The paradox: How a film can be both breathable AND waterproof (XFOIL fly)
- Feb 20
- 3 min read
Anyone who's ever slept in a tent (or even under a tarp) knows the phenomenon: you wake up and your sleeping bag/quilt is damp inside. It hasn't rained – it's condensation. In the ultralight realm, this is primarily one thing: annoying.
This problem – trapped moisture – is universal. It doesn't just occur in bivouac sacks, but also with temperature fluctuations in transport packaging or during the storage of goods. There, it can be more than just a nuisance and can lead to mold or damage.
The solution in both cases is often a " breathable film ". But the term raises questions. How can a film "breathe" and – the biggest paradox – be waterproof at the same time?
The answer is not magic, but precise technology: microperforation . And not our invention, but one that has long been used in industrial processes.
What "breathable" really means
A film like our XFOIL fly is not "breathable" like a complex membrane clothing. It is a highly developed polyolefin film. It acquires its special properties through a mechanical process.
The film features ultra-fine perforations ( hole size: approx. 50–80 µm (0.05–0.08 millimeters)). This perforation solves part of the condensation problem through simple physics.
1. The function: Steam out (respiratory activity)
Water vapor (gaseous H2O) consists of tiny, individual molecules. These molecules are small enough to pass through the 50-80 µm pores unhindered.
Therefore, if warm, humid air forms inside (whether tarp, tarptent or packaging), this water vapor can actively escape to the outside before it turns into liquid condensation on the cooler inside of the foil.
The result: Condensation is reduced by approximately 40%.
This does not mean, of course, that no condensation will form at all . The amount of condensation always depends on many factors: the humidity, the difference between indoor and outdoor temperature, the ventilation (airflow), and how much moisture is produced indoors (e.g., through breathing).
XFOIL fly cannot change these factors, but it helps to more efficiently transport the resulting moisture (vapor) away before it condenses on a large scale as water droplets.
2. Protection: Water in (waterproof)
Herein lies the apparent contradiction. A liquid raindrop is a gigantic structure compared to a water vapor molecule.
Due to the surface tension of the water, a droplet cannot penetrate the 80 µm opening. It simply beads up and rolls off. The film can withstand a water column of over 6,000 mm and is therefore completely impervious to rain or splashing water.
Wide range of applications: From tarps to transport
This unique property – vapor-permeable from the inside, waterproof from the outside – makes a micro-perforated film the ideal solution for a wide variety of applications.
In the outdoor and MYOG (Make Your Own Gear) sector: This foil is intended for everything above ground level. For tarps or ponchos, it reliably keeps out rain while drastically reducing annoying condensation dripping inside. Liners or transport bags for worn clothing also benefit from it.
In industry and transport: The same effect protects high-value goods. When packaging pools, furniture, or machinery, a micro-perforated film prevents damage from splashes or rain, while simultaneously allowing residual moisture or condensation to escape. This actively prevents mold growth and corrosion.
Conclusion: Two slides for two problems
The choice of the right material depends on the problem that needs to be solved:
1. XFOIL (Standard Groundsheet):
Problem: Dampness and steam from below .
Function: 100% waterproof AND 100% vapor-proof.
Application: As a groundsheet (soil protection).
2. XFOIL fly (Breathable film):
Problem: Rain from outside and/or condensation from inside .
Function: Waterproof (against rain) AND vapor-permeable (against condensation).
Application: For everything above ground (tarps, packaging, covers).
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