Condensation inside bivy bags

Often when people sleep they curl up slightly and so the footbox spends a lot of time empty.

Feet don't bang out much heat and the footbox is a relatively large amount of fabric for a small amount of heat.

In 'perfect' otudoor conditions with and without any sort of cover I've consistently woken up to find that a sleeping bag has dew around the footbox area and nowhere else.

Sometimes the reason is obvious – the dew starts just where my feet don't go in the bag…

What a good subject line.

> Why is it that the footbox gets more condensation than the rest of the bivy? Do the feet really sweat that much more?

My understanding, thanks to some research I undertook a while back in the name of my own overactive glands, is that feet, along with your armpits and maybe even hands do in fact sweat more which may contribute to other factors.

Bumping an old thread…

I understand that below a certain point condensation inside a bivy bag is pretty much unavoidable. Can anyone help me understand at what point that starts to be a problem? From what I’ve read on these forums I think it’s maybe somewhere around 35 degrees. Does the problem go away if the temp drops low enough, say below 25 degrees?

It turned to ice on the interior of my bivy. I'm not really sure of the temps. The ranger at a park 9 miles away said their temps were 25 degrees and it was a couple degrees colder where I spent the night. I'm glad the ice didn't form inside my bag, or on it like it would have if I slept under the stars. I'm now seriously considering using VBL clothing to sleep in.

My experience with the Montbell bivy wasn't nearly as good. Only used it one night. Yosemite in March. A clear starry night. Temperature dropped a few degrees below freezing. Woke up in the morning with the inside of the bivy wet, and so was the shell of my sleeping bag.

The sun came out the next morning so I was able to dry everything out before I packed up and headed out. It made me wonder if I hadn't been better off sleeping under the stars with no bivy?

Kristin,

I had a similar experience with the Montbell bivy in Point Reyes. After sleeping under the stars, I woke up with a lot of condensation inside the bivy however the outside was bone dry.

Kristin and Casey:

MontBell sells two different bivy models. Curious, which ones were yours?

Some thoughts based on my un-scientific experiences. Where does most condensation come from? Breathing. That is why single wall shelters get so much condensation on the walls. I have had instances where my shelter walls were a sheet of ice in the morning.

So if you have a lot of moisture in the bag, you might be breathing into the bag during the night. If the bootbox of the bag is dry and you have a lot of condensation on top of the bag, and under the bivy you might be breathing into the bivy. You have to keep your head out of both. Roger Caffin did me a big favor by suggesting an insulated baclava for sleeping. Unconsciously, I have/had a tendency to tuck my head into the bag during sleep. Now I make sure the drawcord of my bag or quilt to drawn tight enough so I don't do this. The other factor for me is that I do not do well in cold weather. Especially my feet. Now I wear my down jacket to bed. As I warm up, I take it off and push it down to the bottom to help keep my feet warm. This helps me keep my head out of the bag. I am thinking about getting some lightweight down booties for really cold weather to help with this.

Just remember to keep your head out :)

"My guess is that the rest of the bag gets more heat from your body. The footbox is the coldest part."

This-

Put a hot water bottle in your bag and you will see the
condensation disapear, the heat drives it further out.

Anne et al, keep in mind that "VBL" means "Vapor Barrier LINER," emphasis on Liner. Not bivy. The WM Hotsac is a sleeping bag liner. It isn't intended to protect a bag from outside moisture; it is intended to protect the bag from your sweat, and to stop your evaporative heat loss. It will be humid and moist in the liner. That's a good thing, though, assuming you use it as intended. Also note that the AMK sack is an emergency bivy, ie intended to use as a stand-alone. If you use it over your sleeping bag you'll trap all kinds of sweat in the bags insulation; bad news.

Your feet really do produce a ton of sweat. I'll have to look up the numbers, but IIRC each foot produces something like 150ml in half a day (like I said, I could remember this wrong, but seems about right in memory). No, I don't have a corrollary w/the rest of your body. But feet do have a ton of sweat proportionately for body surface area. Also consider that footboxes are likely to be shoved into contact w/inner tent walls…

Edit: EACH foot can produce MORE than a PINT of sweat per day…

http://health.howstuffworks.com/skin-care/information/anatomy/question514.htm

Ben – The bivy I had was the one that is more of a sleeping bag cover – no zipper. Weighed in at 6 ounces, breathable waterproof material. I loved the light weight and simplicity of it, but not the condensation in the morning.

Nick – I do remember feeling cold at night and snuggling down as far as I could into my sleeping bag, thereby probably breathing into the bag and making the condensation situation worse.

Anybody – would I have been better off sans bivy on a cold, starry night. Or would I have woken up with the sleeping bag wet with dew? Any theories?

Kristin:

I asked because (as mentioned), Montbell sells 2 different bivies (or bag covers as Montbell calls them) — both of which are lightweight and zipperless:

1.UL Sleeping Bag Cover – 7.2oz / $175

2. Breeze Dry Tec UL Sleeping Bag Cover – 6.3 oz / $125

No direct experience, but ironically, the more expensive one is Goretex laminate and likely much less breathable than the cheaper Breeze Dry Tec model. The weights between these two are just 1 oz. apart — and what with manufacturing deviance, it may not be possible to tell them apart solely by weight. Just wondering if per chance you got the old technology Goretex model?

Sorry if I’m just rattling on the obvious…

Ben,

I used the Breeze Dry Tec UL Sleeping Bag Cover. And I'm almost positive I didn't exhale into it.

Too bad, I had high hopes for it. But it just goes to show that when conditions are right — condensation can happen…

"Too bad, I had high hopes for it. But it just goes to show that when conditions are right — condensation can happen."

Me too, I just had a night with bad condensation (not my first) with my MLD Superlight and was beginning to look at the Breeze Tec sleeping bag cover. I never breath into the bag or bivy.

One of my hiking partners and I in the past used OR sleeping bag covers with GORE DryLoft fabric on the top for years and never had condensation in all kinds of conditions but it weighs 18 oz.

> EACH foot can produce MORE than a PINT of sweat per day…

can but not necessarily will. Not while sleeping, I think. In the winter I usually layer VBL socks over wool ones; in the morning the wool socks are just barely damp. They actually feel almost dry. Doesn't look like feet perspire that much when not working. I don't think that's the main reason for the footbox condensation.

Inaki, I agree in the sense that a foot won't necessarily produce a pint or more of sweat a day… However, I also think it would be a bit misguided to assume that feet don't produce much sweat based on a sample size of one, ya know? To counter your observations, for example, my socks get quite damp just lounging on the couch at home. That's just me, though, and certainly isn't true for other people. Feet do tend to produce a lot of moisture; that might or might not be the problem w/the footbox condensation, but I'd be surprised if it wasn't related.