Hydrostatic head myth-busting: why 30,000 mm is not automatically better
The most frequently cited buying criterion for outdoor gear, and the misconceptions that persist
Hydrostatic head myth-busting: why 30,000 mm is not automatically better
The most frequently cited buying criterion for outdoor gear, and the misconceptions that persist
Hardly any buying criterion for rain jackets and tents is cited as often as hydrostatic head, and hardly any is so misunderstood. Does 30,000 mm sound better than 10,000 mm? Not necessarily. Here, you will find out how the test works, which myths persist and what really matters when buying.
What hydrostatic head really measuresContents
Hydrostatic head measures pressure, or more precisely, the water pressure a material can withstand before droplets penetrate it. The test is simple: a piece of fabric is clamped beneath a cylinder that is gradually filled with water. Depending on the standard, the pressure rises by 100 to 600 millimetres per minute. As soon as the third drop becomes visible on the inside, the test ends. The water level reached is the hydrostatic head.
In figures: a hydrostatic head of 10,000 mm means a 10-metre-high column of water could stand on the fabric before anything penetrates it. That equates to around 1 bar of pressure. Sounds impressive. But what does that mean in practice?
| Description | Hydrostatic head | Pressure |
|---|---|---|
| Light rain | approx. 1,000 mm | |
| Average jacket | 10,000 mm | |
| Premium membrane | 20,000 mm | |
| Gore-Tex standard | 28,000 mm | |
| Sympatex maximum | 45,000 mm |
Myth 1: A higher hydrostatic head = better qualityContents
The biggest misconception of all. The Austrian Camping Club puts it aptly: Did you also think that the higher the value, the better? Then read on. Excessively high hydrostatic-head ratings for tents are often marketing: real-world use rarely demands such extreme values.
The silicone paradoxContents
Now it gets interesting: a silicone coating rated at 1,200 mm is often more waterproof than a PU coating rated at 4,000 mm. The reason is that silicone is smoother and structurally denser than polyurethane. With PU, the hydrostatic head can halve after intensive use, then continue to fall. With silicone, it remains almost constant.
| Coating | Typical hydrostatic head | Long-term performance | UV resistance |
|---|---|---|---|
| Silicone (SI) | 1,200–3,000 mm | Remains almost constant | Very high |
| Polyurethane (PU) | 3,000–10,000 mm | Can halve | Low |
| SI/PU hybrid | 3,000–8,000 mm | Good | Medium |
PU therefore needs a considerably greater safety margin than silicone to deliver the same long-term performance. A high figure on paper can quickly melt away in practice.
Myth 2: The EU waterproof standard is relevant in practiceContents
Under EN 343:2003, clothing is considered waterproof from 800 mm (Class 2) and Class 3 from 1,300 mm. It sounds sufficient, but for outdoor use it is practically worthless. Switzerland's EMPA assumes that functional material is only truly waterproof from 4,000 mm.
| Standard | Requirement | Practical relevance |
|---|---|---|
| EN 343 Class 2 | from 800 mm | For light drizzle |
| EN 343 Class 3 | from 1,300 mm | Legally waterproof |
| EN 343 Class 4 | from 2,000 mm | Minimum requirement |
| EMPA Switzerland | from 4,000 mm | Waterproof in practice |
| Outdoor recommendation | from 10,000 mm | Suitable for prolonged rain |
USA vs Europe: the difference in testing methodsContents
Another stumbling block: the USA and Europe use different testing methods. European manufacturers test new products, while American brands such as Big Agnes and MSR simulate five years of use. This means that 3,000 mm under the US standard performs significantly better in practice than 3,000 mm measured in Europe.
Myth 3: a jacket rated at 30,000 mm is guaranteed to keep you dryContents
Wrong. Even Gore-Tex acknowledges that this test is not relevant to real-world use. When worn, there is no static pressure as there is in the laboratory. EMPA investigated clothing with a hydrostatic head below 120 mm: under simulated rainfall, it remained completely dry. Hydrostatic head is a theoretical maximum value that does not reflect real conditions.
The real weak points of a rain jacketContents
Hydrostatic head tells you about the fabric, not the construction. The critical points are elsewhere: seams, zips and hood design. Every needle creates tiny holes in the membrane. Uncovered zips are entry points for water. And the hood cut determines whether water runs into the collar.
Strengths
- Taped or welded seams prevent water entering through needle holes
- Waterproof zips or storm flaps protect the openings
- Well-designed hoods with a peak and adjustment options keep the collar dry
- High-quality DWR treatment makes water bead off before it reaches the membrane
Weaknesses
- Untaped seams let water through, regardless of the hydrostatic head
- Uncovered zips are the most common cause of wet inner layers
- Poorly fitting hoods channel water directly onto the neck
- Worn DWR treatment leads to wetting out and a feeling of dampness
Practical pressure levels in everyday useContents
What many people underestimate: everyday activities create surprisingly high pressure. An 80 kg person creates around 1,000 mm of pressure when lying down. When kneeling, this rises to around 14,000 mm. This explains why rain trousers need a higher hydrostatic head than jackets.
| Activity | Pressure generated | Recommended water column |
|---|---|---|
The underestimated role of DWR treatmentContents
A common misconception: the jacket is leaking, when often only the DWR coating has worn out. DWR (Durable Water Repellent) is a surface treatment that does not affect the membrane. Gore-Tex makes it clear: a waterproof jacket remains waterproof even when the DWR wears off.
