By The Numbers Quickie: Meet my Shakedry Replacement-The Finetrack Photon WPB

Here is a little analysis that looks into the impact of air permeability and ventilation. Ventilation measures the air exchange through a garment under various conditions. It is based on a study by Havenith, et al and can be read here.

In this study, a person was dressed in coveralls with 3 air permeability levels. Ventilation in the jacket was measured using tracer gas with the test subject standing still or walking, still air or 2.2 mph air (about hiking speed). And jacket openings open or sealed.  The data from the study is in the following table:

I looked at the data and concluded it pretty well reflects what I said above about air penetration of a fabrics and also agrees pretty well with my article, cited above. Interestingly, the P1, low permeability fabric is about 1 CFM/Ft2, so its performance is at the high end of electrospun membranes.

So, I submitted the data table to ChatGPT.  I asked what it thought of the data and what conclusions can be drawn.  I also asked another question to clarify the role of pumping, which we will see matters more than anything else. I think it makes for an interesting read, so here is the discussion.  I hope not to nerdy, but the readers may disagree.

Interesting use of AI and interesting conclusions.

If pumping is so important, to get less condensation inside then youde want a looser garment with gaps at the bottom, sleeves, pit zips.

If you wanted to stay warm and condensation wasn’t a problem, have elastic at bottom to eliminate any gap, and at sleeves.

When I’ve used pit zips I never found them to be effective, but considering pumping, pit zips may actually be useful

Oh, I remembert hat study. Interesting given this discussion.  Of course, as the study is made under controlled conditions. But I remember that this study or something like this showed me the effect of pumping and the importance of the overall design of an outerwear jacket. Where you have ventilation openings is important and the longer you can get them open, the longer that they may have an effect. Even if that effect is very, very small, I noticed that that may have a positive effect on my  comfortlevel.

I’m a poncho user if the conditions allow because for me all this is pretty immaterial in any jacket or anorak once temp > ~ 15C, I’ll just poach like a chicken.  Even with a poncho, I grab the front and pump it back and forth to flow air in and out the arm holes.  Makes a nice difference.

15 C = 59 F – yeah, at that temp I’ll sweat if I wear very much.  It’s tough to stay dry at that temp.

There’s an overlap between sweating from too much insulation for the temp, vs the rain jacket not being breathable enough

A solution is to take off my rain jacket.  My shirt will get wet but because it’s warm, I’ll stay warm.  Eventually, my shirt will dry out.  Keep as much as possible of your clothing in your pack where it will stay dry

That?s exacttly my idea: at that temp, usually I don’t  need any rainprotection, even when it rains. As long as my core temp remains stable, I don’t get cold and I stay comfortable. And after all, that is your actual purpose.

A poncho is a great example.  A poncho is waterproof and generally vapor impermeable–not breathable.  Neither wind nor vapor can go through the fabric. A poncho is comfortable due to its ventilation, which occurs through pumping and wind-driven air entering from the open hem and leaving the hood and sleeve openings (or a combination of each).  If wind blows on the poncho, it deforms and pumps.  If you are walking or just breathing, your body moves the fabric, and it pumps. Of course, the downside of a poncho once it is cool is that you cannot readily control the movement of wind through the poncho, and you have limited control over ventilation due to its pumping. The poncho can lose a great deal of body heat when you are trying to stay warm.  In a jacket, you can close your zips, tighten your hem, velcro your sleeves and pull your hood tightly around your head with your adjustment strings. This will reduce pumping so that air exchange with the outside is minimized and pumping will mainly cause internal air to circulate within the jacket.  When the jacket is tightened up, MVTR is still available to dump moisture.

We can observe the trade-offs between the ventilation mechanisms inherent in ponchos and jackets and determine which might be better suited for the conditions we expect.

There’s an overlap between sweating from too much insulation for the temp, vs the rain jacket not being breathable enough

 at that temp, usually I don’t  need any rain protection

I don’t even touch the rain jacket until low temps and in order to avoid hypothermia and only in high wind where the poncho is a no go.  At those lower temps, MVTR & pit zips start to have some real value because of the temperature difference inside vs outside the jacket

I use the poncho even when its warm to keep water from running down my butt and causing chafing.  Its also good if using a pack without waterproof fabric to pick up less weight, but I relegated that pack to winter duty

My Poncho design has a full length opening down the front. Plenty of ventilation when I need it, or nice warmth if I close it up.

Photo from L Gibson – copies of my pattern

My wife Sue on Mt Jagungal. The cairn she is ‘touching’ is actually covered in ice. No contact!

Cheers

I always felt that was very practical design but I’d prefer more open sleeves to trade off some dripping in for more heat escape.

Bluefield makes a (weird, heavy) knock off

Something like this in 1/2 to 3/4 length uncinched wide (open end, uncinched for pumping) sleeves made from UL ultra sil or similar might be ideal, but unfortunately doesn’t exist

It would take something like that to budge me from my 6.5oz Sea to Summit Ultra-Sil Nano Tarp Poncho which also doubles as an emergency shelter on day hikes

Edit: this might be getting closer but the zipped one doesn’t look like it would fit over a pack: https://3fulgear.com/product/accessories/ultralight-tarp-poncho/

Thanks Stephen for starting another interesting discussion, and everyone else for all the interesting posts!  A couple questions and thoughts:

There was a statement that with urethane membranes, MVTR rises when they become water saturated.  My understanding was that while ePTFE fabrics like Gore Tex lose most of their MVTR when they wet out, some moisture still escapes, but with PU membranes, none does.  I’ve seen this several times, including on Backpackinglight.  Is this wrong, or incomplete, or … ?

