Air permeability vs. moisture vapor transmission rate (MVTR): which one impacts moisture transport more in wind and rain jackets?
Aug 3, 2021 at 9:00 am #3723776
This study compares the performance of four jackets made with fabrics that span a wide range of air permeability rates and moisture vapor transmission rates (MVTR). The study shows that significantly greater moisture removal can be achieved as a function of jacket MVTR than jacket air permeability.Aug 5, 2021 at 6:23 pm #3723986StumphgesBPL Member
These results are stunning. It sounds like, in cool and cold conditions, a windshirt with high MVTR and low CFM, will provide the best performance in windy conditions. This is really good news, and very surprising.
The accepted wisdom has been that high CFM windshirts are the only hope of getting moisture from exertion out of our clothing layers. But high CFM windshirts don’t actually block wind, so what is the point? Further, as explained here, they will only vent more moisture through their permeable fabrics in the presence of wind, so again, what is the point? I suspect the point (and the existence of jackets like the Airshed, with ~60 CFM, has to do with the part near the end, where it’s explained that MVTR is disabled when outside conditions are warm and/or humid. In those conditions, we want some air to come blowing through the fabric to help cool us, and hopefully to convect some moisture out as well. These types of windshirts are also bugproof, so provide that additional utility.
Personally, I find that I only wear winshirts below 50F if it’s windy and pretty much always wear one when hiking at 35F and below, even in still conditions. At 40-50F, highly permeable windshirts (~35CFM) can be very comfortable, but if the wind gets over 20mph I can get chilled.
Knowing that fabrics that actually block wind (low CFM), but have great MVTR, exist allows for the selection of a low CFM, high MVTR windshirt for cool and cold conditions. Windshirts like this provide continual protection from wind, preserve a boundary layer of warm air, and so keep a microclimate near my skin that is pretty stable, whether the wind blows or no. Relying on MVTR works well in cool and cold conditions, because there is a high temp gradient from inside to outside. And if I climb a hill, I can vent increased moisture produced by opening the front zip. I’d rather open a window than have leaky walls.Aug 5, 2021 at 6:52 pm #3723989
I like your closing analogy! I wish I had thought of it.
Thanks for reading.Aug 7, 2021 at 2:43 pm #3724191Kevin OBPL Member
Thank you for writing these reports up. What is the difference between your MVTR findings and Montbells 80,000g using “JIS L-1099 B-1”. I don’t know if there is any comparability between tests but I am curious about differences in the tests to produce different results. Thanks again for your work here.Aug 7, 2021 at 4:40 pm #3724197
Glad you asked. There are 6 main standards for measuring vapor transfer through fabrics or garments. The two standards that seem to be most commonly used are ASTM E96 and JIS L1099. Each of these have four variations. Each variation produces different numerical results. The lowest MVTR numbers are produced by the ASTM test. The highest numbers are produced by the B1 version of the JIS test. The B2 version of the JIS test is probably a bit lower. The various tests impose a wide array of vapor pressure differences across the test fabric. They also use varying air flows where an air space is present in the test configuration. In those tests where an air layer is not present in the test apparatus, the resistance of air is eliminated. The resistance of any present air layer can exceed the performance of the fabric itself and the air layer resistance depends greatly on air movement velocity, from the use of a fan. Faster air flow offer lower resistance. These and other differences in the test configurations account for the wide variation in test results. There have been a number of academic studies that attempt to relate one standard to another, generally without great success. In a study by McCollough, the average for all fabrics tested using the E96B standard, an average MVTR of 772 was found. For JIS L1099 B2, the same study found an average of 12662. In this study, the highest value fabric using the JIS standard produced a result of 13421. Clearly, the fabrics included in this study did not perform nearly as well as today’s best MVTR fabrics. My test, which uses my permeation kettles produces results that are closer ASTME96 values than JIS-1099 B-1. Many product manufacturers, who actually disclose test information will use the JIS LL1099 B1 or B2 tests. This test produces very high numbers and can be completed very quickly:15 minutes. These two features make this a very popular test. My rule of thumb for understanding test JIS L1099 B1 or B2 results compared to my tests, which are very rough are as follow: For B1 or B2, poor MVTR performance is <20000. Excellent MVTR performance is >60000. Right now, for my test, excellent performance is above 3000. Poor performance in my test is below 2000. The other main standards measure vapor transfer resistance. These tests use completely different measurement units and instrumentation than those that present MVTR results. I have seen enough examples to relate those tests results (measured on a sweating guarded hot plate or a permeation cell) to my test or any of the other MVTR tests.
