A waterproof-breathable laminate has to resist liquid-water penetration while allowing moisture vapour to move away from the wearer or enclosed space. Those are different functions measured by different methods. A high headline MVTR, a low RET, or a high hydrostatic-head value can be useful—but only when the test method, conditions, membrane thickness, and finished construction are known.
Quick answer
Do not compare waterproof-breathable TPU membrane samples by one headline number. First align the method, units, climate, specimen, membrane thickness, and laminate stack. MVTR reports mass transport under a defined procedure; RET reports resistance to evaporative heat transfer in a sweating guarded-hotplate test, so lower resistance means easier vapour transfer under that method; hydrostatic head measures resistance to liquid-water penetration. Final approval should test the actual laminate, seams, and product after washing, hydrolysis, flexing, and aging.
A hydrophilic non-porous TPU membrane can transport water vapour through sorption and molecular diffusion without open pores. Its performance therefore depends on the moisture gradient, chemistry, thickness, temperature, humidity, adhesive coverage, face and lining fabrics, and conditioning history. “Non-porous” does not mean “non-breathable,” and “breathable” does not mean air-permeable.
Waterproof and breathable are separate axes
Liquid rain and water vapour are not the same transport problem. Hydrostatic pressure challenges the construction with liquid water. Breathability methods create a vapour-pressure or thermal-driving force and quantify transport or resistance. A laminate can perform strongly on one axis and only moderately on the other.
Metric | What it asks | Direction often preferred | What must accompany the value |
|---|---|---|---|
MVTR / WVTR | How much water vapour passes through a specimen under a named method and climate? | Higher within the same aligned method may indicate greater vapour transfer | Method, units, temperature, humidity, air velocity/orientation, specimen, thickness, conditioning |
RET | How much resistance does the sheet or assembly create to evaporative heat transfer in a sweating guarded-hotplate test? | Lower RET means lower water-vapour resistance within the method | ISO edition, apparatus, steady-state conditions, specimen orientation, laminate stack, conditioning |
Hydrostatic head / resistance to water penetration | What pressure can the fabric or laminate resist before the defined water-penetration endpoint? | Higher may indicate greater resistance within the same method | ISO 811 or other method, pressure rate, test side, conditioning, endpoint, seams or unseamed specimen |
Air permeability | Can air flow through the construction under a pressure difference? | Application-dependent; not equivalent to moisture-vapour transfer | Method, pressure differential, specimen construction, coating/film continuity |
Values from different methods are not conversion equivalents. A cup test, an inverted-cup test, a desiccant method, and a sweating guarded hotplate establish different boundary conditions. Even two results labeled “MVTR” may not be comparable if temperature, relative humidity, air movement, orientation, or sealing differs.
How a hydrophilic TPU membrane moves moisture
Many breathable TPU membranes are designed as hydrophilic, non-porous films. Water molecules are taken up on the higher-humidity side, interact with hydrophilic segments, diffuse through the film, and desorb on the lower-humidity side. The driving force is the water-vapour concentration or chemical-potential difference across the membrane.
Because the pathway is molecular rather than an open pore network, membrane chemistry and morphology matter. So do thickness and temperature. A thinner film may improve vapour transport but reduce robustness, pinhole tolerance, handling strength, hydrostatic resistance, or process margin. More hydrophilic chemistry may increase moisture transport yet change water uptake, swelling, hand feel, blocking, mechanical retention, or hydrolysis behavior. The balance must be validated in the laminate.
MVTR: compare the complete method, not the label
MVTR or WVTR is usually reported as mass of water vapour transmitted per area per time. The number is only meaningful with the method and test conditions. Cup orientation can place liquid water, humid air, or desiccant on different sides of the specimen. Temperature and relative humidity set the driving force. Air movement changes the boundary layer. Edge sealing, exposed area, and specimen orientation can also affect the result.
- Ask for the exact standard and method variant, not only “ASTM” or “ISO.”
- Confirm units and whether the result is for standalone film, a two-layer laminate, or a complete face/adhesive/membrane/lining assembly.
- Record membrane thickness and adhesive type, coat weight, pattern, and coverage. A nearly continuous adhesive layer can become the controlling barrier.
