Bivy moisture dam

It also seems that a waterproof flap or curtain suspended at the shoulders would seal the head area from the quilt area.  This would be very easy to do, and pretty effective.

Yes, it would be quite effective … at sealing IN all the moisture from your body. Your bag could get to be quite wet that way.

Yes, I have tried this idea – once, as a tiny Boy Scout. Took days to dry my old SB out.

Cheers

I think one of us may not be following the other, but I’m not sure which :)

I take your response to mean that most of your body’s moisture (insensible perspiration) exits the bivy via the bug net over the face, rather than passing through the rest of the upper layer of fabric.  That’s not how I understand things to work, but then again, I’ve never actually used a bivy.

To clarify my proposed design:

I’m making a splash bivy with a waterproof floor of PU coated polyester (RSBTR Membrane Silpoly), an upper of breathable DWR nylon (RSBTR Membrane 10 nylon taffeta) and a window of bug netting over the face.  I’m suggesting sewing a piece of waterproof fabric along the edge of the bug net closest to the feet, spanning the width of the bivy, maybe 12″ – 18″ wide (or long, pending which way you care to think of the dimensions) that will hang down inside the bivy below your chin.

Is that basically what you tried and found it got your bag wetter than not using it?

Re-reading my OP, it could be taken as a vapor barrier on the outside of the quilt spanning chin to foot.  I would expect that to give the results Roger described.  That is not what I meant.

To me, sounds like what Roger was trying to say (correct me if I’m wrong!) is that a lot of condensation in a bivy comes from your body, not just your breath. So, while your moisture dam would be keeping moisture from your breath away from your sleeping bag, it would be trapping moisture from your body even more than without it. Which makes sense to me.

One idea I liked is to put a long narrow strip of bug netting down the center top of the bivy, stopping maybe a couple feet before the foot end. This keeps most of the protection from rain splatter and condensation while increasing the breathability substantially. As long as you aren’t using the bivy as a standalone shelter, it should work pretty well.

One idea I liked is to put a long narrow strip of bug netting down the center top of the bivy, stopping maybe a couple feet before the foot end.

That’s similar to what Lance Marshall did. Seems like it would work well:

http://backpackinglight.com/forums/topic/96337/

How does a 3’x1′ piece of waterproof fabric draped across my neck trap moisture from my whole body?

There’s a lot more water from breath than your body, so not breathing into your bag should reduce moisture.

It seems like a water proof breathable or DWR fabric dam like you described would help.  Maybe not sil because it doesn’t breath?  But the rest of your bivy will breath.

Do people with bivies breath into the bivy?  It seems like condensation on the inside of the bivy would be a major problem..

Try it and report back?

I predict it will make condensation worse.

Many conditions,   the lifted netting will be venting significant vapour which has been generated by your body. Transferring it out more effectively than the top fabric. Your waterproof strip will block this flow.

I might be wrong.  Try it!

My quilt has a WPB top strip, samething, but doesn’t require using a bivy.

Interesting idea. Reminds me of the old “draft collars” on winter sleeping bags.

But as Roger indicated, the restriction of airflow inside the bivy may cause more harm than good. The best way to mitigate vapor is through convection. Blocking some of that airflow could become problematic. However, if somehow you could design the collar out of a spongy material so it can keep the bivy shell from actually touching your bag while reducing (not eliminating) airflow, but manage to create an airspace between the two materials, I believe would help the most. Based on my experiences, I believe the best way to manage moisture in a bivy is to make sure the the bivy shell over top of my sleeping bag has an air gap, even if it means I place a towel in between them. In some types of weather, no matter what, condensation WILL  occur… somewhere in the system. How you deal with it is the key.

Regardless, one is “supposed” to always leave an opening in their bivy for their breath to escape. (This is what bivy manufacturers recommend and certainty what I was always taught to do.)

I think this idea is worth investigating further. The ideal situation would be to have the head of the bivy open or only covered with mesh. But in a scenario where the head section of the bivy needs to be closed, one is relying solely on the breathability of the fabric to get rid of moisture.

The fabric needs to get rid of moisture from two sources: mouth and body. I think it’s safe to assume that our breath produces more moisture than our body. In absolutely ideal conditions, WPB fabric might be able to transport all of that moisture effectively, but such conditions are rare. Yet it may be the case that the WPB fabric can transport our body-produced moisture, but not our breath-produced moisture.

