The paradox: How a film can be both breathable AND waterproof (XFOIL fly)
Updated: Sep 30
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 isn’t magic but precise technology: microperforation. It isn’t our invention – the technique has long been used in industrial processes.
What "breathable" really means
A film like our XFOIL fly isn’t “breathable” in the way a membrane jacket is. It is a highly developed polyolefin film that gets its special properties from a mechanical process.
The film has ultra-fine perforations with a hole size of approx. 50–80 µm (0.05–0.08 mm). This perforation solves part of the condensation problem through simple physics.
1. Vapour out: breathability
Water vapor (gaseous H2O) consists of tiny, individual molecules. These molecules are small enough to pass through the 50-80 µm pores unhindered.
So if warm, humid air builds up inside – under a tarp, in a tarptent or in packaging – the water vapour can escape to the outside before it turns into liquid condensation on the cooler inside of the film.
The result: Condensation is reduced by approximately 40%.
Of course, this doesn’t mean that no condensation forms at all. How much forms always depends on many factors: humidity, the difference between inside and outside temperature, ventilation (airflow) and how much moisture is produced inside, for example 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. Water stays out: waterproofness
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 film is designed for everything above the ground. On tarps or ponchos it reliably keeps rain out while greatly reducing annoying condensation dripping inside. Liners or transport bags for worn clothing benefit as well.
For an example of a self-built film tarp, see our ultralight tarp made of xfoil.
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 films for two problems
The choice of the right material depends on the problem that needs to be solved:
1. XFOIL (Standard Groundsheet):
Problem: Moisture and vapour from below.
Function: 100% waterproof AND 100% vapor-proof.
Application: As a groundsheet (protection from the ground).
How XFOIL differs from other groundsheet films such as window insulation film is explained in our guide to ultralight films.
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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