- Confirm the test side and orientation. Hydrophilic transport can respond to which side sees the higher humidity.
- Compare values only after climate, apparatus, specimen construction, conditioning, and calculation are aligned.
RET: a resistance result from the sweating guarded hotplate
ISO 11092:2026 covers steady-state measurement of thermal resistance and water-vapour resistance using a sweating guarded-hotplate, often called a skin model. RET is a resistance metric, so a lower value indicates less resistance to evaporative heat transfer under the specified conditions. It does not directly equal a cup-method MVTR number.
RET is especially useful when a buyer wants to compare sheet materials or multilayer assemblies under a controlled heat-and-moisture transfer setup. It still does not predict comfort by itself. Garment fit, ventilation, activity, insulation, sweat rate, face-fabric wetting, seam tape, and climate can dominate the wearer experience.
Hydrostatic head: resistance to liquid-water penetration
ISO 811:2018 specifies a hydrostatic-pressure method for determining resistance of fabrics to penetration by water. The result depends on the test side, pressure-rise procedure, endpoint, conditioning, and specimen. A standalone film can show strong resistance while a finished laminate leaks at a pinhole, needle hole, weld, seam tape edge, crease, abrasion spot, or delaminated region.
Hydrostatic head is not the same as real rain exposure, kneeling pressure, flexed footwear, or pressurized protective covers. Use it as a controlled screening and quality-control tool, then add product-specific tests for seams, folds, abrasion, pressure points, and dynamic use.
Film data and finished-laminate data answer different questions
Layer or feature | Possible effect on test performance | What the buyer should control |
|---|---|---|
Face fabric and water-repellent finish | Wetting can raise evaporative resistance and change perceived comfort even when the membrane remains intact | Fabric construction, finish, contamination, laundering, surface wet-out |
Adhesive web, dot, powder, or film | Coverage and chemistry can restrict vapour transport or create local stress | Activation window, coat weight, pattern/coverage, cure, peel after aging |
TPU membrane | Chemistry and thickness control transport, strength, hydrolysis response, and processability | Grade, thickness tolerance, orientation, pinholes, blocking, lot traceability |
Lining or backer | Adds resistance, protects the membrane, and changes hand and moisture storage | Fiber, knit/weave, basis weight, finishing, direction |
Seams, welds, and tape | Can become the dominant leakage path or a local low-breathability zone | Seam design, tape/film compatibility, temperature/pressure/time, peel and leakage |
Finished product design | Fit, ventilation, panels, folds, and pressure points control real use | Representative prototype, wear/use simulation, field feedback tied to lab data |
Why durability testing can reverse the initial ranking
A membrane sample may begin with excellent vapour transport and water resistance but change after lamination, washing, hot-humid aging, folding, abrasion, body oils, detergent, or seam processing. Test the construction after the exposures that matter, not only before them.
- Measure initial thickness, mass, appearance, MVTR or RET, hydrostatic resistance, and laminate peel using named methods.
- Process the candidate on the intended lamination, coating, welding, or seam-taping equipment. Record temperature, pressure, dwell, line speed, and cooling.
- Condition and age the laminate using the actual wash, humidity, heat, flex, abrasion, sweat, detergent, and storage scenarios.
- Repeat breathability and water-penetration tests using the same orientation and method. Recheck peel, dimensional change, pinholes, cracking, blocking, and surface appearance.
- Test seams and finished prototypes under pressure points, folds, movement, and realistic product assembly. Investigate leakage location rather than recording pass/fail alone.
- Run production trials across roll width, roll length, lots, and normal process variation; define incoming and finished-laminate controls.
Application-fit questions
Application | What normally drives selection | Validation emphasis |
|---|---|---|
Footwear bootie or lining | Flex, seam density, perspiration, hydrolysis, bonding, tight folds | Flex-after-aging, seam leakage, peel, RET/MVTR of the actual lining package, wear points |
Outdoor apparel | Rain exposure, comfort during activity, laundering, wet-out, seam tape | Initial and washed RET/MVTR, hydrostatic resistance, seam leakage, peel, face-fabric wetting |
Waterproof socks or gloves | High flex, close skin contact, seam complexity, wash | Repeated flex, sweat/detergent, seam integrity, pinholes, comfort and drying |
Protective covers and mattress products | Large-area lamination, body vapour, cleaning, pressure points | Hydrostatic resistance, vapour transfer, peel, disinfectant/cleaner exposure, puncture |
Technical textile or outdoor gear panel | Welding/lamination, abrasion, folds, long storage | Seam/weld peel, pressure retention, flex, blocking, hydrolysis, temperature cycling |
Where a breathable TPU membrane may not fit
- The product needs high bulk air permeability rather than water-vapour diffusion through a continuous film.
