Last Updated on November 17, 2023 by Amanda Bowden
The Definition Of Hydrostatic Head: Plus Lots More Useful Things You Need To Know About It!
Have you been trying to find the perfect waterproof jacket and encountered the term hydrostatic head?
What does it mean, and why is it crucial in the realm of waterproofing?
In this article, we’ll dive into the essentials you need to know about the hydrostatic head.
We will unravel its significance and explore its connection to the performance of waterproof jackets.
Importance Of Hydrostatic Head In Waterproofing
Imagine standing in the pouring rain, trusting your jacket to keep you dry.
The effectiveness of that protection is intricately tied to the hydrostatic head.
The hydrostatic head measures a fabric’s resistance to water penetration when it’s put under pressure.
The higher the hydrostatic head rating, the more water pressure the fabric can put up with before allowing water to seep through.
Understanding The Hydrostatic Head
It’s All To Do With Hydrostatic Pressure
To comprehend hydrostatic head, it’s essential to grasp the concept of hydrostatic pressure.
This pressure is the force exerted by a fluid—such as water—due to the weight of the fluid above it.
In the context of waterproofing, this force is what your jacket fabric must withstand when exposed to rain or other wet conditions.
The higher the hydrostatic head, the greater the force the fabric can endure, making it less likely for water to penetrate.
What Measurements Are Used For The Hydrostatic Head?
Th hydrostatic head is normally measured in millimeters (mm).
Sometimes the measurement will be given as hydrostatic head(HH) instead, but when this is the case it just means millimeters.
The measurement is an indicator of the height of a column of water that the fabric can support before water starts seeping through.
For example, a waterproof jacket is tested and it can withstand the pressure exerted by a column of water 5,000mm tall. It can do this before it allowing water to seep through so it has a hydrostatic head of 5,000mm.
The hydrostatic head is a good way to compare the waterproof-ness of different fabrics. And, importantly, to have an idea of how waterproof something is before you commit to buying it.
It Has An Important Relationship To Water Resistance
The relationship between hydrostatic head and water resistance is pretty straightforward—the higher the hydrostatic head, the more water-resistant the fabric.
But getting the optimal water resistance requires a delicate balance, taking into account factors like breathability and comfort.
It is possible to get a 100% waterproof jacket, if it was made with something like plastic. But, this would not be breathable at all, or particularly comfortable to wear.
If your waterproof is not breathable, and you are active wearing it, sweat will build up and you get wet from within.
In the coming sections, we’ll explore how this balance is struck and how understanding this will help you choose the right waterproof.
What Do The Different Hydrostatic Head Ratings Mean?
Industry Standards and Ratings
Waterproof jackets are often labeled with hydrostatic head ratings, expressed in millimeters (mm).
Common ratings include 5,000mm, 10,000mm, 20,000mm, and beyond.
But how good are each of these ratings?
Here is a chart that explains how waterproof the hydrostatic head ratings are in real terms:
| Hydrostatic Head Rating | How Waterproof Is it? |
|---|---|
| Less than 5,000mm | Water resistant, but not waterproof. |
| 5,000mm | This is the lowest hydrostatic head rating to be classed as waterproof, but it will only be good for light rain. |
| 10,000-15,000mm | This is a medium range waterproof. It will be good for days out with a moderate amount of rain, and will meet the mandatory kit requirements of some running races. |
| 20,000mm or greater | This is the best level of hydrostatic head. It’s the ideal level in a hiking, cycling, or running waterproof jacket. It will be good in sustained heavy rain and, as long as it’s accompanied by sealed seams, will meet the mandatory kit requirements for trail and ultra running races. You can get waterproof garments with a hydrostatic head of 20,000mm that have a good amount of breathability. But, as you go higher, the breathability may suffer. |
Related: The Best Trail Running Waterproof Jackets
Don’t Forget About Breathability
It is tempting to believe that a higher hydrostatic head rating is always better, but the reality is more nuanced.
The best choice depends on the types of activities you’re likely to be using your waterproofs for, and the weather you’re likely to encounter.
If the waterproof jacket you like has a very high hydrostatic head it may be at the expense of breathability.
It’s very important to consider both the hydrostatic head ratings and the breathability ratings together.
If you are sitting around fishing, or hanging out beside a tent, then the breathability rating won’t matter so much. In that case it would be best to choose the highest hydrostatic head rating you can.
If you are running, climbing, or hiking up mountains, the breathability ratings are very important. That’s because as you do these activities it raises your core body temperature and you start to sweat.
If you are doing sweaty activities you want to choose a waterproof with a high breathability rating too, so some of the moisture you are creating can escape. This will stop your jacket getting wet from the inside.
The optimum waterproof will have high ratings for both the hydrostatic head and breathability, and modern fabrics are getting better at this. But getting both does tend to be more expensive!
Climate and Weather Considerations
The climate you’re likely to be out and about in is extremely important for determining the ideal hydrostatic head for your waterproofs.
If you live somewhere, or are travelling to, an area with frequent and intense rainfall, a higher rating is imperative.
On the other hand, if you live somewhere with a milder climate you can get away with a lower hydrostatic head, and opt for maximum breathability instead.
Factors Influencing Hydrostatic Head Performance
Seam Construction
Even if your waterproof jacket has an impressive hydrostatic head, if the seams are not up to scratch you may get water leaking through.
If you get a waterproof with sealed seams it will be much more effective at keeping you dry.
Related:
- The Best Women’s Waterproof Running Jackets in the UK
- The Best Men’s Waterproof Running Jackets in the UK
DWR (Durable Water Repellent) Coating
Many waterproof jackets, trousers, and even jumpers and leggings, are given a durable water repellent (DWR) coating on the outer layer.
The DWR coating repels water, so instead of absorbing into the fabric it beads on the surface.
Over time, DWR coatings can unfortunately wear off, making your waterproof less effective.
When water stops beading on the surface of your waterproof it’s time to have a go at restoring it.
Read this helpful article from REI to learn how to care for, and revive, the DWR coating of your waterproof: Rainwear: Durable Water Repellent (DWR) Care.
Runner | Writer | Science & Nutrition
🏃♀️ Hi, I’m Amanda at Running Matter, where love of running meets science and adventure!
👟 I’m in my element when out on the trails.
🌿 I have a BSc (Hons) First Class in Natural Sciences.
I love writing about how the latest science can improve running performance and nutrition. I’m curious about the human body’s limits and fascinated by ultra running.