Bivy Condensation Sources

Yes – perspiration from your body, also your breath if you have your mouth inside the bivy (don't).
I have found that, generally, I lose about 2 lbs of weight per night. This leaves my body in the form of urine, sweat, and vapor from my lungs.
I guess that about a pound of that is urine, so that leaves about a pound from lung and skin vapor. The vapor from the skin migrates outward until it reaches an area where it is cold enough to condense, certainly the bivy inside surface and also the outer layers of insulation in the sleeping bag.

I have a friend that swears his bivy is wetter when he drinks adult bevrages. He thinks it is coming out his pours in the night- I think he wet the bed. Sometimes I wake up damp and other times not. I have been unable to determine what makes the difference. One thing I do know is that when I use the bivy with my quilt there is never any condensation- ever. Why?

Human sweat is the source of most of the water.

If the sleeping bag is too warm for the conditions this makes matters worse. If the night air is water-laden then it will be bad as well.

In the summer I ran some experiments – a breathable bivy vs. no bivy on a dry night.

The sleeping bag had about the same amount of moisture in it and the same external dampness around the footbox (which is cooler than the rest of the bag).

I've not found a solution – apart from different bivys for different weather: No bag for dry summer nights, a Pertex cover for when I have rain cover and an Aquabloc (DIY) bag for when rain is expected.

Remember many breathable fabrics (such as Gore-Tex) work by having water condense on the interior surface and then migrate through the fabric.

All,

Thank you for your insightful replies.

Given that condensation seems to be a fact of life with bivys (unless someone invents the "eternally dry" bivy), is it better to pair the bivy with a synthetic sleeping bag since the bag will retain its insulating properties even when wet (in comparison with down bags)?

Do most people pair their bivys with synthetic-filled sleeping bags over down?

Just thinking out loud now… If the condensation in a bivy is due to perspiration I can see how a bag too warm for the conditions might cause increased condensation. Perhaps a "colder" bag plus a silk liner (or 2) would allow for better internal heat/humidity regulation (???)

The marginal difference is very low between a cold bag and a hot bag. I’ve tried both!!!

If you have to zip in so that you are breathing into the bag the condensation is a lot worse- don’t go there!

The last time I tested this I was using a cold-weather bag in moderate conditions. After 2 nights of using inside an Aquabloc Bivy bag and one night in the open air during dry cool (8C/48F at night) conditions the bag had taken on around 6oz of moisture. The bag was far too hot for the conditions so that gives you an idea of worst-case.

Down and Synthetic both seem to suffer equally. The major differnce is on the fabric of the sleeping bag itself – if the sleeping bag has something like a pertex or better outer then it will survive better than if it had just a nylon untreated outer.

But this amount of dampness does not have much impact on insulation properties and damp down feels to me no worse than damp synthetic. A good sleeping bag keeps any slight dampness away from the body anyway.

It’s always going to be a case of managing the condensation rather than avoiding it.

The two science bits to remember are that you are:

1. You are a hot body radiating both moisture and heat.

2. Every piece of fabric between you and the outside world that is colder than your body is a potential surface for condensation.

UK conditions tend to be dampish and warmish for most of the year and so I tend to just work on ways of minimising condensation as above. One trick I do use is to have some sort of exposed hood/micro-tarp over the head of the bivy
Solo Micro Tarp so that I don’t have to zip in and can have a relatively weather-free area for my head and cooking.

Allowing time to air the bag in the morning is always key…

Are there any recommendations for down sleeping bags that would be compatible with a bivy bag? How about for bivy bags as well?

A combination that works for me is:

Jacks R Better No Sniveller quilt
Bozeman Mountain Works VAPR Bivy Sack

When needed, supplemented with:
Cocoon UL 60 Hoody and Pant

Let me jump in on this…

I started a thread on down vs. synthetic a while ago that turned into quite the debate. But… I noticed you said that zipping all the way into the bivy is BAD. I guess that makes obvious sense – which bums me out cuz I spent the night in the backyard in my MLD bivy (no field experience in it yet) and I was SO much warmer with the bivy cutting out 100% of the breeze on my face. I guess I need to have adequate head gear to make it so I can sleep with my face outside, right?

Also, if I got a Nunatak quilt, would Quantum or Epic be better for moisture managment in a quilt?

Ryan,

Would you be able to post a link to the down vs. synthetic thread? I would be very interested in reading through it.

Thanks.

Mike, I really like that set-up.
Imagine a cuben fiber micro-tarp with a MLD Event bivy!
Looks like you could set it up anywhere.

It's a setup I like for mountain use. It keeps the worst of the weather off, stops major draughts and gives great views.

It can be set up just about anywhere – if you cannot bang pegs in you use the bivy alone. That makes it wonderful for terrain that may be difficult for conventional camping or tarping

I'm not too worried about trimming weight off it beyond where it is. For mountain use I want kit that's no-fuss weatherproof and this shape is really easy to get pegged down and windproof and at 700-800g for tarp + bivy + pegs that's good enough for me.

But it's not something you'd want to use day-in-day-out.