In summary, there is rough comparability between my tests and the other main MVTR tests. For test results using a sweating guarded hot plate or permeation cell, I cannot provide comparable data.Aug 8, 2021 at 10:16 am #3724216Weekend Gear GuideBPL Member
Great article Stephen, thanks for sharing!
As you know, some manufacturers like The North Face and their Futurelight WPB product are marketed with very high MVTR of 75K and CFM of 1.5 most likely using a test standard that is very much in favor of their marketing strategy.
At the same time, other manufacturer’s like Patagonia do not publish MVTR with the following reason:
“The Quality Team then went one step further; they took the exact same piece of fabric and sent it back to the exact same lab to be retested. Wouldn’t you know it, the second laboratory tests did not reliably match the first set. Obviously, if independent laboratories couldn’t provide similar results from the exact same fabric, comparisons between different fabrics (and brands) were impossible. This discrepancy is the reason we don’t publish hard and fast numbers; any comparison with other fabrics would be meaningless.“
Can you comment on whether your test methodology is able to provide consistently the same MVTR result, with a tolerance of +/- 10 g/m^2/24hr?
If so, then have you considered submitting your test methodology for peer review and getting it standardized within ASTM or JIS, to replace the inconsistent current test methodologies which prevents Patagonia from publishing or marketing their MVTR results?Aug 8, 2021 at 12:21 pm #3724223
Thanks for reading! If only some action I take could influence major manufacturers to publish performance data obtained through standardized tests. I wish my voice were that loud! The best I can do is test their products, determine how they perform and compare with similar products on the market. I don’t think the marketing status quo of selling with unsubstantiated claims will change any time soon. But, that is not really the point of my article. I am trying to provide the community with a basic understanding of how garments can support their needs in the field and that will be a constant theme of my articles on BPL.Aug 8, 2021 at 1:35 pm #3724227Max NealeBPL Member
@maximumdragonflyLocale: Anchorage, AK
Stephen, thank you, as always, for your investigations. I have a few specific comments on the methods and the overall comment that I recommend that you expand research in this area to test the conclusions drawn from this study.
1. The small sample included one rain jacket and a few low air perm windshirts. A high air perm windshirt was not included. I recommend including the Patagonia Airshed and similar. I also recommend including more rain jackets with different types of membranes. Therefore, increase the size and diversity of the sample.
2. Conduct the study in the rain.
3. Conduct a similar study over a longer duration (e.g. 4 hours) in the rain after DWR has worn off all garments.
4. Consider adjusting the research question. This study is relevant to two broader questions: (1) how do you stay comfortable in the rain? and (2) how to stay comfortable in variable windy conditions?
A rain jacket has to be durably waterproof. I have found that most of the ul rain jackets are NOT durably waterproof. Further, since the face fabric wets out (DWR fails quickly), breathability/MVTR falls to zero–it;’s a lark. Shakedry and similar no contact fabrics aren’t durable yet. Patagonia’s H2NO membrane is not durably waterproof based on my experience using it for long duration activities in the rain. (HH test is a short-duration lab test that does not reflect the realities of life in the wild.)
On staying comfortable in variable windy conditions – I abandoned low air perm windshirts (e.g. Houdini and Houdini Air) in favor of the Airshed in 2017. I use that every day all year and then add a rain jacket on top when it’s rainy or very windy (and more insulation, of course).
5. Another research question to consider for future studies: what is the optimal air permeability for a windshirt? I find the Airshed (60 CFM) works well for me. I’d love to try something with a higher CFM that retains similar attributes (strong, light, chest pocket, some stretch).
Cheers, MaxAug 8, 2021 at 7:34 pm #3724280Roger CaffinBPL Member
@rcaffinLocale: Wollemi & Kosciusko NPs, Europe
Vapor pressure is a function of both temperature and relative humidity.
A small point here. Vapour Pressure itself is a function of temperature only, but the VP across a fabric does also depend on the (external) RH.
