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 .
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