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

[Indrit S]

I also tried to understand a little bit better Stephen’s results in terms of CFM/MVTR and the fact that the very same people behind this website appear to not really believe on it or at least as if they have failed to take the results into account (which is quite  interesting, you publish a serious series on your own website to finish not following them) but I received a hastily answer above…

[Roger Caffin]

IMHO:
your weather is not my weather
Your terrain is not my terrain
Your physiology is not my physiology
Your walking speed is not my walking speed
Your comfort level is not my comfort level

So why on earth should my clothing suit you?

(Silnylon poncho over a Taslan windshirt, but with a high rate of climbing.)

Cheers

[Philip F]

Hey there – I have some real world testing anecdotes to add.

I have an ultra lightweight arcteryx shake dry top and a highly air permeable Patagonia Houdini.

I’ve run 5 miles at a reasonable pace using both back to back on similar days with similar pace in the same trail -40 degree temps, no wind, cloudy, and the shake dry leaves my shirt totally soaked underneath, while the Houdini is nice and mostly dry.

something with air permeability is going on that your tests aren’t measuring.

my pace wasn’t anymore then 6 miles an hour and I doubt that amount of “wind” would cause that degree of evaporative differnce

 

[Bill Budney]

If your Houdini is highly air permeable, then it must be pre-2012? I don’t think Stephen has been able to get his hands on one of those to test. It may also have very high MVTR.

[Philip F]

No it’s a couple years old only.

[Philip F]

My point is that that’s study lists them as relatively the same MVTR and the only differnce is air permeability – this does seem to have an effect on under-garment humidity more then the study would have us believe

[Bill Budney]

Newer Houdini’s have been testing with very low air permeability.

Any chance it is a Houdini Air?

That would fit your description (and question) better.

[Philip F]

It may well be.

Again – wanting to point out without getting bogged down in details that either way it’s air permeability will be higher than shakedry with likely similar MVTR.

I have quite a few air permeable garments and all of them have similar improved moisture Managemnt performance over shake dry.

MVTR in my experience is not the only variable if we are to believe as the tests show that it is similar across these groups.

if we do believe it’s similar then air permeability is actually important, or MVTR is a poor measurement tool or measured incorrectly in the study

[Bill Budney]

I had similar thoughts about my OR Ferrosi soft shell, so I sent it to Stephen for testing. The result was that the soft shell MVTR was very much greater than ShakeDry. It was consistent with Stephen’s hypothesis. Yes, the soft shell was also more air permeable, but I’m persuaded that the MVTR makes the difference in comfort.

Your experience is interesting, because Houdini (and Houdini Air) both test with lower MVTR than ShakeDry. Perhaps Stephen will weigh in (or at least consider the question for his next article).

[Philip F]

Agreed. I would be interested in a possible explanation that still holds MVTR as the greatest factor in breathability.

 

 

[Ryan Jordan]

There are a few things going on with wind shirts in real-world use that don’t have much to do with wind (which is the focus of Stephen’s recent CFM vs MVTR work):

1. Wind shirts are made with thinner and lighter fabrics. This increases bellowing, which pumps air in and out of the garment.
2. High CFM garments (in a closed system at least) create higher air pressures (generally not measurable with consumer-grade sensors, which can’t really measure down to 1/10th mb) that create air pressure differentials across the fabric membrane up to 0.5 mb or higher. This is enough to create an air pressure gradient that could be pushing some moisture out.

Moisture vapor movement across a fabric results from passive diffusion (vapor pressure driven) and active convective movement (air pressure driven). The latter is a process that we don’t fully understand yet and is the one missing link we still have in this discussion. But after doing the math, I think a few tenths of mb of air pressure differential is enough to have a noticeable impact *if* a garment has a CFM rating of at least 30 or so.

[Philip F]

Makes sense, thanks.

So high CFM garments do play a role

[Ryan Jordan]

Yes, they play a role. But we don’t know to what extent, air pressure gradient, induced moisture transfer contributes to the overall breathability of a garment.

[Philip F]

We’ll certainly my real world experience would indicate to stick with wind shirts and not shake dry

[Stephen Seeber]

Hi Philip:

I appreciate your comments.  If you review this article carefully, I comment on how the garments were worn.  In all runs, the sleeves and bottom hem closures were tightened and, as needed, retightened during each run.  In addition, the hood was always worn, and the drawstring tightened.  This was done to minimize the influence of pumping and flapping during the runs.  I also discussed the impact of fit on the results.  The results were consistent across the original three jackets I tested but showed a significant change when I added the Arcteryx jacket.  I attributed this impact, without detailed study, to the difference in fit for this jacket:  it was closer fitting than the others. Of course, I also measured all the relevant weather conditions and determined their impact on the results.  I also measured the baselayer weight before and after each run as a surrogate for how much water was retained for a particular jacket.  Finally, I continuously measured my level of effort in terms of speed, calorie expenditure and MET level during each run.  Of course, I also measured each jacket’s air permeability and MVTR.  Then I did the statistical analysis, whose results were not close when determining the impact of MVTR vs. air permeability.

