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Bivy condensation help
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john chong
(johnch) - F
Bivy condensation help on 01/02/2011 03:33:35 MST Print View

Happy New Year everyone!

This past week, me and my buddies backpacked in the Redwood National Park, and met a lot of rain.

I took my Golite Poncho Tarp along with my newly acquired Equinox bivy sack to use to block the sprays and splatters from using the poncho tarp in a half pyramid style. I got so much condensation, that it soaked through my WM summerlite in the footbed area and completely soaked the hood. One night, it was clear skies, and yet, I still had a lot of condensation in the bivy, and wet sleeping bag.

Is there a proper way to use this?

The temp went as low as 30, and I was wearing a nanopuff hoody along with a down vest to keep warm.

If I get condensation everytime I use this bivy, I'd rather just sell it and get a bigger poncho tarp for more coverage. Thanks for the help.

eric chan
(bearbreeder) - F
questions on 01/02/2011 07:54:50 MST Print View

john a few quick questions

- did the you have issues on both the clear and rainy nights
- were you sweating inside the bag ... ie overly warm
- did you make sure to breath outside the bag and bivy
- did you wear the vest and nanopuff on yr upper body inside the bag .. were either damp
- where was the campsite
- was there a breeze and was your tarp well ventilated
- was there condensation on the inside of your bivy in all areas, or just the foot and head
- was there condensation on the inside of yr tarp

BPL actually found the equinox very breathable back in 2006 ... i suspect what you are dealing with is a limitation of any bivy in certain conditions


John S.
(jshann) - F
Re: Bivy condensation help on 01/02/2011 08:59:34 MST Print View

Condensation at foot and head ends only sounds like moisture coming from you. If you also had condensation on the bivy/bag surface over most of the top then it would sound like dew.

Joe L
(heyyou) - MLife

Locale: Cutting brush off of the Arizona Tr
bad neighborhood on 01/02/2011 09:11:47 MST Print View

In those wet conditions, as the temp drops, the condensation will be everywhere, not just on the outside of your tarp. As mentioned, more air circulation might help, but a different campsite or a different hike might be better.

On cars these days, the AC runs when the defroster is on because the AC strips the moisture out of the air. Your breath and damp clothing would fog the cold windshield otherwise. Got an AC in that pack?

john chong
(johnch) - F
Bivy on 01/02/2011 13:23:48 MST Print View

Hey all. Thanks for the reply.

One night, i was sleeping in a halfpyramid setup with the bivy zipped all the way up, and the hood closed tight, with an opening for just my mouth. Another night, I slept with an A frame set up with plenty of ventilation, and as well, I kept my bivy closed the same as before. Later through the night though, I woke up to find a soaked sleeping bag hood and wet sleeping bag surface. I then had to open the bivy all the way to let it breathe some.

@eric chan: I did wear my nanopuff and vest inside the sleeping bag. they were both dry. I made sure to put them on at the very last second before i headed into my bag. Shelter area was a good spot where there was a lot of blockage of wind, and some rain. I didn't feel too warm. Felt like a good mix of clothes to make my sleeping bag warm enough. I believe I'm a cold sleeper and I need extra clothes for my sleeping bag to get me down to the 32 rating of my bag.

@john shannon: How do you prevent dew from building up in the hood area?

@Joe Longbotham:So is there no point in taking a bivy in conditions like the Redwood since it's very humid as it is and it's impossible to avoid condensation?

Diplomatic Mike

Locale: Under a bush in Scotland
Bivvy condensation. on 01/02/2011 13:43:43 MST Print View

I've done a fair bit of bivvying John, and usually had to deal with some condensation issues. I started using an ID all eVent bivvy a couple of years ago, and the difference between eVent and other fabrics has been very noticable. My eVent bivvy has been amazing. I started getting condensation after a lot of use, but washing the bag sorted it.
I hike in Scotland, so i'm usually dealing with wet conditions.
My experience is only with 1 or 2 nights at a time in a bivvy bag, so have no experience of longer trips useage.

Roger Caffin
(rcaffin) - BPL Staff - MLife

Locale: Wollemi & Kosciusko NPs, Europe
Re: Bivy on 01/02/2011 14:16:37 MST Print View

> How do you prevent dew from building up in the hood area?
That's not 'dew', that's your breath.

Read through the Forum channels on this. You will find many discussions, all of which point to the problem that your bivy was blocking the escape of moisture and letting it condense. The discussions will also explain how and why this happens.

> So is there no point in taking a bivy in conditions like the Redwood since it's very humid as it is and
> it's impossible to avoid condensation?
Close to the truth. If it is humid and cold, a bivy just makes your sleeping bag wet.