Regarding the Helly Hanson polypro garments, when I first heard of these I was interested for the same reason, hoping they would not wet out.  But I found on online review stating that in extended rain, the exterior fabric did absorb water in places.  (Alas, it never seemed to occur to the reviewer that polypro might have such an advantage, so they did not go into detail.)  I also contacted HH and while they answered a few questions, once I asked questions about this potential advantage (as I recall, does it wet out, how much water can it absorb per m2, and what happens to MVTR if it does wet out), they did not respond.

As far as I can tell, this is not what they were aiming for.  Rather, they see the advantages as that it does not need reapplication of DWR, and it’s environmentally friendly (unconvincing claims, in part because they compare to new polyester, not recycled).  Perhaps they also made it because they want to differentiate their product and/or love polypro.  When I finally had a chance to try it on, it was unpleasantly stiff and the neck/hood opening was going to lose any battles with pumping or wind.

Also, the line chart produced by ChatGPT suggests an interesting hypothesis.  It seems entirely possible that half or more of the ventilation benefit of going from 1 cfm to 86 cfm could be obtained just by going from 1 to 2.  If so, we’d come back to David’s question about what levels of measured performance correspond to perceived changes.

Finally, I continue to wonder if comfort differences between different wpb and wind shell garments aren’t due not only to MVTR but also to what happens to the moisture that doesn’t escape.  I suspect that some such garments absorb and/or wick moisture more than others and stay comfortable longer.  This seems more likely with looser weaves (and maybe even membranes with bigger pores?), so it could be that there is a correlation between CFM and absorption/wicking, with the latter contributing some/most/all of any perceived increase in comfort.

In this case, the knit lining of the Finetrack jacket might be a big plus.  Then again, the porous woven outer fabric might wet out quickly and a lot, inhibiting moisture transfer and taking a long time to dry.  I wonder what Finetrack thinks of wringing their garments out … .

Hi Andrew S.  I will try to answer a few of these questions. Gibson demonstrates that MVTR for hydrophilic membranes can increase as the RH ratio increases across the membrane.  In most of his examples they go from poor to mediocre as relative humidity increases on one side.  Porous membranes generally do not exhibit this behavior unless a PU membrane is laminated to a porous membrane, as was done with some variations of Goretex.  This behavior has nothing to do with membrane performance when one side is saturated, such as may occur as a result of saturation at a membrane face when a shell face fabric wets out.

Can vapor transfer through a wet-out face fabric?  I have never tested this.  It would make an interesting test, and one I should try sometime.

I think you are referring to the little table I presented above concerning the impact of pumping under different conditions.  That table shows that in the test, increasing air permeability from 1 to 86 had virtually no impact on ventilation in a low breeze.  This demonstrates that increased air permeability is not responsible for increased ventilation.   Increased ventilation results from pumping.  Comparing the walking and standing numbers produces an outsized impact on ventilation.  Open apertures or wind increase from 0 to 2 MPH did not.

Can vapor transfer through a wet-out face fabric?  I have never tested this.  It would make an interesting test, and one I should try sometime.

+1

Maybe asking the same question

Does water vapor go through clothing and condense on the wetted out layer of water

It seems like this is minimal because when my jacket wets out, I get more water collecting on the inside surface of wpb

Water vapor will condense whenever it encounters the dew point. The dew point is a function of temperature and relative humidity.  I think we established previously that the MVTR of your jacket is lacking.  This means that vapor that travels through your clothes will accumulate at your jacket faster than it can pass through.  This causes the RH humidity at the inner face of your jacket to continuously increase.  This will eventually end in achieving dew point on the inner surfaces of the jacket.  Even unzipped, you will probably get condensation in the sleeves, which will ventilate poorly.  Maybe you should wear the Neoshell jacket that you made.  Or, make a new jacket that has higher MVTR.  How about uncalendared Hyper D that you treat with Nikwax?  Very good MVTR, moderate air permeability, and light rain resistance.

Yeah, good idea

I have this plan to use the neoshell which has better MTVR, and the rsbtr which is worse, in similar conditions and see what happens in real life.  It’s going to be a while before it rains again

This means that vapor that travels through your clothes will accumulate at your jacket faster than it can pass through.  This causes the RH humidity at the inner face of your jacket to continuously increase.  This will eventually end in achieving dew point on the inner surfaces of the jacket.  Even unzipped, you will probably get condensation in the sleeves, which will ventilate poorly.

Exactly what happened with my Paclite jacket. As long as the DWR functioned fine, I had no condensation problem. But from the moment DWR started to fail and fabric started to wet out, I had condensationproblems which was rather uncomfortable when walking with bare forearms underneath it.

That same jacket had also shoulderparts in 3L GTX XCR with a differently woven face fabric (plain weave vs. ripstop) and I seem to remember that plain weave wetted out later then the ripstop. Maybe also something to look at it: how does the weave influence the functioning of the DWR?