By the Numbers: The Myth of Air Permeability in Windshirts

Another great article in an epic series. Thanks for all the hard work, Stephen. This kind of attention to detail makes BPL unique.

I am a convert: It’s MVTR (and ventilation) rather than air permeability that we should seek in wind garments. Now if only we could get manufacturers to post their measurements…

Perhaps your windshirt chart, Table 2 from your MVTR article, and Outdry Extreme Mesh test deserve a dedicated thread for easy reference, as well as updates as new information comes in? They are the best available references when shopping for shells.

Other observations from your article:

• At least one soft shell (Ferrosi) outperforms all currently-tested windshirts in MVTR. There is a substantial benefit for the few extra ounces (which are insignificant when wearing the shell). Plus pockets are nice.
• Roger Caffin is right (once again) that blousy is better for cooling. The industry trend toward “athletic fit” is more about style than performance (except maybe above treeline or when climbing with tackle).

Hi Bill:

Thank you for the kind words.  I have been thinking that a sticky posting of my performance data for windshirts and WPB garments would be useful, especially if I could add new listings over time.  If the powers that be are interested and can do that, I can provide the data.

How does the Ferrosi compare warmth wise to say a windshirt and alpha 60 or 90 combo?

Does it serve the same purpose?

Mostly it does the same thing as a windshirt. I find the Ferrosi more comfortable over a wider temperature range than, say, a BD Alpine Start. But the difference in warmth at cooler temperatures is small. If you wear insulation under your windshirt, then you will probably want insulation under your soft shell.

The fabric is relatively light; just not ultralight. They make shorts and Summer-weight pants out of the same stuff.

Another great article Stephen, thank you.

I am very much enjoying my Ferrosi too, although it really isn’t cold enough here to test properly yet!

The aerodynamics findings are fascinating, and it is almost obvious when you think about it! I am now questioning the location of the vents we use on our “top of the line” waterproof garments, they most likely do not work at all in the way we intend or believed.

Can you explain, in layman’s terms, the difference between vapour pressure and air pressure, specifically inside a garment? And is this related to RH% at all?

Thanks again, Scott

I also purchased the Ferrosi Hoodie, thank you for sharing.

Yeah, another great article.  You make a good argument for MVTR being most important to prevent sweating.

On your recommendation I got some Neoshell for a (blousy) rain jacket and I find it to be very resistant to sweating.  This is consistent with your theory that high MVTR is most important.

The Neoshell also has an excellent DWR coating and hasn’t wetted out in even steady rain.

I will now have to see if I can get the fabric from a Ferrosi jacket

“I am now questioning the location of the vents we use on our “top of the line” waterproof garments, they most likely do not work at all in the way we intend or believed.”

yeah, my thought too.

I have a jacket with pit zips and never found them effective

“I am now questioning the location of the vents we use on our “top of the line” waterproof garments, they most likely do not work at all in the way we intend or believed.”

yeah, my thought too.

I have a jacket with pit zips and never found them effective

To add to this. I am in Colorado at the moment. I went snowboarding the last few days with temperatures around 40. I was wearing a Marmot Minimalist Goretex shell with just a long sleeve shirt underneath and shell pants. It didn’t take long to heat up inside with the pit zips wide open the entire time. Having the sleeves closed up didn’t help.

Once a certain temperature is achieved it’s game over without unzipping the front and letting the heat out of the sleeves.

yeah, unzipping the front provides a lot of ventilation compared to pit zips

not that I’m dissing people that like pit zips.  Many people love them and consider them critical.

Jerry and Baja Bob, some clarification, I work for a company that makes apparel for mountain biking, so a  slightly different use scenario. We removed pit zips a number of years ago and changed to forearm and chest vents. The idea of these was less about dumping heat and more about creating a chimney effect. I personally have never found them that effective and now wonder if that is more down to the aerodynamics at work (stagnation and diversion) than anything else. I do tend to open these vents after warming up, so I suppose more to dump heat but of course their positioning doesn’t really help with that.

All great stuff to think about if nothing else!

My Ferrosi anorak Mens size M is 12.4oz. To heavy to carry on multi day backpack trips for me.
thom

Thanks Stephen, really nice work and this will help a lot of people.

Having established that air permeability isn’t a very useful metric, and MVTR is, have you been able to estimate how high MVTR needs to be in order to be useful?  For example, evaporation rates vs temp/RH differences for different MVTRs?  It would be interesting to know if its worth buying a garment because MVTR is 3200 vs 2500 for another.

Now that you’re all set up, any plans to measure the efficacy of pit zips?  Real world conditions could be emulated with mannequin arms extended in trekking pole position, a backpack mounted to block flow behind the garment to the chest, and measures taken under the arm pits, and repeated at chest.

Really looking forward to what comes next.

Hi All:  Here are responses to various comments:

Scott: The pressure differences that I refer to are typically between the interior and exterior of the windshirt.  I have discussed vapor pressure differential in a bunch of these articles.  If you want to figure it out for your conditions, use this online calculator.

