Demystifying the Rain Shell: Hydrostatic Head and MVTR
You may ask yourself, “Where does that highway go to?”
And you may ask yourself, “Am I right? Am I wrong?”
And you may say to yourself, “if the water gets through, is it not waterproof?”
We’ve all experienced that sinking feeling when cold raindrops start to seep through our rain jacket. I thought this was waterproof? Why am I getting wet? What does waterproof even mean? Aren’t all rain jackets waterproof? If that’s the case, why are some ten times as expensive as others? The answer to each of those questions may be the same: hydrostatic head.
Hydrostatic Head is the industry-standard measure of waterproofing. Abbreviated as HH, hydrostatic head is how companies in the outdoor industry measure just how waterproof a material is. Hypothetically, if you piled a pillar of water on top of a rain jacket, how tall would that pillar be (in millimeters) before the water seeps through the fabric? Logically, a higher column of water equates to greater pressure on that fabric. Every fabric has a point of failure where the pressure becomes too great and that column begins to seep through.
The hydrostatic head is measured at the moment the water crosses to the other side of the material. Again, it is measured in millimeters (mm). So, a jacket that has a hydrostatic head of 20,000 can withstand a column of water 20,000 mm, or 20 meters, tall.
Rain jacket brands historically tested this quite literally with a pillar of water. Since then, technology improvements have meant it is now measured with a machine that mimics water pressure. Apparently, creating a 30-meter tall water column was getting tedious.
So, what does “waterproof” mean? The industry standard is that a material can be waterproof with a HH as low as 1,500 mm. Many tent rain flys have a hydrostatic head between 1,500-3,000mm. Tent flys are typically exposed to much less pressure than a rain shell, which often requires a HH of at least 10,000 but more likely 20,000. This is because rain shells have to endure pressure from climbing harnesses, backpack straps, or someone leaning against a tree. For reference, sitting on the ground creates roughly 2,000 mm of pressure, while kneeling with the weight centered can create upwards of 10,000 mm. This is why rain jackets often first begin leaking near the shoulders, where the pressure of a backpack compromises the hydrostatic head of the material.
It’s that easy! All you have to do is look at one number to determine the quality of a rain jacket. The bigger the hydrostatic head, the better the piece!
While that is true, you have to consider what a rain shell is doing. If it prevents water from entering from the exterior, it is most likely preventing water vapor from leaving the interior, too. This means sweat builds up within your layers and you may not cool off since the sweat is not transferred to the exterior of your jacket. This is the next consideration for a rain jacket: breathability, or transferring water vapor from inside your jacket to out.
A common measure of breathability is the Moisture Vapor Transfer Rate, or MVTR. This is an industry standard that measures how much evaporation happens within a square meter of material over the course of 24 hours. A higher number means more moisture evaporates, which corresponds to a more breathable material. This number can range from 10,000 to over 30,000 MVTR.
The trick now is to find a jacket with an adequate HH for your needs while still offering a high-enough MVTR for the activity you are doing. Let’s put these lessons into practice and evaluate a few of the rain jackets you’ll find at RRT:
- Rab Kinetic 2.0: 10,000mm / 35,000MVTR
- Rab Arc Eco: 20,000mm / 15,000MVTR
- Rab Downpour Eco: 20,000mm / 20,000MVTR
- Rab Meridian: 28,000 mm *
- Rab Kangri: 28,000 mm *
*MVTR not available
As you can see, the Kinetic 2.0 has the lowest waterproof rating. However, it has the highest breathability, which is why the Kinetic is an excellent piece for active summertime use climbing, cycling, or running when breathability is more important than long-term waterproofing. On the other hand, the Arc Eco and Downpour are more waterproof, with the Downpour being more breathable than the Arc Eco. Compare those to the Meridian or Kangri jackets, which are better suited for multi-day storms and extreme conditions with top-notch HH ratings but likely aren’t as breathable.
Armed with this understanding, you’ll be prepared to buy the rain jacket that best suits your needs, whether you’re looking for a highly-breathable active jacket or long-lasting shell. Stop by Roads Rivers and Trails to meet the rain jacket that fits your next trip and have an expert answer your follow-up questions.
by: Dalton Spurlin