But without effective DWR, the outer fabric soaks up water, known as wetting out. The result: the jacket becomes heavy and clammy. The film of water blocks breathability. Water vapour condenses on the cold outer layer. You get wet from the inside, even though the membrane is waterproof.
- 1
Check the DWR condition
Let a few drops of water fall onto the jacket. If they bead off and the outer fabric stays dry, everything is fine.
- 2
Know when to refresh it
If the material soaks up water and darkens, the treatment needs refreshing.
- 3
Wash the jacket
Before reproofing, wash the jacket with a specialist technical wash to remove dirt and residues.
- 4
Refresh the treatment
Apply a spray-on or wash-in reproofer and allow it to dry according to the instructions. Heat often reactivates the DWR.
The physical limits of membranesContents
Even the best membrane has limits, regardless of its water column rating. Moisture transport needs a temperature difference of at least 15 degrees between the inside and outside. At outside temperatures above 20 degrees, or with too many layers, transport slows down or stops working altogether.
| RET value | Rating | Typical membrane |
|---|---|---|
| 0–6 | Very good | Sympatex (RET 1.5) |
| 6–13 | Good | Gore-Tex (RET 3) |
| 13–20 | Satisfactory | Standard membranes |
| over 20 | Unsatisfactory | Budget coatings |
The RET value (Resistance to Evaporating Heat Transfer) is a better indicator of breathability than the hydrostatic head. The lower the RET, the better the moisture transport. For sport, we recommend a RET below 13.
Practical recommendations: which hydrostatic head for which use?Contents
Recommendations for rain jacketsContents
| Intended use | Recommended hydrostatic head | More important than hydrostatic head |
|---|---|---|
| Everyday use/city | 5,000–10,000 mm | Fit, hood construction |
| Hiking | 10,000 mm | Taped seams, breathability |
| Trekking with a backpack | 20,000 mm | Reinforced shoulder area |
| Mountaineering/extreme conditions | 20,000+ mm | Construction, DWR quality |
High-quality rain jackets start at a minimum of 10,000 mm, well above the standard. For 95% of all outdoor activities, a jacket with 10,000–20,000 mm from an established manufacturer is sufficient, provided the seams are taped and the DWR is high quality.
Recommendations for tentsContents
| Area | Minimum value (DIN) | Recommendation |
|---|---|---|
| Flysheet | 1,500 mm | 3,000+ mm |
| Tent floor | 2,000 mm | 5,000+ mm |
| Ultralight tent | Lower values | Prefer silicone coating |
For tent groundsheets, the hydrostatic head matters more than for the flysheet, as kneeling and lying down create concentrated pressure. For ultralight tents with lower ratings, look for a silicone coating: it stays waterproof for longer than PU.
Who needs which hydrostatic head?Contents
Hydrostatic head: when more, when less?
Ideal for
Outdoor enthusiasts who want to make an informed buying decision rather than be swayed by marketing figures.
Not ideal for
Not relevant for indoor-only use or if you only head out in sunshine.
Checklist: what to look for when buyingContents
Hydrostatic head alone is not a reliable buying criterion. These points matter just as much, and often more:
- 1
Construction before figures
Taped or welded seams matter more than an extra 10,000 mm of hydrostatic head. Check the seams at the shoulders, hood and under the arms.
- 2
Check the coating type
For tents, choose a silicone coating for durability. For jackets, check the membrane technology.
- 3
Question the test standard
American ratings are more realistic than European ones. Do not compare figures from US brands directly with European ratings.
- 4
Consider DWR quality
A good water-repellent treatment makes a real difference to comfort. Ask about the DWR technology or read long-term tests.
- 5
Check breathability
For sporting activities, a RET value below 13 is recommended. The MVTR value (g/m²/24h) should be above 10,000.
- 6
Factor in brand reputation
Reputable outdoor brands can withstand prolonged heavy rain and often offer better warranties.
Frequently asked questions about hydrostatic headContents
Conclusion: hydrostatic head is an indicator, not a seal of qualityContents
Hydrostatic head is a useful guide, but not a reliable buying criterion on its own. At first glance, it seems simple: the more waterproof a fabric is, the less breathable it is. In reality, it is more complex.
30,000 mm is not automatically better than 10,000 mm: construction and coating type are what matter. The EU standard of 1,300 mm is not enough for outdoor use. Silicone with 1,200 mm can perform better over the long term than PU with 4,000 mm. DWR is often more important for how dry you feel than the membrane. And seams and construction are the real weak points, not the material.