If you want to know about how air pressure differences that create wind come about, use this link.  There are more relevant articles on this site.  Regarding air pressure inside a jacket, I would generally assume the pressure to be around zero in any units you choose.  If you know wind speed in meters/sec, you can convert to approximate pressure with this formula: Velocity Pressure=(WS/1.29)**2.  WS (Windspeed) is in units meters/second, VP (Velocity Pressure) is in units of Pascals.  This formula misses the impact of static pressure, which will generally be minimal for this use.  It will get you in the ballpark.

Vapor pressure is related to temperature and humidity.  Air pressure may be impacted by humidity but mostly from the forces described in the link above.

Jerry and others-Pit Zips:  Don’t expect miracles from pit zips.  If you want a cooling breeze and you are walking at 2-3 mph, in calm winds, you won’t get much benefit.  If you are above the tree line and the wind is blowing at 20 mph, you will get plenty of benefits.  With pit zips open, the garment MVTR can increase dramatically, depending (as always) on vapor pressure differential and what you are wearing between your skin and the jacket.  If you are wearing a backpack that restricts air movement or vapor movement toward the pit zips, then you will get less impact.  Of course, if you are wearing a jacket when it is too warm, or you are working too hard, take it off.

Baja: Skiing or boarding in spring conditions is tough when you start in cold temps and then it becomes warm enough to make corn.  At some point, you need to get rid of your wind layer, or you will sweat too much.  Unfortunately, at the resort, you may have no place to stow it, so you will sweat while wearing it, pit zips or not.  Remember:  Be Bold, Start Cold.  And stow a Montbell Tachyon in your pocket.

Thom: I think everyone who describes the Ferrosi remarks on the weight.  It is a beast for a wind shell. Nevertheless, for aerobic activities where you need some wind protection, it works very well.

David: The tested MVTR allows us to compare performance amongst garments.  It is much harder to apply these numbers to field use because there are many variables.  There is no drawback to a high MVTR if it is achieved with a reasonable Air Permeability rating.  This way, you can enjoy wind protection in high winds but still have the best chance of eliminating vapor.  For me skinning up mountains in the winter is about the most aerobic thing I do.  I found that my base layers remained dryer using the Ferrosi instead of my Neoshell jacket, as described in the article.  If I expect to get rained on, I will wear my now-discontinued Montbell Shakedry or my Neoshell jacket.  A word on MVTR.  The number from a test depends on the specific test.  For my test method, 3000 is pretty good.  4500 is great.  Most of the published data use JIS L1099, B2.  Here, 60000 is good.  80000 is excellent.  Often, 20000 is a result from this test.  That level of performance is pretty bad.  Not sure I can actually do lab testing of pit zips.  It is something for me to think about.  It would be a pretty complex thing to measure.

Thanks Stephen.  I wonder if the Ferrosi also works well because it absorbs sweat which lets wind provide evaporative moisture removal from the surface, allowing it to replenish and draw sweat off the body.  Like a good fleece or base layer.

I have a nordic ski top which I hike in dead of winter that does this but also has a wind block front to avoid freezing from wind.

Thanks, Ryan…well done.

<p style=”text-align: left;”>Thank you for an interesting and thought provoking article. I have moved away from skinning uphill with a conventional shell on. The R1 Techface breathes really well and is wind resistance for most skinning. When I get to an exposed summit or ridge then the shell can go on over for the descent. Only problem is wet snow which is not too common in MT. Has any one else experimented with this?</p>

Hi Stephen,

Thank you for yet another thought provoking and educational article. Do you have MVTR numbers for the airshed or other pertex quantum air fabric?

OR doesn’t say much about the Ferrosi fabric

“bluesign® approved 46% Recycled Nylon, 40% Nylon, 14% Spandex”

“durable yet lightweight, it’s weather resistant yet breathable”

does it have a membrane like WPB fabric?

Jerry:

No membrane.  Just woven nylon with plump yarns.  I don’t really understand how it scores so well on MVTR.  Spandex may contribute during use, but when I do the test, I drape the fabric and try to avoid stretching it.

Here is a photomicrograph, no stretch and backlit.  Very little light comes through, demonstrating low porosity.

Here is the same sample when stretched.  Much more light penetration and, therefore, far more porosity.

The fibers have little twist, which should increase permeability.

If you figure out who makes the fabric and it you can purchase some, let us know.

then it’s probably not real waterproof

Jerry:  No, it is not.  Its hydrostatic head is 183 when new.  The used one that I tested showed substantial degradation from that.  When skinning while snow was falling, it would not wet through.  I would not wear it in the rain if I hoped to keep dry.

Thanks Stephen,

a great follow up to your previous work.

Hydrostatic head does not tell the full story. In light rain, Ferrosi is better than real rain gear.

I suspect it is related to David D’s suggestion about absorbing water; allowing body heat to cook off the moisture before it reaches the skin.

That behavior is not unique to the Ferrosi. In my experience, 60-40 parkas and other soft shells/windbreakers are more comfortable in light rain than real rain gear. Their superior breathability seems to be more important than pure water-proofness.

(The ultralight windshirts that I have tried mostly do not behave the same way; possibly excepting the BD Alpine Start. An example is my EE Copperfield. Although it has higher HH than my Ferrosi, the Copperfield provides almost no noticeable rain resistance.).

Agreed that Ferrosi isn’t a rain jacket. Heavy rain overwhelms whatever resistance it has.