Vapour Barriers?

[Bearbonesnorm]

If you've ever slept in a bivvy bag then there's a good chance you'll have discovered the joys of condensation to some degree. If the conditions are right (wrong) then you might as well be sleeping in the rain without said bivvy ... this got me thinking and the more I thought, the more I liked the idea of trying a VB liner to see whether it could help.

VB are usually used in very cold conditions and over consecutive nights. The idea is that they prevent any moisture produced by your body from reaching your sleeping bag where it can freeze and over a few days / nights turn your best fluffy down into a proper non-insulating mess.

All the thinking I did up there ^^^ has made me wonder whether they might help reduce condensation within a bivvy bag. Now, I know that this will involve sleeping inside something which isn't breathable but from what I understand the effects can be lessened with a little management and thought (more thinking). Besides reducing the amount of moisture in the air between the outer of my quilt and the inside of the bivvy bag I'm also hoping that it should bump up the rating given that evaporative heat loss will be greatly reduced.

On the grounds of scientific endevour I'm going to give it a go and see what happens, I imagine the outcome will be either:

It works fine, a little clammy maybe but reduced condensation and a warm night.

I'll drown in a stew of my own bodily fluids and the levels of condensation won't change.

So, before I embark on this voyage of discovery has anyone actually got any experience of using a VB ?

[Cheeky Monkey]

None, but you've got to wonder if you sweat enough to impact the down and are then keeping it in a bag immediately over your body what that will do. It sounds unappealing. Bit if blather about it on hammockforums.net

Look forward to hearing how it goes.

[welshwhit]

I like the thinking here, but I have no experience of VB.

I was always under the impression that a fair bit of the moisture that goes to condensation was from my breath, so have always used a balaclava or similar? Do you reckon this would have an effect?

Saying that, I do sleep hot and always am aware that my limbs may be sweating too much! Last time I was out I thought my foot box was a bit damp, so will watch with interest!

Drew

[Bearbonesnorm]

I was always under the impression that a fair bit of the moisture that goes to condensation was from my breath, so have always used a balaclava or similar? Do you reckon this would have an effect?

If the air you exhale goes into your bag then you'll nearly always have issues with condensation no matter what you do ... things have got to be really bad before I'll enclose myself fully in a bivvy bag

Saying that, I do sleep hot and always am aware that my limbs may be sweating too much!

One of the things you're trying to limit inside a VB is sweating, you're aiming for comfortable but not hot. Even without accounting for sweat or moisture in your breath, the average person will produce between 600ml and 1L of water each night ... it's this water that we're trying to keep 'trapped' inside the VB and in the process create a micro-climate where we'll be warm, comfortable and hopefully only a little damp

[Ray Young]

I was always under the impression that a fair bit of the moisture that goes to condensation was from my breath, so have always used a balaclava or similar? Do you reckon this would have an effect?

If the air you exhale goes into your bag then you'll nearly always have issues with condensation no matter what you do ... things have got to be really bad before I'll enclose myself fully in a bivvy bag

A human adults lung area is approximately 70 square metres and 6.2% of the air we exhale is water vapour so sleeping with your head inside your bivi bag is not a good idea, .

[Zippy]

VB are usually used in very cold conditions and over consecutive nights. The idea is that they prevent any moisture produced by your body from reaching your sleeping bag where it can freeze and over a few days / nights turn your best fluffy down into a proper non-insulating mess.

^^^ This

I did some reading into bivvy bags and condensation when for about 80% of my bivvy trips with the hunka I had condensation, then stumbled upon the VBL; I think there's even been a discussion on here in the past.

Anyway my key thoughts are:
- Only really needed where the moisture will bugger up any insulation and you can't then dry it out easily.
- The VBL next to your skin sounds like the best idea, as if you have a layer of clothes on first, I can see these getting really sweaty and ewww!
-Once wearing the VBL, the body should get accustomed to the conditions and I hear should sweat at a lower rate as it can't get ride of all the moisture easily.

My personal conclusion in the end was that you're sleeping in something non breathable, I can never see it being the most comfortable situation and therefore if I can solve condensation through having a more breathable setup then that's what I'd do (seems to be working so far). From what I've read, the conditions are going to have to be pretty bad to need a VBL.

[Bearbonesnorm]

Only really needed where the moisture will bugger up any insulation and you can't then dry it out easily.

Could easily be anywhere in the UK at anytime of year on a multiday trip

The VBL next to your skin sounds like the best idea, as if you have a layer of clothes on first, I can see these getting really sweaty and ewww!

I'm going to try both ways - bollock naked and thin synthetic base layers. There seems to be much debate on this.

Once wearing the VBL, the body should get accustomed to the conditions and I hear should sweat at a lower rate as it can't get ride of all the moisture easily.

Yep, my understanding is that moisture is released through the skin to help keep the surface slightly moist. In a VB the skin should be fairly moist anyway given the high humidity, so the body reduces the amount released through the skin ... I don't actually think it's sweat though.

Obviously I'm planning to use it in different conditions than intended, so it might go very wrong but I'm interested to see whether it might:

Reduce condensation on the outside of my sleeping bag / inside of bivvy bag.

Allow me to use a lower rated sleeping bag than I would usually for the conditions.

Give me that 'just washed prune' effect from tip to toe.

[Zippy]

I'll be very interested in hearing about how you find it. I think I'm exploring the more breathable / still works when wet coz it works like fur (paramore/buffalo) philosophy which I guess is the other end of the scale with general waterproof type stuff inbetween I'm very impressed with these momentum90 / pertex microlight type materials for their windproofness yet huge breathability, and as materials tech keeps improving, and I MTFU more, I'm less bothered about getting wet and more interested in comfortable temperatures (which I guess a VBL does, just you create the moisture rather than outside sources)