Measurements of fabric performance are helpful only if they can predict field results. As we will learn in this article, Air Permeability Measurements (APM) do not replicate the forces at play when wearing a garment. If one of your objectives in selecting a windshirt is eliminating moisture vapor, your selection should rely more heavily on Moisture Vapor Transmission Rate (MVTR) than Air Permeability Measurements.
In August 2021, I published an article investigating whether a jacket’s Air Permeability Measurement or Moisture Vapor Transmission Rate was more critical for removing moisture vapor produced by a user. In that article, I concluded that MVTR was responsible for removing seven times more moisture than APM for the garments tested. MVTR testing can provide a more useful prediction of comfort in the field than APM.
Unsurprisingly, my conclusions concerning MVTR have been met with some skepticism. The ability of APM in windshirts to remove moisture during high-level activities has received extensive discussion in both the Backpacking Light community and the outdoor clothing industry for years. These discussions can be misleading and reflect a misunderstanding of the forces at play for removing moisture vapor from a garment. Part of the problem is that none of those discussions included measurements of MVTR, so no one could say what was responsible for performance in the field.
In this article, I am going to revisit this issue. I will provide data from my tests demonstrating the forces pushing water vapor from jackets. I will show how these forces work with market-leading windshirts. I will also describe the findings from other researchers who have studied this issue. Critically, I will demonstrate how aerodynamic forces limit air penetration of windshirts.
Here is what we will see:
- Elevated wind speed provides the force to drive air through an air-permeable garment and remove moisture vapor. When wind contacts an obstruction, such as a hiker in a windshirt, the airspeed at the contact point drops dramatically due to airflow stagnation and diversion. Consequently, much of the force that could drive outside air through a garment is lost. Standard air permeability measurements do not reflect this behavior.
- Compared to air blowing onto bare skin, any windshirt dramatically reduces the volume of moving air that can remove water vapor. In the absence of high winds, a typical windshirt allows almost negligible penetration of ambient air. Moreover, the limited volume of air that makes it through a windshirt can only remove vapor after following a tortuous circuit through inner layers and then back out of the windshirt. Standard air permeability measurements do not capture this difficult journey.
- The force produced by vapor pressure differences from a hiker’s skin to the environment can be many times the force available from wind-related pressure on a windshirt. High vapor pressure difference helps to account for the magnitude of vapor reduction due to MVTR vs. APM described in my 2021 article.
- Two studies that support my conclusions will be briefly described later in this article.
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Discussion
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Probably the best way is if you had an external document (like a Google Spreadsheet) that was in the first post. Then you could update the spreadsheet at will and it would be linked in the first post. I can edit the first post any time you want. How does that sound?
Edit/Clarification: That could be the first post in this thread or you could start a new thread. Whatever makes sense.
I can do that. I will PM you.
@Scott Emmens. That is interesting, when cycling in temps around freezing my forearms get oppressively hot and clammy whilst my chest might be cold and damp.
Cycling in cold weather is weird because your core temp is high but periphery might be cold – so you can sweat into cool skin, making hypothermia a real possibility when you stop riding .
I personally find that a nylon wind garment without a membrane keeps me warmer in both cold-dry and cold-wet conditions as I sweat terribly in membrane garments.
Cycling companies, except your own excellent company, rarely make wind jackets without some kind of windstopper membrane which I think is short-sighted.
I’d only wear a WPB garment when cycling in biblical rain, a short commute or when touring as it’s really uncomfortable to ‘boil in the bag’. A nylon top with high MVTR would be perfect for cycling IMO. I have a quantum pro windshirt that I am trialling on the bike as winter closes in in Australia.
@Sean P I believe that cycling does really introduce some weird phenomena for apparel too! Forcing your (mostly) stationary body through cold air is kind of unnatural I suppose. I also wonder if air permeability of fabric is a little different as you often are moving at higher speeds than quoted, even given stagnation and deflection.
I am also convinced that the moving air, over wetted out waterproof fabric is the cause of most customer issues with the fabrics not performing (other than a lack of understanding). The fabric is getting noticeably colder and therefore condensation is forming more easily. I’ve come to this thinking after a customer’s experience – only one sleeve of an undergarment getting wet when riding into a cold, wet, cross wind. I waterhead tested the garment on both sleeves, chest, shoulders and back. The fabric and seam sealing were sound yet he got a very wet (merino) undergarment sleeve, just the one mind you!
It’s complex for sure.
Our windproof fabric is Pertex Quantum, it works extremely well, I just wish the DWR would last longer…
@Scott Emmens.
wetting out seems to be the bête noire of water resistant/waterproof garments. I take it that MVTR is reduced markedly in the presence of liquid water on the surface of a garment?
The internet consensus is that ‘breathability ceases’ when a textile wets out but I didn’t think that it was that simple.
I’m also looking at testing grid fleece/alpha under both pertex quantum and equilibrium this winter on the bike: it’s an outstanding way to test for extreme hiking conditions!
Stephen, thanks for sharing again another great informative article.
Regarding:
How would you compare the following RET values from Rab to MVTR and what you would consider “great” “excellent” “good” “fair” and “poor”?
