Bivvy Sack Blues — The Case of the Condensation

I have never used a Goretex Bivy sack, but have used an eVent one in varying conditions, high humidity, low humidity, down to -10 C (14F) and have only experienced a little condensation around the foot area and if my face is covered by the sleeping bag. To overcome condensation around the feet I have used vapor barrier socks when temps are below zero C. I have found that even with bivys with a momentum top there is also some condensation in the foot area.

All waterproof breathable fabrics need to be cleaned has yours been cleaned recently?

Apparently Ryan Jordan has done research on the humidity inside a bivy, hopefully he will publish it in due course.

Just because you have the mouth of the bivy sack open does not mean you will avoid getting condensation further down inside the bivy sack. The moisture coming off your body will diffuse outwards through the SB or Quilt until it hits something which is below the dew point. And then it will condense. Basic physics.

Whether the surface it condenses on is silnylon, GoreTex or plain fabric does not matter: if it is below the dew point condensation will form. Of course, once you have a film of water there the 'breathability' of a membrane goes to pot. Tough.

In your case it would seem that the bivy sack was the right temperature for condensation to form. So it did.

> Are Gore-Tex bivvy sacks just a bad idea in cold weather?
Actually, the real problem is that the surface of the outer layer (a bivy bag in this case) was too close to your SB. If you had been in a tent you would still have experienced the same condensation, but it would have formed on the inside of the tent, well away from your SB. And your SB would probably have stayed fairly dry.

Now, one might object to this for two reasons.

* 'But I want to use a bivy bag, not a tent.' No problem, carry on. You will just get a wet SB.

* 'What if there is wind: it will shake the condensation down on me.' Actually, it is more likely that the wind will blow the moisture out of the tent, keeping the inside of the tent dry. I have experienced that many times in the snow.

Cheers

2x what Roger said.

Roger,can you take this a step further.

How would it work with no bivvy OR tent. Would condensation end up back on the SB (I'm guessing yes) or disappear into the night.

Here's an article the membership is crying out for!!

Rod

IIRC, it should condense inside the bag/quilt since the dewpoint would be moved to the shell of the bag/quilt instead of the shell of the bivy/tent.

'But I want to use a bivy bag, not a tent.' No problem, carry on. You will just get a wet SB.

3x what Roger said

Rod, I think there may already be an article on that, and will try to find it.

I agree with most of the posts above, but think i've found the best option.
I've used various GoreTex 'stand alone' bivvys over the years, and always had some condensation to deal with. Cleanliness of the bag is important. If it's dirty, it won't breathe.
I bought the Integral Designs all eVent bivvy/bag cover a couple of years ago and it has proved superb.
In my opinion, the non-breathable floor of most bivvys contributes to condensation more than anything else. The ID bivvy being 100% Event, has proved to be in a different class. Maybe a bit of dampness, but nothing more.

Mike I agree completely, full eVent bivy means less (or no) condensation. Interestingly when using an ID Unishelter with a MLD Grace Solo I had a lot of condensation on the tarp, which I assume means that the Dew point was on the tarp not the bivy, which may be a solution when the environmental conditions warrant. That is use a lightweight tarp to move the dew point from the bivy to the tarp. YMMV

Adding a heat source to the bag, IE hot water bottle, will
help drive moisture further out too.

Sometimes a closed
goretex tent or bivy (or jacket for that matter) works better with the vents sealed up so their is a temperature differential.

another useful tip is to put your sleeping pad inside the bivy to reduce contact with a non breathable floor.

Having slept in full non-breathable bivys, sometimes THEY
produce no noticeable condensation. The dew point must
be closer in the insulation where it is not noticed.

Wind is your friend if you have a WPB bivy as it draws
moisture away.

I've never had luck with Gore-tex breathing as advertised.

If anyone wants to get in on the all-eVent bivy, IIRC (& perhaps I don't) it's going to be discontinued… might wanna get one while you can.

Why is that Brad? I think this is one, if not the best, overall bivy sacks ever made. I wonder if Rab / ID will be producing something similar with a Rab tag?

> I've never had luck with Gore-tex breathing as advertised.

Me neither. I avoid it.

> How would it work with no bivvy OR tent. Would condensation end up
> back on the SB (I'm guessing yes) or disappear into the night.

Tricky, very tricky!
If we assume that the outside world is *cold*, it would most likely end up inside the SB/quilt. Basic physics. Yeah, rather unfortunate, but a good reason to sleep slightly cool.

Cheers

Thank you, everyone for all your comments, and in particular to Roger for the science.

Adding a heat source to the bag, IE hot water bottle, will help drive moisture further out too.

Interesting. I might have to try that.

Thanks also for the idea of keeping the pad in the bivvy, but I already do that.

The bivvy is relatively new. I think I've used it five nights total on the trail. I could clean it, but it's really not dirty that I can see. This is my first GoreTex bivvy. How often should a GoreTex bivvy be cleaned? I always use a ground sheet underneath it.

If the hot water bottle trick doesn't work, I'll probably have to bite the bullet and get some additional gear. I've been hearing good things about tarp tents, although I was hoping not to have to spend more $$'s.

HJ

"How often should a GoreTex bivvy be cleaned?"

As often as necessary, and not a bit more!

I've never cleaned any MYOG Goretex bivvy sack in over thirty years, and they don't seem to leak or have problems. Now, keep in mind that the fabric is the Goretex that we had back in 1978-1982, which I think was Generation One or something. The Goretex garments were getting taped seams during that period.

From Goretex garment washing, I discovered that I had to follow the factory directions exactly. Of course, on my Goretex fabric used in my bivvy sacks, there were no care directions.

–B.G.–

Jim, point of interest… the Big Agnes Fly Creek is a double-wall tent w/vestibule for less weight than most bivy sacks.

David, I dunno. Didn't see it in the lineup for spring, though… not sure if it was an oversight (mine or theirs) or actual omission.

Hey Jim – I was wondering – just how wet is your bag after sleeping in the bivy? I live in your area and I have an REI minimalist bivy which I think has a goretex-like material. I've only used it a few times so far and it does reliably produce wet spots on the surface of the bag. However, I've always assumed that it has sufficient breathability that things shouldn't get any worse than that.

If you were sleeping under a tarp and had a Bivy made out of a material that was only wind proof would the condensation form on the tarp?

Waking up this weekend with frost on the inside (foot end) and outside of my Bivy was interesting. I'm sure condensation is the major reason why I'm more than comfortable all night and wake up near cool early morning, even when the temp hasn't really dropped at dawn. It must be that the down is getting slightly damp and losing some loft.

I really enjoy sleeping in just the Bivy on cold, clear nights so I want to get past this condensation problem.

there's a great bunch or articles on condensation in the techniques section … i recommend reading them