Tom Clark
(TomClark) - MLife

Locale: East Coast
Re: Bivy condensation help on 01/02/2011 16:58:23 MST Print View

I am leaning more toward using a tarp that provides enough cover for rain and splash and leaving the bivy at home. My WM bag has a wind resistant, water resistance outer, so waht does the bivy add for the weight and condensation? I do like the bivy when using my quilt since I move around alot at night. Regardless, I plan to leave the bivy top open.

eric chan
(bearbreeder) - F
IMO on 01/03/2011 02:00:58 MST Print View

hey john ...

Looks like you did most things right ... what i suspect happened was that the dew point was in yr bivy ... i think that the reason most of the condensation ended up at yr feet was because yr feet dont produce as much heat as yr core ... so with less heat pushing out the dew point was somewhere in yr system in that area, see the pics below ... same with the head, exhalled breath often gets trapped ... IMO a synthetic jacket inside a down bag isnt optimum for moisture management, especially if there's moisture in the jacket ... lack of ventilation in the bivy exaberates the problem

chances are that the temp dropped below the dew point somewhere in the morning

you can try
- use yr bivy unzipped, you only need splash protection at the head and feet
- don't use a bivy, ventilation will be improved, though i guess is certain situations the dew point could end up in yr bag rather than the bivy ... putting yr rain jacket (better ventilation) over yr feet and yr windshirt on the head will likely give you enough protection in an A frame pitch
- increase the heat inside that bag as that will push out moisture, it wastes fuel but a hot nalgene might help at night , heat warmers may also work
- put yr synthetic jacket over yr sleeping bag, rather than wearing it inside ... this wil act like a partial synth overbag ... you jacket may get damp instead, but since its synth you can dry it on the move ... remember that moisture travels outward, where do you want it to end up?
- use a VBL

just keep on testing in yr back yard as much as possible

sometimes condensation, especially in freezing and humid conditions, are just a fact of life, you just need to deal with it

i personally use synth bags so i dont need to worry about it as much

for more ...

No fabric, however, can completely eliminate condensation inside a bivy sack. Any time cold temperatures are mixed with a warm body that emits perspiration, and that body is confined in a small space such as a bivy sack, condensation will occur. Most often, condensation occurs in the head end of the bivy sack (where a significant quantity of moisture from respiration is difficult to ventilate) and the foot end of the bivy sack (where a lack of body heat creates very cold surfaces on the bivy sack shell, causing even minute quantities of moisture vapor to condense into liquid moisture.

Edited by bearbreeder on 01/03/2011 03:57:15 MST.

Alex H
(abhitt) - MLife

Locale: southern appalachians or desert SW
Re: IMO, Thanks Eric on 01/03/2011 07:59:48 MST Print View

Eric, I just wanted to thank you for the great post including the links to Will's article and the Cold Weather Clothing article. I have been doing extensive internet research on this subject of bivy condensation and those two are excellent and I had not found previously.

As a long time bivy user it is really frustrating that one cannot find a material that will work in all conditions especially those tough near freezing/humid conditions. From my experiences and those of others all over the internet, all bivies have condensation/frost from time to time. The ones with the least mentions of condensation are all eVENT like the ID All eVENT bag cover (like Mike Reid above) and the OR Advanced Bivy. The Pertex (Quantum/Momentum) topped ones without bathtub (minimal) silnylon floors are next best. There seems to be something about the waterproof floors that leads to more condensation.

Edited by abhitt on 01/03/2011 09:43:13 MST.

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Dew points on 01/03/2011 08:59:59 MST Print View

Yeah, that's as good as it gets for clothing.
I would also note that with an increase in volume of the internal air, the less effect breathing and insensible perspiration will have on dew point.
An example, a large vs a small bivy. For comparison:
Lets assume we use a 10'x10'x6' tent for a volume of 600ft^3
Lets assume we use a 3'x6'x1' bivy for a volume of 18ft^3
If we perspire at the rate of 15cc/hour, this is can easily be absorbed by the large tent under a wide range of tempertures and dew points.
If we perspire at the rate of 15cc/hour, this is not so easy for the small bivy.

The only time they would be the same is when the dew point inside matches the dewpoint outside to start.

The amount of surface area also has an effect. If the surface area is doubled, we can expect condensation to be reduced or eliminated, except at the dewpoint.

Ventilation has the same effect as increasing surface area. If we take the bivy and
prop it up on one end while opening the other end, the heat (and moisture) are allowed to escape. We CAN create a ventilation "chimney" that will remove moisture. But, as always, we need to balance this with heat lost through driving it.

So, in conclusion, without supplying some form of energetic modification to the dewpoint, you WILL get some condensation.

David Olsen

Locale: Steptoe Butte
Re: Dew points on 01/03/2011 10:38:06 MST Print View

"So, in conclusion, without supplying some form of energetic modification to the dewpoint, you WILL get some condensation."

This is called "wind" and is one of the circumstances where bivys excel.

