The Vapour Barrier Liner (VBL) thread.

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Keith_Beef

Native
Sep 9, 2003
1,331
237
51
Yvelines, north-west of Paris, France.
A very interesting thread, and since I have only very limited experience of spending time outdoors at those kinds of temperatures I am learning a lot of new stuff here.

I think that the lowest I've experienced asleep is probably not less than around -5°C to -7°C, and in waking hours probably not less than -20°C on a very few occasions (though -10°C to -15°C quite a few times in both the USA and in Russia).

Now it seems to me that if you have a vapour barrier layer over a thin base insulating layer, then the thin base layer will get wet. Grey Owl has explained that even at rest this will happen as the body tries to maintain a humid environment around the skin. Next you have some layers of insulation around that which stay dry because the vapour barrier layer prevents any body moisture from getting into it. Now at -20°C the air is going to be bone dry, and so that outer layer will stay dry and not lose any of its efficiency through damp.

I can understand all that, and I think it sounds very reasonable.

Now to the bit about waking up and wanting to take off the vapour barrier layer... I think that I would want to get out of the clammy base layer, and into dry underclothes. I think that at -20°C, if I was out of the wind, I would be able to do that in less than a minute, with a good insulating mat to sit or stand on.

This would leave me comfortable, back in daytime clothing that I can open and vent to allow me to dissipate "metabolic heat" generated through exertion. Leaving my night-time base layer exposed to the -20°C temperatures which would instantly (or near enough) crystallize all the water as ice. Now I can beat the frozen stiff clothing with a stick and shake out all the ice, leaving it dry. This would be the change of underclothing for the next morning.
 

Teepee

Bushcrafter (boy, I've got a lot to say!)
Jan 15, 2010
4,116
4
Northamptonshire
@ID and Shewie; to be fair guys, it wasn't the VB socks that made my feet smell. They can melt steel at the best of times.


Great post Grey Owl :)

@ Andy. Interesting thoughts. I have a Wiggys -20 bag. I can attest to the breathablity of their bags, I've found it better than my other synth bags and compared to a friends equivalalent rated cheaper bag, it stays massively drier.
I decided to test the breathabilty of it last winter. I slept in it for 6 hours at -16c with no bivi bag. I weighed it before I left and after getting home. It gained not one gram according to the scales which means it evaporated everything. I find it feels warm and dry in extended use. All of my down bags gain weight though on extended trips whatever the temp. I'm no big fan of Mr Wigatows claims, its blown out of all proportions but he does sell very well made and hardwearing bags that perform IMO.

Drying boot liners isn't necessary if you use VB socks, they stay bone dry. My experience of my VB socks (I've used RBH Vaprthrm and plastic bags) is that if I don't sweat, they actually stay quite dry inside. In my first use of them, I wore them for 3 days without taking them off and you could feel the moisture but it wasn't excessive. No worse than inside leather boots and normal socks.


I feel that VB usage will never be universally liked.
For example, take the DD Travel hammock. Its a non breathable hammock and a vapour barrier. Some find they wake up in pools of sweat every time, some don't and use them very happily.



Good to see you here Macentyre, welcome to the forum. I'm Turnerminator on HF. Your input is very welcome on the cold stuff ;)
 
I'd like to propose a different view of the technical aspects, and ask a question.

Your skin has a high water content. In extreme cold, the ambient air has a very low water content. The difference between the high concentration of water vapor at the surface of your skin, and the low concentration of water vapor in the ambient air, is the driving force for water vapor to move through your clothing. It is not just the heat differential, but also the humidity differential that drives the water vapor.

So the clothing doesn't move any water vapor... it only resists the movement of water vapor. The extent to which it resists vapor permeability is an attribute of the material. The clothing between your skin and the ambient air acts as a resistance to the movement of vapor across the differential. Highly permeable materials provide low resistance. Vapor Barrier Liners provide high resistance.

"As GO said, at low temperatures, your body does not produce sufficient heat to push the moisture out of the fabric before it freezes." That depends upon how much resistance your clothing provides to the movement of water vapor.

Our bodies move water to the surface of our skin as insensible and sensible perspiration for the purpose of expelling excess heat. It costs us energy, but we are exothermic, or warm blooded, so we need a way to rid ourselves of excess heat.

So, how does a VBL clothed winter trekker lose excess heat during periods of high activity? I believe he must vent, which means he vaporizes water.

If so, then the difference between VBL clothing and wool is that VBL clothed winter trekkers dump excess heat in batches or locally, where wool wearers dump excess heat continuously across their entire surface area.

Please correct me if I have got it wrong... it's tough to discuss this in the States, as VBL occupies a position just below politics and religion.

- MacEntyre
 
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mrcharly

Bushcrafter (boy, I've got a lot to say!)
Jan 25, 2011
3,246
33
North Yorkshire, UK
"As GO said, at low temperatures, your body does not produce sufficient heat to push the moisture out of the fabric before it freezes." That depends upon how much resistance your clothing provides to the movement of water vapor.
- MacEntyre
I don't think you are correct here.

