The Breathability Gap: WPB Membrane Numbers, 2026

The Gap Is Now 4×

Stephen Seeber tested 21 current waterproof-breathable (WPB) jackets and fabrics using a permeation kettle, then compared those results with his 2021 dataset of 18 jackets. The spread between top and bottom performers has widened to approximately 4×. The cause: PFAS chemistry is leaving the market and taking the highest-performing membranes with it.

The full study — “By the Numbers: The New Breathability Gap in WPB Jackets and What the Breathability Tests Get Wrong” — was published at Backpacking Light on October 22, 2026 behind a membership paywall. What follows covers the membrane taxonomy, the test method, and what the numbers mean for shell selection.

Two Membrane Families

WPB jackets rely on one of two strategies.

Microporous membranes — expanded polytetrafluoroethylene (ePTFE) and newer expanded polyethylene (ePE) — work by physical structure. Billions of pores are smaller than a water droplet but larger than a vapor molecule. Vapor moves through by diffusion and pressure differential. No heat activation is required.

Monolithic hydrophilic polyurethane (MHPU) membranes have no pores. Water vapor passes through the polymer matrix by absorption and diffusion — a chemical process that depends on temperature and moisture gradient. The membrane must be warm and moist on the inside to transport vapor efficiently.

Until recently, most high-end WPB fabrics used ePTFE (Gore-Tex, eVent, earlier Toray Dermizax) because microporous transport is fast and largely temperature-independent. PFAS chemistry — specifically PTFE processing and the DWR treatments applied to face fabrics — is now under regulatory and voluntary phase-out pressure across Europe and North America. That pressure is redistributing the market away from ePTFE.

What the 2026 Numbers Show

Seeber ran every sample on a permeation kettle at a constant vapor pressure differential of 0.300 psi (2.069 kPa). The kettle method is not a standard commercial test, but it is consistent and reproducible across his dataset. All figures below are in grams of vapor transmitted per square meter per 24 hours (g/m²/24h) at that differential. Do not compare them with manufacturer-quoted JIS or ISO figures, which use different conditions and produce substantially higher numbers.

High end (2026 dataset):

Seeber characterizes the top performers as microporous. The Outdoor Vitals Tushar Laminate 2 result is the anomaly: the manufacturer claims MHPU construction, yet it measures near the top of the microporous range. Seeber treats this as evidence that membrane trade names are unreliable guides to actual performance.

Low end (2026 dataset):

2021 benchmarks:

The 2026 spread runs from roughly 1,000 to 4,000. That ratio appears larger than the 2021 dataset showed.

Gore’s Membrane Progression and the ePE Transition

Gore introduced expanded polyethylene (ePE) membranes for the Autumn/Winter 2022 season, announced September 21, 2022, with Arc’teryx, adidas, Patagonia, and Salomon among the launch partners. The ePE membrane is marketed as PFC-free (with caveats) and carries a lower carbon footprint per the Higg MSI metric.

Seeber’s kettle results show what that transition costs in breathability:

• Arc’teryx Beta LT 2021 (Gore-Tex ePTFE): 2,040 → Arc’teryx Beta SL 2026 (Gore-Tex ePE): 1,760 — a 14% drop
• Arc’teryx Beta AR 2017 (Gore-Tex Pro, ePTFE): 2,800 → Arc’teryx Alpha SL 2026 (Gore-Tex ePE Pro): 1,760 — a 37% drop

Both 2026 Arc’teryx jackets land in what Seeber describes as the overlap zone (roughly 1,700–2,200 g/m²/24h), where microporous and monolithic are difficult to distinguish by MVTR alone. The Alpha SL result is notable: it descends from a Pro-grade fabric that once measured near the top of the dataset.

Why Standard Tests Overstate MHPU Performance

The real-use gap is likely larger than Seeber’s kettle numbers suggest, because the tests behind manufacturer MVTR claims systematically favor MHPU membranes.

The dominant standard is ISO 11092 (also published as ASTM F1868), the sweating guarded hotplate method. According to SATRA’s published technical bulletin, the test holds the plate and chamber air at 35°C with 40% relative humidity — isothermal, warm conditions. Under those conditions, an MHPU membrane is warm and well-humidified: precisely the environment where chemical diffusion transport works best.