There are many ways to measure a garment’s ability to transfer moisture vapor.
Oh, I fully agree. They all measure slightly different things.
As Stephen says himself:
There are 6 main standards for measuring vapor transfer through fabrics or garments. The two standards that seem to be most commonly used are ASTM E96 and JIS L1099. Each of these have four variations. Each variation produces different numerical results.
A cynic within the industry once commented to me that you pick the version which gives you the most commercial advantage. The general public would not have a clue anyhow.
CheersAug 9, 2021 at 2:57 pm #3724395Brett PeughBPL Member
For Stumphges’s flip side I have been using my Westcomb Crest Hoody more as a sun and bug shirt. The CFM is 42.8 and it was one of the highest MVTR that Stephen had tested. It is made out of Pertex Equilibrium. The SPF is 30 and HH is 125ish. I have talked with Westcomb and they said it is a layer that should be worn next to the skin.
I have done this quite a few times over the last few weeks when the temp is from 80-93F. I had zipped the jacket all the way up and had the hood on. I did some walking and was pretty okay. I was sweating but that fabric did pull the sweat off of me. It was sunny each day and I did notice some sweat build up on my head. I also spent a few days moving gravel with a shovel with it on for about 3 hours each day and the temps were 88-93F. I was sweating bullets but the fabric still kept me pretty dry. The fabric remained somewhat damp and that help me also. I had a lot of sweat buildup on my head where some would come down my face but I think that would be prevented with a head band or visor. The color is black so I am sure that added to the heat some. I basically have a farm tan and did not notice tanning on my upper arms, shoulders or head that was not present before.
The jacket itself does not have any DWR so I applied Kiwi’s Camp Dry. It took longer to wet out but since I would be wearing this jacket while moving I will probably not care about that so much.
I also have worn it out when it has been buggy. I have not really noticed any mosquitoes biting me. This may be to the decently tight weave and the Camp Dry may have a little to do with this also.
Westcomb said at one time that they had made a grey version and I wish they did now because I am sure that would mitigate some of the heat I have had to deal with having the black color.Aug 10, 2021 at 10:36 pm #3724510
Thanks for reading and providing some comments.
The thesis of the article is pretty simple: High MVTR beats out high air permeability for eliminating water vapor. However, I do mention in the article that my results could change for high air permeability garments like an Air Shed. But, without a breeze, even an Air Shed doesn’t move much air. At 3 mph hiking speed in still air, the best you can hope for in the Air Shed is about .4 CFM/ft2 of air flow. That is not much. From my experience with highly permeable garments for summer use, in order to get some cooling at hiking speeds, air permeability of over 300 CFM/ft2 is needed. One of the summer shirts I have used this summer is from Japan, called Fine Track. It is over 700 CFM/ft2 and is reasonably comfortable on hot days. Its drawback is that it is black which is a bad color choice above tree line due to solar heat gain.
Rain is not particularly relevant to the test I did: a high air permeability garment is not water proof. A high MVTR garment can be. High MVTR garments are still subject to the laws of thermodynamics and being out in the rain can be challenging, depending on the weather conditions, the layering you are using and your MET rate. I would guess you are a master at being comfortable in the rain and would be interested in your strategy as temperatures rise.
Optimal air permeability: I think that is a myth. Like everything else, comfort depends on weather conditions, the layering you are using and your MET rate. I typically like my wind layer to have no air permeability but the option for ventilation through pit zips, front zips, etc. However, for winter ascents with little to no wind, a highly air permeable base layer often is typically all I need down to 20F.
I am going to go into more detail on these sorts of issues in the article to be published in September. This will focus on active insulation garments. I have read your excellent articles on active insulation garments and think we come to some of the same conclusions.Aug 11, 2021 at 3:34 pm #3724584Bryan BihlmaierBPL Member
@bryanbLocale: Wasatch Mountains
Great article, Stephen!
For fun (I’m a geek), I analyzed your data in MiniTab too. I was mainly interested if any two-way interactions were significant – for example, does the effect of MVTR change depending on the temperature or vapor pressure? Turns out none of the interactions that made sense were significant. So high MVTR is the best for any condition (within the range you tested, of course). FWIW.
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