In order for the water vapour to pass through the clothing before freezing, the air temperature must remain high enough for the water to not start condensing out.

The humidity and air temperature (moving away from your skin) are a gradient. At some point, as the air temp drops, the humidity reaches 100% and water condenses. At another point the air temp drops low enough for the water to freeze.

Now if we had 'perfect' insulation, this would occur on the outside of our sleeping gear/clothing without loss of heat.

We don't.

So there are three alternatives.
1) Have very thin insulation. We'll lose a lot of heat, because our bodies will be heating up the air outside the sleeping bag.

2) Use a VPL - the water vapour remains as vapour and we have high humidity adjacent to the skin. This doesn't condense, because our body heat is keeping it warm.
 
I think we are both correct. The temperature and humidity gradients are controlled by the resistance to heat transfer and the resistance to vapor permeability of the material.

If an insulator offers high enough resistance to heat transfer, and low enough resistance to vapor transfer, it will pass vapor without condensation.

You don't have to give up heat insulation for vapor permeability in all materials.

- MacEntyre
 
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mrcharly

Bushcrafter (boy, I've got a lot to say!)
Jan 25, 2011
3,246
33
North Yorkshire, UK
If an insulator offers high enough resistance to heat transfer, and low enough resistance to vapor transfer, it will pass vapor without condensation.
- MacEntyre
I think you are missing something.

No matter what insulation you are using, there is a condensation point and a freezing point. Using a VPL keeps the moisture the 'right' side of these points.

If your insulation was 'perfect' (infinite resistance to heat transfer) then the freezing point would be on its immediate surface.

Lacking perfect insulation, the freezing point is below the surface - or if the insulation is very rubbish, then some distance from the surface (ie, the surrounding air is being heated by your body heat).
 

rik_uk3

Banned
Jun 10, 2006
13,320
20
66
south wales
Putting all the science to one side for a moment, the bottom line is your kit will gradually get damp or you can wake up damp from the first night?
 
Insulation does not have to be adiabatic (infinite resistance to heat transfer) in order for vapor to pass through without condensing. That is just what happens with a canvas anorak. It is possible that vapor condenses only to sublime immediately, but the effect is the same.

I've never worn VBL clothing. So, putting the science to one side, exactly how does wearing VBL clothing differ from wearing traditional vapor permeable clothing? A trekker dressed in wool with a canvas anorak adjusts his insulation layers to match his activity level, such that his sweat evaporates and he remains comfortable. Does a VBL wearer adjust his insulation layers to match his activity level, such that he does not sweat? And if he sweats he ventilates, right?

- MacEntyre
 

Andy BB

Full Member
Apr 19, 2010
3,290
0
Hampshire
I'd have thought that in heavy exercise, the vbl would work exactly the same as a sweat suit, (which by definition is a vbl) ie intense sweating. Which itself would be debilitating, requiring substantial inputs of water and salt (and some way of drying out the base layer whih is recommended to wear under a vbl.). It would therefore have to be vented, but I'm not sure how efficiently you could realistically vent a vbl. So probably safer to leave the vbl until you're not doing any significant amount of exercise?
 

rg598

Native
Insulation does not have to be adiabatic (infinite resistance to heat transfer) in order for vapor to pass through without condensing. That is just what happens with a canvas anorak. It is possible that vapor condenses only to sublime immediately, but the effect is the same.

I've never worn VBL clothing. So, putting the science to one side, exactly how does wearing VBL clothing differ from wearing traditional vapor permeable clothing? A trekker dressed in wool with a canvas anorak adjusts his insulation layers to match his activity level, such that his sweat evaporates and he remains comfortable. Does a VBL wearer adjust his insulation layers to match his activity level, such that he does not sweat? And if he sweats he ventilates, right?

- MacEntyre
You are assuming that you are in a situation where one can regulate body temperature to an extent where no sweat will be produced. In such a situation, you are correct that it does not matter if you are wearing VBL or just regular clothing. You regulate your temperature in the same way.

Unfortunately, that is not possible in most conditions. As a simple example, no matter what you do, even if perfectly regulated, your body will produce water vapor while you sleep in your sleeping bag. Without the VBL it will get wet when the outside temperature is low enough to prevent the moisture from being pushed completely out of the bag by your body heat. Same thing goes for clothing. Your feet will produce sweat inside your boots no matter how careful you are.
 
Without the VBL it will get wet when the outside temperature is low enough to prevent the moisture from being pushed completely out of the bag by your body heat.
The notion that heat alone pushes moisture out is incomplete. It's the humidity differential , as well as the heat differential, that makes up the driving force. It is not just those driving forces, but also the resistance to heat transfer and the resistance to vapor permeability that control whether the vapor makes it out completely. Even in extremely low temps, there is a combination of these factors that will allow water vapor to pass through without condensing. The question is, does that leave the wearer warm enough? There are several arctic animals that demonstrate this... foxes, wolves, bear, etc.

I've made no assumptions about regulating sweat... it's something I do understand. Just was wondering how a warm blooded trekker in extreme cold thinks differently when wearing VBL clothing. Thanks!

