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Bivy Condensation
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Andrew F
(andrew.f) - F - M

Locale: San Francisco Bay Area
Re: VBL on 11/21/2013 22:46:12 MST Print View

Keep in mind it's totally possible to have large amounts of condensation even if you aren't adding your own moisture to the system (like with a VBL.) I laid my bag and bivy out one night with clear skies and temperatures in the low 40's, then enjoyed a campfire for a few hours. When I came back my bag was soaked - totally wet with visible water droplets inside the sleeping bag shell. The radiant cooling from the clear sky dropped the temperature of the outer layer of fabric below the dew point and the bag quickly accumulated water. Fortunately it wasn't that cold out so I stayed comfortable anyway and the bag was mostly dry by morning from my body heat.

Billy Ray
(rosyfinch) - M

Locale: the mountains
Re: Re: VBL on 11/21/2013 23:03:03 MST Print View

So. If you use a VB liner inside your sleeping bag your down will be in better shape due to less condensation, but you will be moist and likely less comfortable... moist, but warm... except for when you turn over and suck cold air in over your moist skin.

Perhaps a compromise solution would be to have a water proof breathable bivy both inside and outside of your sleeping bag :)

At least that way the moisture crossing over into your down would be reduced to maybe the same rate that moisture escapes from the exterior bivy. :)


Robert Perkins

Locale: The Sierras
Bivy Condensation on 11/22/2013 07:23:36 MST Print View

Thanks Bill! For the 'normal' hiking I do in the Sierras, I think I will stick to not using a VBL. I can always drape my bag on a rock sometime during the day and let it dry out. I can't stand that clammy feeling I get while even in a rain jacket while hiking, let alone lying in a puddle of my own sweat:( !

I'm also thinking about switching to a quilt, or a bag that can be opened wide like a quilt, to allow for ventilation and keep me from breathing into my bag as much. I try to avoid it, but sometimes when I zip it all the way up and breath out of the small opening created, my head will get turned around facing into the bag, which isn't doing me any good for condensation prevention!

Anyone else have some good info?!

Stuart R
(Scunnered) - F

Locale: Scotland
Re: Bivy Condensation on 11/22/2013 09:05:20 MST Print View

I have often wondered whether a bivi (or tarp or tent) made from a fabric with a metallic reflective (low emissivity) coating would suffer less from condensation on clear nights. In theory the temperature of the coated fabric should be a few degrees warmer than an uncoated fabric. Anyone tested this?

Example fabric: Silver Metallic Ripstop from

Paul McLaughlin
(paul) - MLife
Re: Re: Re: VBL on 11/22/2013 15:32:47 MST Print View

Bill D - unfortunately, using W/b layers inside and out doesn't do much for you, because of the dew point issue. Your body produces moisture; it moves through the inner W/b layer nicely because it's going from higher humidity inside to lower outside the fabric (but still inside the sleeping bag). That moisture travels through the insulation until it reaches the dew point. If you are in warm dry weather the dew point is outside your bag, and all is great. In colder/damper weather the dew point is within the insulation, and the moisture condenses before it leaves the bag. So the RATE of moisture production is not the issue, it's where is the dew point - inside the bag, inside the bivy, or outside it all. A VBL, if used effectively, keeps moisture from getting into the insulation in the first place; but it has no effect on where the dew point is. But VBL's can be tricky to use; that's a whole separate discussion.

Alex H
(abhitt) - MLife

Locale: southern appalachians or desert SW
aluminized fabric on 11/22/2013 15:41:25 MST Print View

Stuart that was exactly the conclusion from this BPL article some years ago.

Stuart R
(Scunnered) - F

Locale: Scotland
Re: aluminized fabric on 11/23/2013 13:39:11 MST Print View

Interesting article, thanks for the link Alex

Anthony Weston
(anthonyweston) - MLife

Locale: Southern CA
cuben on 11/23/2013 15:23:25 MST Print View

I find cuben fiber tarps get less condensation than silnylon.