Monday, 8 November 2010

On Hydrostatic Head

Hydrostatic Head is a term used to describe the waterproofness of a particular fabric. For any fabric to be considered fully waterproof it must be able to withstand the pressure of a column of water 1000mm high without leaking. This is quantified as a hydrostatic head rating of 1000. The Ministry of Defence definition of a waterproof fabric is that it must resist a column of water at least eight hundred millimetres high - that is to say a hydrostatic head rating of 800. Most tents have a minimum hydrostatic head rating of 1500 and European weather, it is widely thought, will be repelled by a tent with a hydrostatic head of around 2000mm.

The optimum position for a shelter is that the groundsheet has at least the same, it not higher, a rating than the fly as the serious pressure will come from the knees/feet/backside pushing the groundsheet against the wet ground rather than rainfall. Indeed, the higher the rating, the longer it retains it waterproofness over the short term (during a specific trip) as well as the longer it retains it waterproofness over the long term (during its lifetime).

Hydrostatic Head is not the only issue in shelter design of course - shape plays a part too and this will have an impact on how much of the shelter is actually exposed to wetness. Condensation will also play a part - so ventilation is also key.

Some Examples
My Big Agnes Fly Creek UL1 has a HH rating of 1200mm on all the fabrics used - fly and groundsheet specifically - which means it is technically "waterproof" but is right at the bottom of the range - it is a 3-season shelter at best. Conversely, the Vaude Power Lizard UL has a HH rating of 10,000mm on the groundsheet which means it will likely be waterproof longer in both senses of that analysis.

The Hilleberg Akto - considered to be one of the better All-Season solo tents in existence - has a fly with an HH rating of 2000mm and a groundsheet with an HH rating of 5000mm. This is a true All-Season tent in terms of its ratings (but remember: waterproofness is not the only issue).

The TN Laser Competition has a fly with an HH rating of 3000mm and a groundsheet with an HH rating of 5000mm. It is perhaps the perfect balance of lightweight and waterproofness.

What does all this mean? At the very least, you'll have an idea how waterproof your fabric is. Thus, when pressing a knee into a groundsheet on very wet ground, you might, after a lengthy period of use, experience water ingress on a lower HH fabric. After some years of use, this may also be the case. Some users have reported water ingress on the Big Agnes Seedhouse SL2 after weeks of trekking and serious rainfall. That may be so, but there is always a compromise to be made between fabric and weight. Durability and waterproofness are always going to be issues in UL fabrics until technology advances. Being fully informed is one thing but being paranoid about kit is another.

In the end, backpacking, hillwalking and mountaineering is all about compromises and sacrifices. Know your limits and your destination and pack accordingly. Opinions will differ, of course, especially in this area as we can all survive with differing levels of comfort and we're seeking different theatres in which to spend our time.

For as illuminating a discussion on Hydrostatic Head as you will ever see, have a look at this thread on the BPL forum.

Jim Woods' superb, comprehensive and analytical article on Silnylon can be found here and is just about the clearest rendition of the limits of Silnylon as I've seen.


  1. Jim Woods always tells it as it is. I mentioned misting on my DuoMid the other day and hydrostatic head. The misting is most likely due to that. Jim got stuck into a discussion on this on BPL a while back. It is not the link you mention; Always a good discussion. Roger Caffin used silnylon tents in terrible conditions. So they work fine. just seal them up well.

  2. It seems to me that HH is just one aspect of shelter design and its attributes - the shape, size, weight, features and so on, are others. As long as a shelter has a HH rating that suits your likely/expected weather (ie I am happy to use the Fly Creek for 3-Season use in the UK) then I wouldn't get too hung up on HH. 1200mm for the BA Fly Creek/Scarp 1 is admittedly rather low, and misting may occur for example, but both will still keep out serious rainfall, even for several days at a time. If you stay in any shelter for days at a time, there is going to be sufficient condensation (from cooking and breathing, for example) that HH is going to be largely irrelevant anyway.

  3. The discussion Martin is referring to can be found here:

  4. Does this misting mean that you actually get wet? I would hope that the inner would catch this and stop your sleeping bag getting too wet inside the Scarp1. Saying that my bag gets damp after a long winters night in a tent when it is not raining. We humans give off a lot of moisture over a few hours.

  5. In my experience of misting (not with silnylon tarps and tents, I confess but most UL materials react similarly) moisture from outside tends to sit on the fabric until disturbed (perhaps until the continuity of the meniscus has been broken) and some penetrates low HH fabrics and 'mists' on the inside. The link above to the discussion on BPL is illuminating. Not sure whether the inner would 'catch' it as the inner tends to be a more breathable fabric (on the Akto, for example) which does not necessarily prevent a great deal of water ingress. Not sure about the Scarp however. That said, condensation collects on all tents at some stage (from us breathing) which collects on the inner as well as the fly. It's a balancing exercise.

  6. Interesting post Maz. I Have not had chance to do anything with the Scarp 2 yet due to Flu, so don’t know how it will perform in the conditions we have currently which would have been ideal to test misting..
    I hadn’t come across the Jim Wood article but it is an excellent piece of work. Thanks for sharing it.
    I emailed Tarptent with some questions about sealing and other issues etc and they have ignored me. Doesn’t say much for customer services and future orders from me.

  7. I am planning a night in the Akto when it drops really low - in the back garden. We get really serious winds and some nasty frost here in the North Downs area of Surrey so it'll be a good test. Shame that Henry Shires and Tarptent have not covered themselves in glory lately when it comes to customer service. I suspect that they are swamped by the demand for their products but that's not really a good reason. I am always forgiving of that sort of thing as starting a company like that is a brave endeavour so I wish Henry well - but it's important in a cottage industry, as it grows, to maintain impeccable customer service.

  8. You are more forgiving than me Maz.
    When i had a question prior to placing the order the answer was back within 4 hours.
    Now that i have bought it the priority has obviously changed. I won’t put in print what i think but there’s no excuse for not answering simple questions in a reasonable time frame.

  9. Well, it did have an impact on my decision not to purchase a Scarp 1.

  10. Alan sealing a Scarp or DuoMid is easy. See seam sealing for dummies on my blog. I sadly keep reading comments about a certain company that show a lack of service I have not encountered. I do hope they sort it out.

  11. When you're looking for this kind of thing, I'd check out western hydrostatics, they have a great selection.