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  1. #1
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    Hydrostatic Head rating not enough, or something else?

    I recently completed a tarp made out of 1500mm Hydrostatic Head rated SilPoly. I have read up on what HH ratings mean, and understand that this is typically considered "more than enough for a tarp", which is why I purchased it instead of a higher rated fabric.

    The Setup:

    I got to use the new tarp last week, where it was setup in the rain all day (intermittent rain). The tarp was setup in a diamond configuration under the ridgeline (its roughly 9x9ft, so the diagonal is ~13ft, with tieouts on the ends, and one in the center), because of the diagonal pitch, and the bias stretch, there was some sagging in the center. These "saggy bits" collected a bit of water at some point in the day. If I had to guess... maybe somewhere in the range of half a cup total? Nothing major. Shook it for a moment, and everything ran off.

    The "Problem":

    The area under the tarp stayed dry throughout the duration. I never got any leaks/misting/etc, as far as I could tell, and the hammock and the bug net were dry. However, the underside of the tarp was very wet. So much that it got my shoulder fairly damp getting into the hammock that night, and that was a cold spot later at night (~2am).

    If I was in it, I'd think this was condensation, but the tarp had been setup all day, and I wasn't in it, which "should" mean that condensation is less likely. Also, as I said, the tarp was setup in diamond configuration, and wasn't even hung that low (bottom corners maybe 18in off the ground), and was hung at the top of a hill, where it was fairly windy most of the day. Again, based on my understanding of condensation, those both should reduce the chance that what I saw was condensation.

    The Question:

    Is this normal for 1500mm HH fabric? Is it abnormal? And does this mean that I should be looking at fabric with a higher HH rating if I make a tarp in the future? Or is this just condensation, even though the circumstances "should" mean its less likely to be the cause.

    Thanks everyone .
    Last edited by ocnlogan; 04-20-2016 at 18:23.

  2. #2
    SilvrSurfr's Avatar
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    Sounds like condensation.
    "A foolish consistency is the hobgoblin of little minds." Ralph Waldo Emerson

  3. #3
    New Member
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    With your given HH,
    I would say that is about right and depending on the conditions there may be some condensation.
    Overall it sounds like the tarp did its job but some comfort was sacrificed.
    Fabric that has a higher rating would help the issue at at the expense of more weight, if you wanted too you could treat the fabric yourself to increase the rating. I would not use this set up in a downpour.
    Hanging out in da UP.

  4. #4
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    Thanks for the help so far guys .

    While I've hammock camped a fair bit over the last year or so, most of it hasn't been in much rain (big droughts around here most of the year). Additionally, this is my first sil-tarp. My other tarp is a Kelty Noah 12, which is PU coated. I wasn't sure if Sil-coated fabrics worked a bit differently than PU stuff, or if I had simply leaked through.

    The rain was intermittent through the day and evening. The rain that did fall usually didn't last more than 30 minutes, and and was mostly straight down. Daytime highs of ~55-60f, with lows that night of ~35f. I setup the tarp and hammock at the top of the hill, around 10am, and used it for lounging/relaxing during the day, and then decided to sleep in it overnight (I was at my families cabin, and elected to try out my setup just outside, to try my new TQ, tarp, etc, so then I could bail if anything bad happened). During the day I did not notice any water on the underside of the fabric.

    I got into the hammock around midnight, after chatting inside to family during the reunion, so the temperature was already low, and the water on the underside of the tarp was already present before I even got in, and started generating my own condensation.

    So its pretty common for the inside surface of the tarp to be that damp then?

    If so, I'll be more careful in the future. I just wanted to make sure it wasn't because I'd somehow surpassed the HH rating.

  5. #5
    SilvrSurfr's Avatar
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    Condensation is pretty common for any tarp, in my experience. I've experienced condensation with silnylon, polyester and cuben fiber tarps.
    "A foolish consistency is the hobgoblin of little minds." Ralph Waldo Emerson

  6. #6
    Senior Member bartlax4's Avatar
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    Sounds like knowing the humidity level and dew point may have helped figure things out. Likely condensation with a fabric that tested HH at 1500.
    Jared

    Mid-Atlantic Mountain Works
    www.mid-atlanticmountainworks.com

  7. #7
    Senior Member Firesong's Avatar
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    Sounds like condensation on the tarp. Not moisture coming through.

  8. #8
    Herder of Cats OutandBack's Avatar
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    You might try a backyard test with a lawn sprinkler to test your tarp.

  9. #9
    Senior Member Carrico's Avatar
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    Sounds like condensation, I've had that happen more then once, especially on warmer days with rain and cold nights.
    By all means, let's argue about whether or not a hammock will hurt a tree. All the while ignoring the fact that there is an island of garbage the size of Texas floating in the Pacific ocean. Or how about the fact that over 75% of the world's nuclear reactors are leaking...

  10. #10
    Senior Member Trees company's Avatar
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    Maybe the fact that the tarp was wet all day with wind blowing over it caused an evaporative effect which would slightly cool the tarp . This would allow the humid air to condensate without human breath causing it. This is just my thoughts though you were actually there.
    visualize whirled peas.

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