- The required vapour-transfer target cannot be met after the necessary adhesive coverage, lining, coating, or seam construction is added.
- Continuous hot-water, steam, aggressive chemical, or sterilization exposure exceeds the grade and laminate evidence.
- The application requires a certification or protective-performance claim that has not been tested on the finished product.
- A porous membrane, coating, monolithic non-TPU film, or vented construction better matches the transport mechanism, process, durability, or cost target.
What to send GOAFLEX for sample review
- Final application, climate, activity or vapour source, water-pressure exposure, expected life, and failure mode to prevent.
- Requested MVTR/WVTR, RET, hydrostatic-head, air-permeability, peel, wash, hydrolysis, flex, and aging methods—including editions, units, and conditions.
- Membrane thickness and width, standalone film or laminate, face fabric, lining, adhesive chemistry, coat weight/pattern, and lamination route.
- Seam, welding, or tape process; equipment; temperature, pressure, dwell/line speed; and required seam-waterproofing method.
- Current sample data, test reports, roll defects, leakage maps, peel failures, and retained results after washing or aging.
- Regulatory, restricted-substance, skin-contact, protective-clothing, food, or medical requirements for the exact product and market.
For a membrane or laminate review, send the application stack, test methods, film thickness, processing conditions, and durability targets to hello@goaflex.com. For seam and process selection, continue with Weldable TPU Film and Coated Fabric. The two guides work together: this article aligns performance metrics; the companion article addresses film/coating structure and welding validation.
Image credits
Image 1: Drop of water on water-resistant textile (100% polyester). Creator: Petar Milošević. Source: Wikimedia Commons source. License: CC BY-SA 4.0.
Image 2: Sightseeing in the rain in Porto, Portugal. Tourist wearing outdoor jacket hood, visiting a viewpoint with Ribeira do Porto in background.. Creator: Marek Ślusarczyk (Tupungato) Photo portfolio. Source: Wikimedia Commons source. License: CC BY 3.0.
Image 3: Mountain Safety Research (MSR) hooded Gore-Tex rain jacket, c. 1976, "Seattle Style: Fashion/Function" exhibit (2019), Museum of History and Industy, South Lake Union, Seattle, Washington, USA.. Creator: Joe Mabel. Source: Wikimedia Commons source. License: CC BY-SA 4.0.
Image 4: Environmental testing samples. Creator: Textile and Fibre Technology, CSIRO. Source: Wikimedia Commons source. License: CC BY 3.0.
Image-use caution: All images are existing search-sourced Wikimedia Commons photographs with verified commercial-reuse licenses. They are contextual. They do not prove GOAFLEX ownership, creator endorsement, the exact chemistry of any photographed surface or textile, or performance of any GOAFLEX grade. Brand names visible in contextual images remain the property of their owners.
References
- ISO 11092:2026: ISO source — Current sweating guarded-hotplate framework for steady-state thermal and water-vapour resistance of sheet materials and multilayer assemblies.
- ISO 811:2018: ISO source — Hydrostatic-pressure method for resistance of fabrics to penetration by water.
- Chang and Liu, Review of Waterproof Breathable Membranes (Materials, 2023): Open-access paper — Peer-reviewed open-access review of porous and hydrophilic non-porous membrane mechanisms, performance factors, and textile applications.
- SATRA, measuring thermal and water-vapour resistance: SATRA source — Independent test-lab context for the sweating guarded-hotplate and its relationship to sheet-material evaluation.
- GOAFLEX weldable TPU film and coated fabric guide: GOAFLEX companion article — Internal companion article for seam process, coating/laminate structure, and finished-product validation.
Intellectual property notice: If any information or material is believed to violate intellectual property rights, contact GOAFLEX so it can be reviewed and addressed.
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