Sonder Bikes
Sympatex is a waterproof membrane used in walking boots and outdoor kit. Here's how it works, how it compares to Gore-Tex, and why we chose it.
The membrane inside a waterproof walking boot determines how dry your feet stay and how comfortable they remain over a long day. Here is what it is, how it works, and what sets it apart from other membrane technologies, including Gore-Tex.
How Sympatex works in a walking boot
A waterproof membrane sits bonded to the inside of the boot upper, between the outer material and the lining. Its two jobs: stop liquid water entering from outside, and let water vapour from your foot escape.
Sympatex (and similar technologies like TePor) achieves this through a hydrophilic (water-loving) molecular structure rather than through pores. Water vapour is absorbed into the membrane and transported through it along molecular chains, driven by the difference in temperature and humidity between the inside of the boot and the outside air. There are no holes involved. The process is physicochemical rather than mechanical.
What this means for your feet on the hill
Performance stays consistent over time. Many waterproof membranes work through micropores — tiny holes too small for water droplets but large enough for vapour molecules to pass through. Those pores can gradually clog with dirt, salt from sweat, and detergent residue, reducing breathability as the boot ages. Sympatex has no pores to clog. The mechanism that keeps your feet dry on day one is the same mechanism at work years into the boot's life.
Breathability is dynamic. The driving force behind moisture transport in Sympatex is the difference between internal and external conditions. The greater that difference, the more effectively the membrane works. On a hard day on the hill — sustained output, cold air outside — the membrane is at its most effective, precisely when you need it most.
Sympatex is made from polyether-ester, a non-fluorinated polymer. It contains no PFAS compounds at any stage of manufacture or in the finished membrane. At end of life, the membrane is fully recyclable. More than a quarter of its raw materials come from renewable, bio-based sources.
Sympatex performance figures:
- Hydrostatic Head: 45,000mm.
- Windproof: certified to DIN EN ISO 9237
- PFAS-free across the full membrane and manufacturing process
- Fully recyclable at end of life - the membrane is a polyether-ester (like a PET bottle).
Sympatex has been used in performance outdoor kit since the 1980s.
What about Gore-Tex boots?
Gore-Tex is the most recognised name in waterproof membranes and has been widely used in outdoor kit since the 1970s. It is made from ePTFE (expanded polytetrafluoroethylene), a fluoropolymer stretched into a microporous structure. It performs well across a wide range of conditions.
Where it differs from Sympatex: the ePTFE membrane is fluoropolymer-based throughout. Gore has been phasing out PFAS compounds from its DWR coatings in recent years, which is a step in the right direction, but the membrane composition itself remains fluoropolymer-based and is not recyclable at end of life.
For walkers used to seeing the Gore-Tex logo on kit, Sympatex matches it on the two things that matter most on the hill: waterproofing and breathability. On composition, recyclability, and manufacturing chemistry, they are different products.
Tepor is another hydrophilic membrane used in walking boots. Like Sympatex, it works through chemical diffusion rather than micropores and is PFAS-free. Performance figures sit at 10,000mm HH, suited to three-season and general walking use.
Why use Sympatex in footwear
The choice to use Sympatex in our top of range mountain boots was the baseline requirement: the membrane had to work as well as anything else available in real UK walking conditions. It does. Beyond that, building boots without fluoropolymers in the membrane, without PFAS in the manufacturing process, and with recyclability designed in from the outset, that is how we think kit should be made.
You can read more about our approach to materials: Making Responsible Choices When Buying a Waterproof.
When you might not need a waterproof boot
A waterproof membrane is not always the default choice. In warm or dry conditions, any membrane traps heat inside the boot. A non-waterproof boot breathes more freely and dries faster if it does get wet through. For summer trail running, fast-and-light approaches in good weather, or walking in hot climates, a non-waterproof boot is often the better call.
For most UK year-round use, particularly in upland or wet terrain, a membrane earns its place.