– GORE-TEX 2-Layer with insulated construction: RET < 9
– GORE-TEX 3-Layer construction RET < 9
– GORE-TEX PACLITE® Plus 2.5-Layer construction RET < 9
– GORE-TEX PACLITE® 2.5-Layer construction: RET < 6 (this most likely is an error. should be <9)
– GORE-TEX Pro Most Breathable: RET < 6
– GORE-TEX Active 3-Layer construction: RET < 4
Thanks
Hi Weekend:
Unfortunately, I cannot really answer your question. I have been able to relate my results to JIS L1099, B1 results. This is the standard most companies use for the MVTR data they publish. However, I have been unable to accomplish this task for Gore RET data. RET data is produced using a sweating-guarded hot plate. This method is very different from my test method or any of the major testing protocols. That does not mean it is inferior to other tests. RET is different and not readily comparable. Also, simply knowing the general product description doesn’t help much because the MVTR of a Gore-Tex product WPB is heavily influenced by the face fabric properties. As face and liner fabrics get heavier, MVTR goes down. The only way I can answer your question is to test a range of Gore garments for which specific RET values are also published. I have found RET data for a limited number of Gore products, which are no longer produced. Further, existing data may have limited value since Gore is now eliminating EPTFE in their membranes, so who knows how the replacement garments will perform. The preliminary data I have seen are not very encouraging. Some of these new product have hit the market, but I have not tested any yet.
Hey Weekend:
Here is one point of reference, maybe. I went to the Rab website to see what they listed. One was a Men’s Kroma Cirque. Gore Active, 30 Denier. In a this article, one of the jackets I tested was a Patag0nia Ascentionist. 30 denier, Gore Active. I measured MVTR of 2400. The Rab jacket may be the same fabric, since it is 30 denier Gore Active. Rab says Ret <4. In my book, a MVTR of 2400 is a pretty poor performer. In looking quickly at their breathability graphs and RET or MVTR values that they publish, there seem to be inconsistencies. If you every come across a RET for Shakedry, then we will know what an excellent RET look like.
Hi Stephen, I don’t know how the Gore RET ratings go but I just stumbled across this page:
https://www.gore-tex.com/en_au/technology/original-gore-tex-products/shakedry
and this slide which states an RET<3 – “putting it firmly in the best possible category”??:
Gore, the masters of marketing.
I’m curious that no one is commenting on the Houdini Air. When I look at Stephen’s chart comparing different windshirts, it would seem that the Houdini Air is relatively high in the permeability category, and reasonably good on the MVTR front. Perhaps because it is lacking in the water resistance dimension? I’m curious if anyone has experience with the Houdini Air in the field. I hike primarily in the Appalachians but during all seasons so have to deal with humidity, precipitation, and a wide range of temperatures.
@Stephen Seeber. Stephen, I am curious as to the relationship between Air permeability (CFM) and MVTR. Intuition would suggest that a low CFM garment would correlate with low MVTR (and vice versa) but experiment (including yours) show that the relationship is not linear in tighly woven textiles even if it might be for, say, for a fishnet baselayer.
What physical prinicple allows a garment porous enough to have a moderate air permeability (such as the Houdini Air you tested) having a MVTR lower than a 0 CFM garment (such as Gore Shakedry)?
The best I could come with from reading was the hygroscopic properties of the textile itself contributing to passage of water (sorption/desorption). I can get my head around a low CFM garment having as good a MVTR as a higher cfm garment (the analogy I thought of was that one does not die of asphyxia when taking shelter behind a rock on a windy day – blocking direct air flow does not prevent diffusion/convection) but my intuition struggles with a lower CFM garment having a relatively higher MVTR.
Intuition suggests that a high CFM garment, in still air, should still have a high MVTR as the porosity of the garment should still allow diffusion. Obviously there are other physical properties at play than sheer pore size – is it the weave etc that also contributes here?
Thanks in advance.
Hi Sean: Please see Figure 6 in the article. I think that goes a long way to explaining the difference in forces pushing air vs vapor through a fabric, including a membrane. Of course, the actual forces trying to move wind through a garment, as explained in the article, are far less than those described in Figure 6.
Fantastic article, thanks Stephen! Do you know if the Ferrosi you tested is the current model?
REI has this to say about the Ferrosi hoodie: “Updated primary fabric is now 46% recycled with movement-mirroring stretch; body-mapped construction features CORDURA® fabric in high-abrasion zones”.
I take this to mean the fabric has changed recently.
The technical specs are listed as “86% nylon (46% recycled)/14% spandex, 90-denier stretch-woven ripstop (bluesign® approved)”.
I have some older Ferrosi pants that are listed as 86% nylon and 14% elastane (which I understand is synonymous with Spandex) so the makeup seems the same, but I’m not sure what else might have changed. I’ve seen some noticable variation in Patagonia capilene so just wondering if you’d expect differences between older and newer Ferrosi garments.
Thanks!
Stephen, I also use wind shirts/jackets a lot skinning uphill on my skis in winter. I feel like jackets with high air permeability help me transfer heated air out of my clothing system, not just moisture vapor. I go uphill pretty hard, so it is difficult to regulate my temperature well. I have found the Houdini Air works best for me because it transmits vapor but also heated air with the high APM.
It seems that controlling the temperature inside your clothing is probably more important for staying dry, by reducing the sweat produced, than measures to try transporting that moisture out of the clothing after your body produces it. -B-b
All I know is my Patagonia Houdini from 2011 (AP of ~35 – If I remember correctly) seems much more comfortable than any other windshirt I’ve used. It also has a slight texture to the fabric – sort of like it was brushed compared to say a Pertex fabric used in a sleeping bag. All these years later and it’s still going strong!
I wish I could find one of these mythical Houdini’s! Do you have a picture you could post?
I have been using the MH Kor Airshell over a OR Echo during winter higher output portions of my hikes and I think it’s a great combo in regards to allowing moisture and heat to vent. The Airshell also has enough of a DWR that brushing against snow laden trees or light sprinkles won’t penetrate. I throw on a Peleton 97, Alpha 60 or R1 Air under the Airshell once I am not moving so much or static and it traps the heat decently. Not good for super high winds as you can feel that coming through.
Here is one of the ’11 Houdini in Brooks Range Alaska. It’s amazing despite how many backpacking pictures I have – how few I have of me actually backpacking!
Thanks Brad. These are hard to find.
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