A backpacker at moderate aerobic output on a cool, wet day is not in that environment. The air outside the jacket is cold and damp. The outer face fabric may be saturated with rain. MHPU transport degrades as the temperature differential between skin and air narrows and as external moisture disrupts the diffusion gradient.

Seeber identifies four specific mechanisms by which standard tests overstate MHPU performance: temperature conditions, the absence of insulating layers in the test setup, the absence of saturated base layers, and direct membrane contact assumptions. The full analysis is available to Backpacking Light members. The article also reviews five test methods in parallel: ASTM E96 (cup test), JIS L-1099 B-1 (desiccant), ISO 11092 / ASTM F1868 (sweating hot plate), ASTM F2298-03 (dynamic moisture permeation cell), and the permeation kettle.

The marketing consequence is significant. In an earlier Backpacking Light forum discussion, Seeber measured Toray Dermizax DT at 1,560 g/m²/24h on the kettle against a manufacturer-listed figure of 20,200 attributed to JIS L-1099 B-1 methodology. He described the escalating manufacturer claims as an “MVTR arms race.” That gap — 1,560 vs. 20,200 — is a test-condition artifact, not a measurement error.

The PFAS Backdrop

The shift toward MHPU is partly regulatory. PTFE-based membranes involve fluorine chemistry. DWR treatments have historically used PFAS compounds. European REACH restrictions and voluntary brand commitments have accelerated the search for alternatives.

Gore’s ePE is one response: a fluorine-free microporous membrane. Early measured results from Seeber’s dataset place ePE meaningfully below comparable ePTFE Pro fabrics, though it still leads most MHPU options. The article identifies Toray’s Torain line as currently leading in measured microporous breathability within the dataset; that claim is Seeber’s own and was not independently verified here.

Recyclability is also entering the equation. The article notes pressure on WPB laminate architecture to support end-of-life recycling, which complicates construction choices further.

Practical Implications by Use Category

Ultralight fastpacking and trail running. Microporous membranes — ePE and remaining ePTFE options — show the highest measured MVTRs under kettle conditions. For high-output use in cool, wet weather, this is where the breathability gap matters most. The 2026 top performers (Papsura FL, Photon, PreCip Eco) are in this zone.

General backpacking in variable conditions. MHPU jackets in the 1,200–1,800 range are adequate for lower-output hiking where moisture management is less acute, particularly with active venting. The Columbia Cassier Pro and Patagonia Torrentshell fall here.

Techwear and urban-to-trail crossover. Use intensity is lower and layering is typically lighter. MHPU and ePE fabrics in the mid-range are a reasonable trade-off for sustainability credentials or a softer hand feel. The breathability ceiling matters less in this context.

What the Dataset Points Toward

Buyers cannot rely on manufacturer MVTR figures to compare shells honestly. Test methods differ. Conditions differ. A jacket rated at 20,000 g/m²/24h under JIS conditions may measure below 1,500 on the kettle.

Seeber’s dataset is, as of October 2026, one of the few comparable cross-brand measurement sets in the public domain. The Outdoor Vitals Tushar Laminate 2 result — 3,680 on a claimed MHPU — suggests the monolithic technology ceiling is not fixed. Gore’s ePE demonstrates that fluorine-free microporous construction is viable; the remaining question is how far the breathability cost can be closed.

The full article, including individual jacket analyses and the test methodology sections, is at Backpacking Light. Seeber’s 2021 dataset remains the baseline for tracking how the membrane market moves from here.

Primary sources:

• Stephen Seeber, “By the Numbers: The New Breathability Gap in WPB Jackets and What the Breathability Tests Get Wrong,” Backpacking Light, October 22, 2026. https://backpackinglight.com/wpb-jacket-breathability-gap/
• SATRA, “Measuring thermal and water vapour resistance,” SATRA Bulletin, June 2019. https://satra.com/bulletin/article.php?id=2310
• “Introducing New GORE-TEX Products With Innovative Expanded Polyethylene (ePE) Membrane For Autumn/Winter 2022,” Textile World, September 21, 2022. https://www.textileworld.com/?p=79729