. Perhaps you could speculate as to why that is.
Update on Lifa. HH has not contacted me. However, I received two short-sleeved shirts. Same fabric as the 48800. Since I was not convinced they would get back to me, I ordered another 48300. This is the one I already own. The fabric in the new short-sleeved shirts is the same as the 48800. The air permeability of the 48800 is 603 and its MVTR is 4920. The air permeability of the 48300 is 1143. I have not measured the MVTR of the 48300. It is probably around 6800. Big difference. I will continue to try to get an explanation of the fabric change.
Hello Stephen,
Were you ever able to sort anything out about the Lifa? The new fabric of the 48300 is the one to get?
Hi Brett: They never got back to me. If you want the higher MVTR and air permeability product, than 48300 is the way to go. However, it is only available in long sleeve and is black. That is what I am usually wearing these days over my fishnet. It is not yet cold enough here to require an Alpha Direct layer over my fishnet.
Thanks Stephen. Would you think the average person would feel much difference between the 43800 and the 48800 since those numbers are getting pretty large. It looks like the 43800 was once in white. I did ask HH and they said the 43800 has no UV protection. It is also discontinued and will no longer be produced in any color. Also, what do you use as a windshirt type thing? Thank you.
As part of multiple layers, I would opt for 48300. You are right; the air permeability of both is quite high. I used the 48800 in white this summer. It served me well enough, but I think 48300 in white if it existed, would have been better. Why not get both and decide for yourself? They are pretty inexpensive, especially the 48300. I am disappointed they are discontinuing the 48300, but I just purchased 3 more, since they are selling for $32. They don’t seem to have much inventory left. Now, only XL and 2XL.
Thanks Stephen. Would you think there would be much of a difference between a black 48300 or a white 48800 since they are so air permeable? Also, Amazon and others have the 48300 but in black and not on sale.
I care about color when I am wearing LIFA as an outer layer. Then I want white to reflect sunlight.
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