With respect, I suggest unless you take similar steps, it is pretty difficult to reach performance conclusions.  We don’t know, from your description, what Houdini you wore: the regular or the Houdini Air. We don’t know their condition, fit, adjustment, etc.  We don’t know how comparable was the level of effort.  We don’t know enough about the weather conditions, especially humidity and wind speed.

The air permeability of the regular Houdini is pretty close to that of the Shakedry.  The air permeability of the Houdini Air is substantially higher than Shakedry.  The MVTR of the regular is considerably lower than Shakedry.  The MVTR of the Houdini Air is a little lower than the Shakedry.

So, I don’t think we have enough information about many factors that could have contributed to your experience.

If you have not read my latest article, you might find it helpful.  In the article I am presently working on, I will, among other things, present the correlation between Air Permeability test results and the velocity of air that actually penetrates a wide range of garments.  I think the results will be surprising, so stay tuned.

 

[Philip F]

Thanks Steven – all of those factors such as flapping, fit, etc would certainly make a difference, but I will say that this is an across the board difference. I have multiple windshirts – Patagonia Houdini air, Patagonia air shed pro, black diamond alpine start – they all subjectively breathe far better then the shakedry – I’ve used them all extensively – running similar courses, backcountry skiing and all the wind shirts just work better – you can say that that’s not scientific and of course it’s not a controlled experiment but one can’t discount what works.

anyway, I appreciate your work and your articles and will continue to look forward to reading them with interest

[Stephen Seeber]

Philip: Interesting.  If you would like to follow up, why don’t you try a little mini-experiment along the lines of what I did for the article and compare performance for your Shakedry and favorite of the various windshirts?  One thing (among many) that I don’t know, is whether Gore made different versions of shakedry for different clothing manufacturers.  If you want, I can test yours and then we will know how it compares to the Montbell Shakedry that I use.  If you are interested, PM me.

[Philip F]

Hi Steven – certainly a fascinating topic but unfortunately not fascinating enough for me to devote the time for setting up a testing apparatus. I feel I have a pretty good idea of what works and dosent in real world scenarios.

Have you considered that the membranes are just plain “hotter” – perhaps since there’s no air permeability they can’t thermoregulate as well, and the extra heat build up manifests as more sweating which overwhelms their MVTR whilst the more air permeable fabrics allow air transport in and out which would allow for a more cooling effect and thus less sweating?

seems as though perhaps there may be more at play then just MVTR numbers alone.

a corollary question would be if you’ve tested baseline characteristics of say a lightweight polypro shirt – if the MVTR was similar to other fabrics in the windbreaker /shakedry range – say 3000 – then we would know that something else is at play because that obviously dosent make real world sense as we know anectodotally that lightweight polypro breathes better then shakedry or wind shirts

[Jerry Adams]

Do you think WPB would work in a tent to reduce condensation inside?

There would be insignificant temperature difference across the fabric, but there could be a humidity difference.  100% humidity inside, if there’s condensation, but less than that outside.

Maybe this question is outside what you studied.

[Roger Caffin]

Do you think WPB would work in a tent to reduce condensation inside?
That is a definite ‘maybe’.
Sorry!

From experience, I would say that ventilation would be far more practical.
My 2c.
Cheers

[Stephen Seeber]

WPB fabrics have been used in a variety of tents.  I have never tried one.  I would like to. I have seen user reports that are, like most things, all over the place. I suspect the answer will always be: it depends on the environmental conditions.  But, as I said, I have no practical experience with a WPB tent.

[Jerry Adams]

Thanks

[Bill Budney]

Ryan has a WPB tent — his Locus Djedi (DCF-eVent) that he says is one of his favorites.

Would he lend it to Stephen for testing?

[Jerry Adams]

Is a WPB tent better because it has less condensation inside?  Because it’s breathable?

An experiment would be to have a waterproof tent next to an identical tent except with WPB.  Humans sleeping overnight.  Does one have less condensation inside in the morning.

I bet there are users of WPB tents, like Bibler, that have this experience.

[Jerry Adams]

I have seen the texture of fabric make a difference.

Like, on my sleeping bag next to my face is some supplex.  There is less condensation there.  The supplex has a rough texture.  The rest of the sleeping bag is slippery nylon.

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