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I'm new to this bivying thing, and last night I tried out my Alpkit tarp / bivy bag combo. It was ace, had a great night's sleep (turns out a nice new sleeping bag is a LOT better than my squished ten year old one).
But in the morning the tarp was completely saturated. Didn't really cause a problem 'cos I was in the bivy, but every time the wind gusted I got spray from the underside of the bag in my face. Now it didn't rain last night, but it was quite close and there was a lot of dew about.
What did I do wrong? I assume that this isn't supposed to happen. The tarp was pegged down to the ground in long grass on one side, was that my mistake? It was also touching the bivy, but only at one end and only slightly. Seems a bit useless if I get wetter with the tarp than without it...
Any suggestions STW?
You need some air flow. There's no need to peg the thing to the ground, leave a 6 inch gap and that'll help. The long grass might also have made the problem worse as it'll hold a lot of moisture which condenses on the underside of the tarp. Using a waterproof groundsheet can lessen the problem.
If it's any consolation, the tarp acted as a 'condensation sink' ... without it, your bivvy bag would likely have been soaked.
Aye it wasn't really a problem, just a bit odd! I'll try pegging it a bit off the grass next time. I guess long wettish grass plus warm me = moisture!