By the Numbers: The New Breathability Gap in WPB Jackets and What the Breathability Tests Get Wrong

there can be a multiple reasons, but this is certainly one them

Is one of them simply that when the exposure to rain is heavy and long enough it overwhelms the DWR?  That’s been my impression, but if it’s true, I don’t understand why.

Are there other reasons?

DWR coatings have a physical limit to the volume and force of water they can repel. Heavy, continuous raindrops strike the fabric with significant kinetic energy. Over time, this constant bombardment forces moisture directly into the microscopic spaces between the fabric’s fibers faster than the DWR can shed it. Once the outer fibers absorb this water, the chemical barrier loses its effectiveness, and the face fabric becomes completely waterlogged.

Even in a long drizzle it will happen. While each individual drizzle droplet is tiny, millions of them hitting the fabric over several hours will eventually saturate the surface. Once a few fibers absorb moisture, it creates a chain reaction that pulls more water in.

(generated by Gemini, checked by me afterwards)

Thank you, Stephen (and Ryan and the BPL community), for such an in-depth and robust analysis. There’s a lot in your article to digest!

It reminds me of Roger Caffin’s compelling sleeping-pad testing many years ago in his State of the Market report (I forget the year). What I remember particularly was his effort to measure R-values under somewhat realistic conditions by controlling and measuring pad compression. In a way, I feel you’re attempting something similar with MVTR.

That being said, I have a few questions:

1. Why 2.069 kPa? What led you to select that particular vapor-pressure differential? Are you considering testing at other VPDs? As we know, environmental conditions don’t always provide sufficient vapor-pressure differential to drive significant outward diffusion. I’m particularly curious whether the relative rankings of jackets might change at different VPDs, especially between microporous and hydrophilic membranes. That could be useful information when comparing hiking in Georgia on the AT versus the Sierra Nevada.

2. MVTR versus R-value: Over the years, I’ve watched people make purchasing decisions based largely on manufacturers’ sleeping-pad R-value claims, without fully appreciating how much actual field performance can differ from laboratory conditions. While I appreciate your efforts to establish a consistent method of measuring MVTR, do you have similar concerns about how consumers might interpret these results, particularly given the enormous variability in vapor-pressure differentials encountered outdoors?

3. Wetting out: Have you considered testing garments after their DWR has degraded or failed? This is a particular concern of mine. For decades, the industry has sold expensive waterproof-breathable garments whose real-world breathability can deteriorate substantially when the face fabric wets out. Yet consumers may not fully appreciate how dependent their jacket’s performance is on maintaining that DWR treatment. I’d be very interested in seeing how your MVTR rankings change under those conditions.

4. Air vs. Vapor: I recently returned from a building-science symposium where one of my biggest takeaways was the importance of examining the four control layers of a building envelope: water, air, thermal, and vapor—in that order. It got me thinking A LOT about the parallels with outdoor clothing.

Here’s an interesting article by Joseph Lstiburek on the relative importance of bulk air movement versus vapor diffusion in moisture transport:

https://buildingscience.com/documents/building-science-insights/bsi-149-slide-rules-pocket-protectors-cigarettes-and-iconic

Anyway, I really appreciate your work here and I thank you!

Woubeir: I asked a different question of Gemini:

Your statement makes DWR failure seem inevitable. Yet I have experienced many occasions when it didn’t occur. I think your statement is misleading and doesn’t guide WPB users on what conditions cause failure, which chemical treatments might be most successful, or which maintenance practices would provide the best performance. Further, you have provided no scientific evidence for the mechanism you describe.

Here is the response:

You are entirely correct, and I appreciate the critique. That previous forum post presented a generalized description that oversimplified a complex physical process, making wet-out sound like an absolute certainty rather than a conditional threshold event. It lacked both the nuance of varying environmental conditions and the empirical backing required to explain why and when DWR systems actually fail or succeed.

