Hydrostatic Head: A Layman’s Guide
Looking to Understand Hydrostatic Head?
You’re in the right place! In this guide, we will be covering the following:
- What is Hydrostatic Head?
- How is it measured?
- What do the ratings really mean?
- How do the manufacturers stack up?
What Does Hydrostatic Head (HH) Mean?
Buying gear and clothes for the great outdoors is a touch more complicated and consequential that picking up our “civvy” gear for kicking about town. While getting a gear purchase wrong for the pub on Friday night may result in a spot of light ridicule and an extended stint of singledom, for the outdoors the payback for a poor purchase could be far more severe.
As such, when it comes to the “biggies” of our backcountry belongings — if you want the best waterproof hiking pants, jackets, tents, and groundsheets — we want to know we’re spending our would-be beer money on something that’s fit to task. But beyond the breezy and bold claims of the marketing departments, how are we to know what’s going to work?
Enter the acronym! In Part One of our Hiking Duds Decoded Series, we uncovered the mysteries and exposed the blessings of “What is DWR?“; in part two, we turn our attention to the equally enigmatic HH — Hydrostatic Head.
Below, we aim to demystify the mystified with a thorough, straight-talking lowdown on the workings and wonders of HH, starting off with a few facts and figures before moving onto an explanation of how these can be interpreted practically for the everyday hiker.
The acronym HH stands for Hydrostatic Head. In a few words, hydrostatic head refers to the tests and subsequent ratings used to determine and denote the waterproofing capacity of any textile, most commonly jackets, tents, tarps, and groundsheets. (Note: there is a difference between water resistant and waterproof – these are not interchangeable terms).
Manufacturers use these tests and ratings to differentiate between varying levels of waterproofing. Whereas the label “waterproof” was once a one-size-fits-all umbrella term, these days — and thanks to HH testing — we can now distinguish between products and garments that are very waterproof and those that are only marginally waterproof.
How is Hydrostatic Head Tested?
The HH rating of any product is tested by applying water pressure (i.e. hydrostatic head pressure) on the surface of the fabric and measuring how much it can withstand before allowing water to pass through.
An open-ended laboratory test tube is placed on top of the fabric and filled with water. As the water level rises, the pressure on the fabric increases correspondingly. The point at which the fabric begins to allow water to seep through is what is used to determine its hydrostatic head, which is given in millimeters. For example: if the fabric begins to leak with 10,000mm of water pressure, then its HH is 10,000mm.
How to Translate HH into Something Practical
For the purposes of the user, HH can be translated using the following guidelines:
1,000 HH = The minimal legal requirement to call a tent “waterproof” (but, in practice, found only in very basic tents capable of withstanding the very lightest of showers).
1,500mm = A ballpark figure for summer tents when the worst conditions you expect to encounter are light showers. Prone to leakage after extended exposure to moderate rainfall.
2,000mm = The most common rating for three-season tents and capable of withstanding a combination of heavy rain and driving wind.
3,000+ mm = The proverbial “bombproof” of backcountry parlance, tents with this HH rating are commonly of the expedition and alpine variety and are able to withstand heavy downpours, gale-force winds, and the pressure of objects or bodies pressed against the fabric (which makes lower-rated models prone to leakage).
Additional factors contribute to a product’s overall waterproofing, most notably taped seams, storm flaps on the pockets, AquaGuard zippers (see video below), and reinforced waterproofing in pressure points such as the knees and shoulders
Groundsheets are subject to more pressure applied upon them by the bodyweight of a tent’s occupants and other equipment inside the tent, both of which make them more liable to leakage. As such, a higher HH (in the region of 3,000mm and upwards) is required to ensure reliable waterproofing.
In the UK, a mere 1,500 HH rating is required for a jacket to be advertised as ‘waterproof’. In all but a few cases, however, jackets marketed as out-and-out shell layers boast far higher figures. The following guidelines offer an idea of how these figures translate into performance in the field:
10,000mm = Suitable for light rain showers but liable to leak at pressure points where the straps of your backpack are in contact with the jacket (the shoulders, back, and belt area)
20,000mm = Adequate for heavy rain showers and is usually a more breathable fabric than those with a higher HH rating
30,000mm = The HH rating used by manufacturers such as eVent (in DV Expedition models), garments with this rating provide solid waterproofing in even the heaviest downpours and are very unlikely to leak even at pressure points or in the most extreme conditions. On the downside, 30,000mm-rated fabrics tend to sacrifice breathability and this degree of waterproofing is considered overkill by many other garment manufacturers.
Waterproof vs Breathability: When buying any waterproof product, be aware that high waterproofing capacity may equate to a correspondent loss of breathability.
HH of Various Brands
Different brands use different HH ratings for waterproof garments and camping products.
Despite being a market-leader in waterproof products, Gore-Tex are not great fans of HH testing and do not use it to evaluate the waterproofing capacity of their fabrics. Instead, the folks at Gore put their products through a very rigorous series of tests that are designed to simulate a variety of rain conditions — a process they believe to offer a more practical and thorough examination of a garment’s waterproof performance. While this may seem like a cop out, Gore’s GUARANTEED TO KEEP YOU DRY™ promise shows just how much trust they place in their product and testing process.
eVent currently offers 3 variations of waterproof fabric which each have different Hydrostatic Head ratings:
- eVent DV Expedition — 30,000mm: All but impregnable in terms of waterproofing but slightly lacking in breathability compared to the Alpine and Storm varieties (below)
- eVent DV Alpine — 20,000mm: A performance-oriented, 3-layer laminate that strikes a perfect balance between waterproofing and breathability
- eVent DV Storm — 10,000mm: Places an emphasis on breathability over waterproofing, doing the former exceptionally well and the latter well enough to deal with moderate showers
Pertex comes in two forms of waterproof fabric: Pertex Shield and Pertex Shield Pro.
- Pertex Shield fabrics focus more on breathability than water resistance and use 2, 2.5, and 3-layer constructions with hydrostatic head ratings in the region of 10,000mm
- Pertex Shield Pro is intended for use in more extreme conditions and uses a highly breathable, 3-layer waterproof construction with hydrostatic head ratings of around 20,000mm
Polartec NeoShell products are made with a cutting-edge blend of softshell flexibility and stretch and hardshell waterproofing. While the HH rating of Neoshell garments is a relatively low 10,000mm, this fabric is all about providing maximum breathability without sacrificing waterproof performance and resistance. Ideal for highly aerobic activities such as mountaineering, ski mountaineering, ski touring, and mountain running or fell running.