If that’s the case, then a moisture dam makes sense. Assuming a perfect seal, one would end up with two humidity “zones.” Below the neck, there would be a low-to-moderate amount of moisture production, and about 15 square feet of WPB fabric (moisture transport should increase linearly with surface area). Above the neck, there would be a high amount of moisture production (from breath), and only about 6 square feet of WPB fabric to transport it to the outside.

So you would have a high humidity zone above the neck and a low-humidity zone below. Quilt users typically rely on knit caps and balaclavas for above-the-neck insulation, and those aren’t particularly sensitive to moisture. Down, on the other hand, is. So I think this is a good idea and worth pursuing.

I recently bought a enlightened prodigy 20 and tested it a couple days ago.   Bivy trapped so much moisture between the quilt and bivy I had ice shards/pellets between the two layers.

I want to use the bivy for extra warmth but not if its gonna trap all the moisture.  Although it’s just freezing.

I think I’m in the same boat as you.

Bivy trapped so much moisture between the quilt and bivy I had ice shards/pellets between the two layers

You would prefer the moisture to condense in your bag ?

in a scenario where the head section of the bivy needs to be closed,  You are almost certainly using it incorrectly, and adding lots of moisture to a bad situation isn’t going to improve your chances. 

in a scenario where the head section of the bivy needs to be closed,  You are almost certainly using it incorrectly, and adding lots of moisture to a bad situation isn’t going to improve your chances.

1. I’m not talking about water-resistant bivies for use under a tarp. I’m talking about standalone waterproof/breathable bivies like the MLD Soul, Borah Snowyside, or USGI Gore-Tex. In a storm, these have to be fully closed. That’s not “using it incorrectly,” that’s using it as intended.
2. What do you mean “adding lots of moisture”? A moisture dam won’t add any moisture to the system, it will only compartmentalize the moisture that’s already being produced.

If your best option is to lay face up and exposed in a storm, and soaking your insulation intentionally becomes your best idea, it may be time to rethink your idea of doing it right.

I’m not sure we’re talking about the same thing. What do you mean by “soaking your insulation intentionally”? I’m talking about starting with a fully enclosed waterproof breathable bivy, which many people use successfully already, and adding an internal barrier to divide it into two independent sections. There’s no “face up and exposed” here.

OP suggested this in the context of a splash bivy with a mesh head, and I agree it’s not a good idea for that. But in a fully WPB bivy, it might help.

if you have a fully enclosed WPB bivy, then you’ll be breathing inside your bivy

Doesn’t that cause a lot of condensation?

It can if the conditions aren’t right. That’s why this moisture dam could prove useful. It would keep the extra moisture from your breath out of the lower portion of the bivy where your insulation is.

Ain’t gonna work.

You could get a more breathable water resistant bivy with a mesh area at the head (and a waterproof bottom) and use it with a tarp. Or no head coverage at all if you don’t mind insects. Even then you’ll likely have some condensation some nights.

Or just get a lightweight tent.

Yes, Must be a misunderstanding.   I thought people were expecting to fully seal a wpb bivy and breath into the bivy all night long, without soaking the sleeping insulation.

Yes, Must be a misunderstanding.   I thought people were expecting to fully seal a wpb bivy and breath into the bivy all night long, without soaking the sleeping insulation.

Not a misunderstanding. Many people successfully do exactly that.

Yup.

People have been successfully sleeping in sealed wpb bivis for decades.

I first did it as a teenager in the 80s in a borrowed goretex bag. Fully zipped up. No condensation. It was dryish and windy.

The gas permeable goretex with a non woven scrim. Like they make mountain bivi  tents from.

Some conditions, there will always be condensation even with the hood open.  Worse with it closed.

bivis using a coated waterproof fabric are not to be closed up, but those of event and gtx are OK

I don’t know if this dam thing will work, but based on analysis of my various bivis have performed in different conditions. I think it’s a waste of time,  and probably counterproductive to the intention. Bivi venting via the bellows effect as one moves about could be obstucted by the dam. If the dam lays flat on your quilt,  as parts of it surely will, then it will trap vapour at those points.

If I ever find the need for a new  bivy I’ll keep eVent in mind.   I have a waterproof breathable bivy, and  it doesn’t breath.

Rene, I don’t know if you’ve made your bivy yet, but I noticed the Nemo Tenshi 2P tent actually has something they call a “condensation curtain”. It sounds very similar to your idea, just done in a tent instead of a bivy.

From Nemo’s website:

The Condensation Curtain™ helps keep moisture created by normal breathing contained in a small portion of the tent. The curtain has a tieback loop for easy stowage when not in use.

http://www.nemoequipment.com/product/?p=Tenshi+2P