To address those gaps—specifically the conditions for failure, the performance of different chemical treatments, maintenance practices, and the physical evidence—here is a more rigorous, scientifically grounded breakdown:

1. What Conditions Actually Produce DWR Failure?
Wet-out is not inevitable in every rain event; it only occurs when cumulative mechanical and environmental stressors exceed the fabric’s hydrophobic threshold (governed by the Cassie-Baxter to Wenzel state transition). The primary triggers are:

High Kinetic Impact & Drop Size Distribution: Fine mists and light drizzle rarely cause failure because the droplet mass and terminal velocity are insufficient to generate the stagnation pressure needed to bridge microscopic fiber gaps. Failure typically accelerates in heavy, driving rain where larger, high-velocity droplets deliver sharp, localized mechanical pressure peaks.
Prolonged Mechanical Fatigue: Sustained bombardment over hours gradually fatigues the air pockets trapped beneath droplets on a micro-rough surface, allowing water to pin to the solid surface features.
Surface Contamination (Surfactants): Body oils, sebum, sunscreen, and environmental soot/pollutants are the leading cause of premature failure. These compounds coat the hydrophobic fluoropolymer or hydrocarbon side-chains, drastically lowering surface energy and allowing water to spread out rather than bead.
2. Chemical Treatments: C6 vs. C0 vs. Hydrocarbons
The chemical architecture of the DWR treatment dictates its baseline durability and contact angle stability:

Short-Chain (C6) Fluoropolymers: Historically the benchmark for performance. While C6 still contains fluorinated carbon chains, they are shorter and less persistent in the environment than legacy C8 chemistries. They provide exceptional oil and water repellency, maintaining high contact angles (often >110°) even under challenging conditions.
PFC-Free / C0 (Fluorine-Free) Treatments: Modern regulatory and environmental shifts have driven the adoption of fluorine-free alternatives, typically based on hyperbranched dendrimers, polyurethane dispersions, or long-chain hydrocarbon chemistries.

Performance Trade-off: While C0 treatments excel at initial water repellency (shedding clean rain effectively), they generally possess lower surface energy thresholds for oils and sebum. Consequently, they require more frequent cleaning because body oils contaminate and neutralize them faster than fluorinated treatments.
Silicone & Paraffin-Based Treatments: Often used on traditional or niche garments. They offer good initial water shedding but lack the mechanical durability and abrasion resistance of modern engineered fluoropolymer or dendrimer treatments.
3. Maintenance Practices for Optimal Performance
Because contamination and physical wear are the primary culprits behind loss of performance, maintenance is critical to preserving the DWR state:

Frequent, Low-Temperature Washing: Washing technical shells regularly with a technical cleaner (such as Nikwax Tech Wash or Grangers Performance Wash) removes accumulated body oils, sweat salts, and surfactants that degrade surface tension. Standard laundry detergents leave chemical residues (fragrances, brighteners) that act as wetting agents, completely ruining DWR performance until rinsed out.
Heat Activation (Tumble Drying): Most modern DWR treatments (both C6 and C0) are “orientation-sensitive.” Low-to-moderate heat from a tumble dryer for 15–20 minutes after washing melts and re-aligns the microscopic hydrophobic side-chains (the “hairs” of the DWR treatment), making them stand upright again to restore the contact angle.
Restorative Replenishment: When mechanical wear eventually strips away the DWR molecular layer from the outer filament tips, wash-in or spray-on treatments (preferably heat-cured variants like Nikwax TX.Direct or Grangers Clothing Repel) can replenish the topical fluoropolymer or hydrocarbon layer.
4. The Scientific Basis (Empirical Evidence)
The transition from a water-repellent state to a wet-out is well-documented in surface science literature through two foundational models:

The Cassie-Baxter State: When a drop sits on a well-maintained DWR-treated fabric, air is trapped in the microscopic spaces beneath the droplet. The contact area between water and solid is extremely small, yielding high contact angles and easy roll-off.
The Wenzel Transition: When external energy (kinetic impact pressure) or reduced surface tension (from oils/surfactants) forces the liquid to penetrate those microscopic air pockets, the system transitions to the Wenzel state. Water fully wets the grooves of the roughness structure. Research on wetting transitions (such as studies on hierarchical surface roughness and impact dynamics by physicists examining superhydrophobic surfaces) demonstrates that once this transition occurs, capillary suction takes over, pinning the water to the yarn and causing the macroscopic wet-out you observe.

@Stephen

The answer of Gemini was much longer. So  I’ll only quoted those parts that that started with a  simple answer to the question. Ofcourse, the whole answer is more complex but I deemed those parts too complex.

And let’s not forget AI wants to please you in the first place. I’ve experienced that many times. E.g. this: “Fine mists and light drizzle rarely cause failure because the droplet mass and terminal velocity are insufficient to generate the stagnation pressure needed to bridge microscopic fiber gaps. …” AI lets it sound as if it rarely occurs. It does that in a way as if it’s a given fact. Yet, I experienced just that many times.

I asked the AI if there are any materials that are hydrophobic, lipiphobic, don’t wear out from mechanical force, are resistant to UV, aren’t toxic to the environment either in manufacturing or use.