- MacEntyre
 

rg598

Native
The notion that heat alone pushes moisture out is incomplete. It's the humidity differential , as well as the heat differential, that makes up the driving force. It is not just those driving forces, but also the resistance to heat transfer and the resistance to vapor permeability that control whether the vapor makes it out completely. Even in extremely low temps, there is a combination of these factors that will allow water vapor to pass through without condensing. The question is, does that leave the wearer warm enough? There are several arctic animals that demonstrate this... foxes, wolves, bear, etc.

I've made no assumptions about regulating sweat... it's something I do understand. Just was wondering how a warm blooded trekker in extreme cold thinks differently when wearing VBL clothing. Thanks!

- MacEntyre
You are right that more things move water out of the clothing, but at low temperatures they don't matter much. Heat is the relevant factor, not because it pushes the water, but because it keeps it in a state where it can move. If your body does not produce sufficient heat to keep the moisture at least liquid, it will freeze inside the insulation.
 
Heat is the relevant factor...
Heat is pretty much a constant for a living human. The colder it gets, the larger the differential, and the greater the driving force for heat transfer. Same with the humidity... pretty much constant high humidity at the surface of a living human, and extremely low ambient humidity in really cold weather.

But this is not what I wanted to discuss... I hoped to learn from those who have used VBL clothing how their thinking is different from when they wear more traditional clothing.

:)
 

Mark Lanham

New Member
Dec 21, 2019
1
0
53
Yorkshire
A very interesting thread, and since I have only very limited experience of spending time outdoors at those kinds of temperatures I am learning a lot of new stuff here.

I think that the lowest I've experienced asleep is probably not less than around -5°C to -7°C, and in waking hours probably not less than -20°C on a very few occasions (though -10°C to -15°C quite a few times in both the USA and in Russia).

Now it seems to me that if you have a vapour barrier layer over a thin base insulating layer, then the thin base layer will get wet. Grey Owl has explained that even at rest this will happen as the body tries to maintain a humid environment around the skin. Next you have some layers of insulation around that which stay dry because the vapour barrier layer prevents any body moisture from getting into it. Now at -20°C the air is going to be bone dry, and so that outer layer will stay dry and not lose any of its efficiency through damp.

I can understand all that, and I think it sounds very reasonable.

Now to the bit about waking up and wanting to take off the vapour barrier layer... I think that I would want to get out of the clammy base layer, and into dry underclothes. I think that at -20°C, if I was out of the wind, I would be able to do that in less than a minute, with a good insulating mat to sit or stand on.

This would leave me comfortable, back in daytime clothing that I can open and vent to allow me to dissipate "metabolic heat" generated through exertion. Leaving my night-time base layer exposed to the -20°C temperatures which would instantly (or near enough) crystallize all the water as ice. Now I can beat the frozen stiff clothing with a stick and shake out all the ice, leaving it dry. This would be the change of underclothing for the next morning.
Hi.im brand new to this site so hope I'm doing this reet but I've been searching so hard for info on vpl's for ages and come across this site.
I've camped year round for over 20 years,use a hilli tent and ME down bag and for most of those 20 years put up with soggy bag come 2am regardless of weather conditions.so nasty when cold out as bag freezes,,tent freezes and you all nod I'm sure at having been there.anyway about 3 years ago I got a vbl from PHD for £80 zoyx!!! But hay presto dry bag.girst use jeez,clammy russly unusual night so after that put microfibre liner inside the vbl and worked a treat.3 years on went on ace snowy camp in lake District and had wet bag every night.so does anyone know if they have a shelf life.they do work for any doubters out there.tent condensation can obviously still wet the bag but I leave inner doors and vents open regardless of weather to keep tent moisture to minimum.phd don't seem to be replying to my question on this concern.grrr
 

C_Claycomb

Mod
Mod
Oct 6, 2003
5,840
915
Bedfordshire
Hello and welcome to the forum.

Coated nylon is usually coated with urethane and urethane will eventually fail. However, this is usually something that takes at least 10 years, sometimes more. Mind you, those figures are when most of the product's life is in a dry storage environment. If stored somewhere a bit damper, and used frequently I suppose it might be possible for hydrolysis to kick in sooner.

This thread is on a similar subject.
https://bushcraftuk.com/community/i...it-drybags-warning.150723/page-2#post-1874213

Is your VPL still air-tight?

As for PHD not replying to you. Have you phoned them, or just emailed?
 

demographic

Bushcrafter (boy, I've got a lot to say!)
Apr 15, 2005
4,297
289
-------------
Vapour barriers are getting to be a basic item as far as UK and onto cold climate housebuilding is concerned.
They stop vapour from going from the inside of the house and condensing in the insulation and so lessening the insulation value. Also that moisture causes rot.
There's usually a breathable membrane on the outside of the house as well, that allows vapour out of the insulation to the outside and the vapour barrier should stop (in reality it generally just lessens) the water vapour from inside from getting into the insulation causing interstitial condensation.

Any housebuilder in the UK who's not on board with this is getting left behind. Which seems to be quite a lot of em.
 
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