Will Rietveld surprising finding in windshirt comparison

[Stumphges]

http://ultralightinsights.blogspot.com/2017/11/gear-review-patagonia-airshed-pullover.html

Will compares the Montbell Tachyon windshirt, the fabric of which I believe Richard Nisley tested at about 10 CFM, and the Patagonia Airshed pullover, which he reports being 5 or 6 times more air permeable (50-60 CFM).

Despite the much greater permeability of the Airshed fabric, Will measured greater humidity inside that windshirt than the Tachyon during hikes he rates as comparable.

What should we make of this? I’m thinking there’s a hidden & confounding variable at play. On the other hand, Will knows his stuff.

[Eric Blumensaadt]

Does the fabric of the more wind-resistant Montbell windshirt wick moisture quickly? If so it may create a cooling effect.

Interesting comparison of not just air permeability but retained moisture.

[Edward John M]

Is the fit of the garments exactly the same?

Perhaps there is a funnel effect with one and not the other venting excess warm moist air at the neck

I find my Montane UL windshirt can feel quite sodden inside at times when no fresh air is getting through the elasticised waist band

[Ryan Smith]

Is Will guessing at the airflow specs or has accurate 3rd party information? Assuming his flow estimates are spot on I assume a different cut in the jackets to be the culprit. I have a Tachyon and a custom windshirt made from Nobull 1 fabric with truly 5-6x more air permeability(IIRC).  I’ve never measured humidity, but the breathability between the two is not even in the same ballpark.

[Stumphges]

Ryan, I believe Richard Nisley tested the Tachyon fabric at 10 cfm. (Although I can’t find the post where he reported his test result.)

Will reports that a manufacturer rep quoted a 50-60 cfm figure to him for the Airshed fabric. Ryan Jordan reported a similar figure from a similar source in his review of the Airshed for BPL some time ago: https://backpackinglight.com/patagonia-airshed-pullover-review/.

Two posters have now advanced ‘looser cut of the less breathable windshirt” hypotheses to explain Will’s findings. If more humidity did build up under the far more air permeable Airshed pullover because it fit snugger than the Montbell Tachyon (and Patagonia cuts the chests of their jackets quite tight, IMO), this would support Will’s explanation. He concludes that garment ventilation trumps fabric permeability.

Could there be some other explanation?

[Bruce Tolley]

Thanks folks for pointing to this article and to Will’s site. Will is yet another long time contributor to BPL, who now appears to be going direct to his audience. Alan Dixon has a pretty good site as well.

[Richard Nisley]

I tested the 2014 MB Tachyon at 3 CFM. Moisture transport is positively correlated with air permeability, assuming roughly comparable fit.

I am in the field on an adventure trip and won’t have access to all of my data until I return 12/17. With that caveat, my recollection is that the US Army tests on WPB garments concluded fabric air permeability was the primary determinate of internal moisture transport and not fit/ventilation.

[Steve Collins]

Very unscientific testing but I was really surprised when I read Will’s article a couple days ago. I have used my airshed for most of this year and I truly find it is significantly better than my montbell or old golight windshirts (all have been 1/4 zip).

I have used the airshed in 95 degree high humidity nights to cool drizzly days. It’s my go to layer in the morning or as a base layer for sun protection. My only complaint is the back is destroyed by runs in the material from pack straps.

Obviously YMMV!

[Stumphges]

Richard, your thoughts on this would be greatly appreciated by all.

Steve, my experience with highly permeable windshirts (e.g. BD Alpine Start, cfm ~ 50-80) has been similar to yours. But I’ve never put a weather tracker inside to check my subjective impression against objective reality:)

The popularity of the Patagonia Nano Air stuff also belies Will’s findings. The face fabrics in that series are very similar to the Toray stuff used in the Airshed and people rave about the combination of synthetic fill with highly permeable face.

[Dan @ Durston Gear]

I appreciate Will’s article. It’s nice to see he put in the work to get some data.

If I had to pick at it, It looks like Will tested the different windshirts on different days. While I’m sure the conditions were similar, a small difference in ambient temperature or humidity (or Will’s condition) might underlie the observed differences in performance.

Based on the starting temperatures, it appears that the weather was colder on the day when the Airshed was tested (28F vs 34F stating temp). This might have a big effect on humidity because these are both likely near the dew point. So the ambient air might have been near saturated on the Airshed day, and thus exchanging the air inside the garment would be of little help. The best you can do with a windshirt is lower the inside humidity to be the same as outside.

It’s also worth noting that the Airshed tracks pretty similar for the first half hour, and then rises. This points to something in the environment changing, moreso than a fundamental property of the windshirt.

I have 3 copies of a windshirt that are exactly the same in every way except for the fabric. The fabrics are 10, 35 and 50 CFM. To me, the difference is obvious. I’ll be puffing up a climb, feel extra sweaty and then look to see which one I’m wearing and it’s always the 10 CFM.

[Stumphges]

Dan, which windshirt is that, if you don’t mind me asking?

 

[Michael F]

What about the Copperfield wind jacket?

[Martin D]

Fabrics are tested under pressure. ASTM D737 calls for a minimum pressure drop of 125pa across the sample. Which is the equivalent of a 15mph wind. I find it odd the standard doesn’t specify a very specific pressure drop as CFM will obviously be pressure dependant and as such to be comparable CFM test results must be quoted with pressure drop.

Anyway, what I wanted to say is that if there is no wind, hence no pressure, both fabrics might behave differently than on the standard test.

[Author]

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