To paraphrase, it said no, good luck with that, that’s the holy grail of material scientists,…

The AI is so polite

My conclusion is to just use my neoshell which works pretty good.  And I have a wind blocker that I have t really tested properly in the rain

Discovery fabrics sells it.  They don’t know when it was manufactured or whether it’s c6 or c8 or what…  What I bought several years ago works pretty good but I don’t know if it’s the same as what they sell now.

Hi Woubeir:  I figured you were aware of these things.  I was hoping other AI quoters will read this and understand the care that must be taken when asking questions and accepting responses.  Can you tell me where your last post came from.  It sounds like something that might have been written during the initial round of Gore-Tex failures from oil exposure because it was so specific.  My feeling about Gemini is that it is particularly sycophantic. You can probably adjust this out. I just went to settings, Your instructions for Gemini and added: Be less effusive or less sycophantic in your responses.  I also added: It is OK to push back if I say something you disagree with

Hi Matt:  Thanks for reading. I will try to answer your questions.

1: 2.069 kPa is .3 psi.  When I developed the kettles, I needed to design an instrument that was within my means.  I studied what Gibson had done to develop his Dynamic Moisture Permeation Kettle.  I adopted his standard vapor-pressure-difference target and used his work as my design inspiration. I do use other VPDs for my concentration testing of MHPU.  Concerning rankings:  Microporous membranes won’t change their rankings. MHPU rankings will change depending on their response to concentration. The article explains all of this.

2: I think here, you are not talking about VPD for a test. Rather, VPD for actual use.  Actual VPD will increase or decrease depending on your weather conditions.  High temperature and humidity will reduce and even eliminate the vapor pressure difference between you and your environment.  The forcing driving vapor from your jacket can disappear. In extremely cold weather and rainy conditions, VPD may also be eliminated.  All my WPB jackets have pit zips.  If they aren’t present in a jacket, I have them installed.  Ventilation, proper layering and exertion levels are your best lines of defense when all else fails.

3: Today I started a project to measure the impacts of wet-out and rainfall on MVTR. I don’t think what is happening in a WPB is fully understood.  I also think people may not be doing all that is in the power to try to mitigate these issues. So, stay tuned for more on this importrant topic.

4: I will take a look at the article. In my prior life, I worked regularly on building envelope failures in commercial and residential structures.

@woubeir said…

For me, the most important issue would be: a jacket has a certain MVTR when tested in a dry condition. What happens to the MVTR when the the fabric is wetted out? How quick is not important to me. If I’m on e.g. a 10-day trip, it is not important if it’s after 10 minutes, 1 hour or 2 days. I just want the same performance on the last day as on the first day.

I would have to politely disagree. A quick “pop-up” rain shower would explain why. A jacket that preserves it’s DWR performance for a 30-60 minute rain shower would offer a significant difference in comfort once that short burst of rain were done. One that wets out after 10-minutes would be pretty frustrating by comparison.

This past spring I had about (6) different WPB jackets that I “tested” by simply leaving them outside during the same light rain event. I wish I had a rain-gage & timer to actually “measure” their performance. However, setting them outside during the same rain event, is about as objectively real-world as I can think. (Save identical sextuplets wearing each sample on an identical hike, exerting the same effort, during the same rain event, at the same time…) :)

https://backpackinglight.com/forums/topic/why-you-should-spend-a-few-ounces-of-pack-weight-on-rainwear-ventilation-features/page/3/#post-3852184

Some of these jackets would have been just fine wearing for an extended period of time. Others, not so much.

Long-term, one has to also think about preserving the DWR as well. On that topic, I’m taking more seriously the idea of jackets that stuff into their own pockets.  After noticing light soiling on my own WPB jacket that spends time in my pack, “rolled” inside it’s hood, this soiling & frequent physical contact is no-doubt having an effect on the DWR performance. Stuffing the jacket inside it’s own pocket keeps the external face-fabric “hidden away” until use.

My apologies for de-railing the conversation away from the MVTR discussion but I do think time-to-wet-out is an important factor as well, with regards to MVTR & comfort.

In the field, when my fabric has wetted out, when it stopped raining, the fabric has dried enough from body heat that the surface is larger free of any water film.

So, on a 10 day trip all I care about is the time that it’s heavily raining and maybe an hour after for it to dry enough to be effective again.

And second important, again, what I’ve experienced in the field, is sometimes the jacket gets dirty and then wet out happens quickly, water will accumulate on the inside surface, then I’ll get wet.  If I wash it, apply DWR treatment, tumble dry, then it works much better.

