Air permeability vs. moisture vapor transmission rate (MVTR): which one impacts moisture transport more in wind and rain jackets?

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.

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.

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.“

https://www.patagonia.com/stories/from-the-pct-to/story-20305.html

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?

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, Max

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.

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.

Christopher, so you’re saying the Airshed (CFM ~70) is so permeable that lots of cool air blows through the fabric and keeps you cooler so you don’t sweat as much? Intrinsic ventilation? That might be more important than any breathability advantage offered by high permeability fabrics, definitely.

But you can get that ventilation extrinsically with a little work: If you take a low-CFM windshirt with extrinsic ventilation (pitzips, or laser perforated armpits, plus front zip, maybe with a snap to keep the zip open but the front from flying open all the way, loose hem, etc.) you can have the same thing – venting to allow more cool air in to prevent overheating. But if your activity level drops and/or the wind comes up, you can batten all those venting hatches and enjoy the low-CFM/high-MVTR fabric actually blocking the cold wind.

With the Airshed, what happens when the sun goes behind the clouds, the wind whips up, and I start going downhill instead of up? I’m going to be reaching for another layer, because 70 CFM doesn’t really block the wind.  And that second layer had better have good breathability, because now my perspiration will be fighting to get through two layers instead of one.

I like high-CFM windlayers when it’s borderline too-warm to wear a windshirt in the first place. Today I wore a 35CFM Squamish in 55F and 20-40 mph winds. The wind wasn’t cold enough to bother me even though my shirt was pretty permeable, and it was nice to have some air coming through without having to fuss with zips and cuffs and hems. But when it gets near freezing and windy, I want a low CFM/high MVTR windshirt with ventilation options to vent heat/moisture when my activity level goes up. And I want that low CFM fabric to actually protect me from the freezing wind. My 35CFM shirt sucks in those conditions.

Good read, I was a bit surprised to see the weight gain of the more air permeable fabrics which led me to wonder if you consider weight of the actual jackets used after activity. The fabric specific construction and finishing could be effecting how much moisture is actually transferring outside and not just into the fabric itself. Stretch Windshells are typically using a twisted or crimped nylon which would tend to retain more moisture inside the yarns. Perhaps there is a domino effect increasing condensation inside the jacket, thus more weight gain of the baselayer shirt. Anyway, after much experience with multiple types of shells I do have a hard time accepting that a shake dry fabric with a WPB film is more comfortable than an air permeable wind shirt so I am just wondering if something else is going on.

However, as air permeability goes up, you have less of a wind shirt. After all, a wind shirt is supposed to block wind.

To put this in simple English: you can not have your cake AND eat it.

But I m sure marketing will continue to claim that you can. It’s your wallet they want.

Cheers

MVTR of both the wind shirt and the shell jacket would deteriorate as the environment temperature gets higher
I don’t think that is correct. I suggest the MVTR of an uncoated windshirt depends on the gaps between the threads, and that will not change as the ambient gets warmer.
If the fabric has a coating I would not call it a ‘windshirt’.

Cheers