James Marco
(jamesdmarco) - MLife

Locale: Finger Lakes
Re: Re: Dew points on 01/03/2011 10:54:57 MST Print View

(Sorry, I got interupted...)
Wind is one of many ways. But, it is not the norm for a hiker to control the wind. I think I would consider this an environmental influence like temperature and humidity. Dew point is generally measured without ventilation, but it is possible to compute it with. 'Corse it has been over 10 years since I worked for the Meteorological people, I could be wrong. Sort'a like wind chill and real temperature. Then we also get in to evaporative cooling and that whole snowball.

Heat can be used directly as in documents in the above post by Eric.

So, there are actually two basic methodes to reducing local vapor pressure.
Direct as in the above post or indirect as in driving some ventilation scheme, as in a "chimney" type ventilation scheme, perhaps the most usefull...

No, I was not thinking of wind, but that certainly would help.

I seem to recall that air pressure can also effect condensation and dew point. I sort'a doubt this is very usefull, though.

Edited by jamesdmarco on 01/03/2011 11:52:54 MST.

john chong
(johnch) - F
Re: IMO on 01/04/2011 01:16:11 MST Print View

Thanks for this info. I never found the articles you posted, and will definitely look into ways to reduce condensation inside my bivy.

I think condensation from the conditions I was in was inevitable since the ground was wet as well, along with the constant rain. Pitching my poncho tarp in a half-pyramid also could've added to the condensation.

From all these articles, I'm kind of discouraged in using my poncho tarp/bivy as a shelter of choice in rainy conditions. Seems like I won't be able to keep my down sleeping bag dry.

Steven McAllister
(brooklynkayak) - MLife

Locale: Atlantic North East
Some advice on 01/13/2011 17:34:57 MST Print View

I only had time to glance over the comments, they were all good.
The biggest improvement I have found is to not use the bivy to block air flow for warmth, but to use it to block splash, spray and spindrift only.

I almost always suspend the top of the bivy under a tarp so air can flow into it. A closed tight bivy in cold humid conditions will soak your bag/quilt big time.

I never allow the bivy to lay over the top of my bag, but instead suspend it with ultralight/thin bungee. This creates a small gap between my bag and the top of the bivy for some air to circulate.

I can still get damp in extreme conditions, but not enough to be dangerous or even uncomfortable.

David Olsen

Locale: Steptoe Butte
Lots of variables on 01/14/2011 10:04:33 MST Print View

Lets look at shelter surface area versus condensation some more.

Say you WANT to make more condensation, ie desert/solar still.

More plastic, along with more water holding ground or vegetation will produce more
water in your cup.

Same can go for your shelter. If you have a larger expanse of coated material over
you and the moist ground, you can potentially end up with more condensation by volume than
if you were in a smaller shelter. If this does not drip onto you,(steep pitch of shelter) this may not be an issue in wetting your insulation, but it will make for more weight in your pack if you can't dry the shelter before you hit the trail.

A larger ground sheet or floor will reduce condensation on the overhead shelter from
moisture coming from the ground, but again you still have more wet fabric to deal with
in the morning.

So, a smaller shelter will produce less condensate from the ground and exterior sources
than a larger one. A bivy will be about the smallest you can go.

David Olsen

Locale: Steptoe Butte
Re: Bivy condensation help on 01/14/2011 10:13:37 MST Print View

"I took my Golite Poncho Tarp along with my newly acquired Equinox bivy sack to use to block the sprays and splatters from using the poncho tarp in a half pyramid style. I got so much condensation, that it soaked through my WM summerlite in the footbed area and completely soaked the hood. One night, it was clear skies, and yet, I still had a lot of condensation in the bivy, and wet sleeping bag.

Is there a proper way to use this?"

Given that you were using a bivysack cover made of basically the same material as your
sleeping bag, the condensation would likely occur even without the bivy. Much of the water source
for the condensation probably came from outside the sleep system. If the cover had been
made of a waterproof material, it might have prevented the outside moisture from wetting
your bag.

Robert Larue
(RobertL) - F
Re: Bivy condensation help on 01/18/2011 21:20:00 MST Print View

David - I really don' think the condensation comes from outside the bivy. I've had side by side comparisons under the same tarp with the same bivies (same temps and wind also!). I get condensation inside the bivy, and my girlfriend does not. It only happens at the footbox. The difference is that my feet sweat more than hers.

I think you can get a little air gap between the sleeping bag and the bivy, and if the outside temp is low enough to cool that space, you will have condensation if there's enough moisture - just like on the inside of a tent.

David Olsen

Locale: Steptoe Butte
Condensation outside and inside on 01/19/2011 09:08:59 MST Print View

You are right in that you get some condensation from your body and breath. However
you can also get outside condensation, dew etc. that can be tremendous in quantity
and is the kind that is helped by waterproof/breathable fabrics.