Seeking evidence-based design and testing methods for PFAS-free waterproof systems that resist shoulder-strap pressure and prolonged cold rain
Most shell comparisons still lean on static hydrostatic head (HH) and cup-based MVTR, which poorly predict performance during 6-12 hours of wind-driven rain, 5-10°C ambient, and sustained output under a 8-15 kg pack. The dominant failures I observe are not through obvious fabric wet-through but via:
- Localized leakage under shoulder straps/hipbelts due to point pressure, fabric bending, and pumping
- Hem/cuff wicking into midlayers
- Condensation overload when external RH 100%, especially during climbs
- Face-fabric wet-out that collapses the humidity gradient despite intact membranes
I am looking for technically grounded input on both system design and field-test methodology that better correlates with these real conditions. Specific questions:
Metrics and thresholds
- What hydrostatic head is actually required to prevent leakage under dynamic shoulder-strap pressures? Has anyone measured strap/hipbelt pressure distributions in the field and converted to equivalent water column (mm) including movement-induced spikes?
- Are there data on post-abrasion HH retention (e.g., after 10-50 hours of strap abrasion or brush contact) that better predict in-storm performance than virgin HH?
- Which breathability metric correlates best with cold-rain use: RET under elevated external RH, sweating guarded hotplate with external side saturated, or moisture buffering capacity of the whole clothing system?
Materials and construction
- Membrane choice for cold, saturated conditions: ePTFE vs monolithic PU vs PU microporous. How do they handle surfactant contamination (skin oils, dirty water) over time, and what cleaning regimen actually restores performance for each?
- PFAS-free DWRs: C0 fluorine-free finishes vs silicone-based treatments vs paraffin emulsions. Which maintain high contact angle after exposure to pack-strap abrasion and detergent residues? Any quantitative contact angle or spray test data pre/post contamination?
- Face fabric architecture: does higher denier, tighter weave, or double-weave shoulder yokes materially reduce kinetic penetration and wet-out under straps without crippling MVTR?
- Seam technology: narrow tapes vs bonded seams vs liquid-applied seam seals on PFAS-free face fabrics. Data on peel/shear durability after repeated tumble-dry cycles (often needed to restore DWR).
Patterning and integration
- Effective strategies to stop hem/cuff wicking into base/midlayers: hydrophobic seam allowances, drop hems, silicone micro-beads as drip gutters, liner discontinuities at cuffs to break capillary paths. Any proven patterns or cross-sections that measurably reduce wick-back during sustained rain?
- Pocket and vent designs that provide usable airflow in near-horizontal rain without becoming ingress points. Are external flaps inferior to laminated “guttered” pocket entries in driving rain?
- Pack integration: shoulder overlays (non-WPB reinforcements), strap covers, or pack-liner vs pack-cover tradeoffs to reduce water loading at strap-shellyoke interfaces.
Footwear and lower-body system
- For all-day rain and trail splash, are non-waterproof shoes plus waterproof/breathable socks outperforming boot membranes in terms of moisture accumulation and drying time? Any weight-of-water gain data per hour?
- Gaiter-pant interface designs that stop capillary rise into socks without compromising stride or trapping condensation at the shins.
Proposed community field-test protocol (seeking critique and refinement)
- Conditions: 2-3 hours walking at 3-4 km/h, 400-600 m ascent/hour on a treadmill, 5-10°C, wind 10-20 km/h equivalent via fans, rainfall 10-20 mm/h using overhead nozzles targeting realistic drop spectra.
- Instrumentation: iButton or similar temp/RH loggers at base-mid-shell interfaces (chest, upper back, shoulders), garment segmentation by pre/post weighing (shell, base, mid, pack straps), plus localized blot tests at shoulders/hip for ingress vs sweat attribution.
- Preconditioning: clean vs contaminated state (light detergent residue and skin oil), and abraded panels at shoulders to simulate 20 hours under a pack.
- Outcomes: time to face-fabric wet-out, internal RH/temperature profiles, total water uptake by component, and leakage onset under straps.
Requests to the community
- Empirical datasets (even small-N) on strap pressure vs leakage onset, post-abrasion HH retention, or water uptake by system component.
- Specific garments or constructions that demonstrably delayed or prevented strap-area leakage in cold rain.
- Best-practice care cycles for PFAS-free shells that measurably restore repellency without degrading adhesives or seam tapes.
- Any published test methods or DIY rigs that capture dynamic pressure and wind-driven rain better than static HH/MVTR.
The goal is to converge on system-level recommendations (materials, patterns, care, and pack integration) that reliably keep hikers warm and drier in cold, sustained rain without PFAS chemistry.