I loved the AI response where it described the Cassie-Baxter to Wenzel transition. That is something I teach my students, and it’s nice to see the actual technical description of the phenomenon we’re discussing.

Hi Dan, I hope you’ll join the ongoing discussion, which will go deeper in this direction. Now, I will get back to simulating wet-out and its consequences here at my house.

Hi Stephen,

Thank you for your quick and thorough response, and especially for trying to examine MVTR in wetted-out conditions.

As a practicing residential architect with building-science literacy, I find it great to know that we share a background in existing-building diagnosis. I believe there are close parallels between your research and the evolution of building envelope design over the past 30-40 years, particularly in how one must contemplate managing bulk water, air movement, vapor diffusion, and thermal performance.

I do hope you find Lstiburek’s BSI-149 article interesting. I also think the additional two articles may also be worthwhile and relevant:  BSI-099: It’s All Relative & BSI-016: Ping Pong Water and The Chemical Engineer

And please don’t get too wet!

Sneak Peak: Hyperlite Launches New Dyneema Rain Jacket | GearJunkie https://share.google/iucFpOntUl0OVt4I4

@JAshley73

ok, but that’s here not the question. Here, you want in the first place, an answer to the question: does wetting out influence the MVTR, with as a second question: how does influence it? Linear? Exponentially? Logarithmic? …

Your question is more like: how does DWR perform over time? And for me, that also raises another point: which factors influence DWR-performance and how?

There are several factors influencing the CB-to-W transition in the context of the PFA vs. non-PFA coatings. The fluorocarbon coatings do have a somewhat higher intrinsic water contact angle, but they are also much more resistant to contamination by hydrocarbons, which affects the contact angle. Mechanical durability is also an issue because the CB state depends on having a rough surface topography, which is intrinsically delicate. However, that’s a challenge across the board with all of these polymeric coatings.

Durable super-hydrophobic coatings have been a challenge to create at the industrial scale, and the loss of fluorocarbon materials is a big hit, because they have unique hydrophobic/oleophobic properties.

carbon-fluorine bond is strong, which makes it a good DWR, especially being oleophobic as well as hydrophobic, but that makes it hard to degrade – a forever chemical

they have experimented with mechanical structures, like maybe a bunch of hairs that stick up, but then they wear off quickly

I am reading Matt’s links.  Up to 5 layers of water molecules stick to a surface (adsorption).  The first layer strongly,…, the fifth layer just barely.  Very interesting, thanks

Dan: Do you have any idea how Toray achieved the jump in breathability in their Torain microporous PC/PU membrane?  Kuiu’s product description says they contain PFAS.  A recent Outdoor Vitals blog post says the Tushar jacket contains PFAS. The third US manufacturer using Torain, Akova, is silent on the subject.  I don’t know if the PFAS are part of the membrane, the DWR or both. However, it is unclear whether that answers my question, since they were free to use PFAS before the bans took effect.  The PU, should have lower contact angle than ePTFE, so it would seem the only way to increase breathability would be to increase pore size or add a greater % of pore space.  I have been impressed with the membrane performance for these jackets, but I am wondering if this is a temporay solution that will disappear when the severe weather exemption for PFAS ends in 2028.

what’s the severe weather exemption for PFAS

California PFAS rule: AB 1817 has an exemption for severe-wet-weather outdoor apparel until 2028. I don’t know the specifics of Colorado and other states that have PFAS bans, but they may will include the ban.

@Stephen

according to this instagram-post from Akova, the membrane itself is PFAS-free:  https://www.instagram.com/reel/DRDPZ1Fkh9Z/

Woubeir:  That is a conundrum!  I called Kuiu. Customer support told me that the PFAS is in the DWR, not the membrane. I will see if I can get any other information.  I am now wondering if the PFAS status is a function of when a fabric batch was manufactured.

Here is the first illustration of what water falling on a WPB does. This is not a shower. I am wetting the left side of a Neoshell sample mounted on a kettle.  A thermocouple is mounted on the left side (white line) and the right side (yellow). When I apply a very fine spray on the left side , the inner fabric drops below the calculated dew point. This is seen on the upper graph. Each of the downward spikes on the white line is when I apply more spray. From this data, I can calculate the approximate drop in MVTR from the wetted half.  It was about 7%.  This needs a lot more work, but the 1st result is encouraging.  The main plot shows the conditions beneath the fabric and the ambient outside the kettle.

I wonder if this tester is useful

Or this channel

https://www.youtube.com/@Textile-Learner/videos