Do a testing program with disposable materials and report back.
Two silvered mylar survival blankets, taped together is a very cheap option.
A bit of background about insensible perspiration;
Found a good description here,
http://www.bushcraftuk.com/forum/sho...99#post1217899
gives some good background. I am sure they won't mind me reproducing it here,
(the Some Science bit it especially good),
Some Science:
We humans have a challenge, we are primarily a tropical creature, that really only thrives when our bodies are subjected to tropical conditions. At the skin/atmosphere interface our body attempts to maintain a humidity of 70-80%. This is achieved through vaporisation of water, or what we typically call sweating. But in reality, what we think of as sweating, is really excessive vaporisation followed by condensation on the skins surface or in our clothing. While we are exercising during the day, we are typically producing large quantities of metabolic heat, in order to maintain an ideal temperature our body reacts by vaporizing additional moisture, and we behaviourally should remove layers to reduce insulation and allow the metabolic heat to dissipate as water vapor before it condenses.
In the winter or cool weather, most of us will wear a base layer, either synthetic or woolen. When functioning properly this layer is capable of 'wicking' the water vapor away from our skin before it has the opportunity to condense as liquid sweat, soak our clothing and possibly result in overcooling. However, once we are at rest our body is producing much less metabolic heat, yet it still needs to maintain the 70-80% humidity at the skin/atmosphere interface. In cold climates we now have some challenges. Now that we have all that information out on the table, let's put it together. Our base layer itself is working against our bodies innate tendency to maintain a high humidity at the skin surface. As we release needed water vapor, our clothing is busy attempting to move this required vapor away from the skin.
To create water vapor is an enormously energy intensive endeavour. To be precise 4.186J are required to raise 1g of water 1C. But to turn that same gram of water into vapor requires 2,257 j/g. The heat of vaporization is not being used to raise the temperature of water, but merely to break the intermolecular bonds between adjacent molecules and allow the to escape as water. What does this mean to us? The more you sweat, the more heat you lose, the more energy you lose. Or another way, the more energy utilized to create water vapor, the colder you will be. So to conserve heat, we need to reduce the quantity of water lost through vaporisation.
Vapor Barrier (VP) Function and Usage:
At the simplest level, a VP restricts the loss of water vapour, and by default creates a microclimate with 100% humidity. If we are encased in 100% humidity in our environment, then our body does not need to release additional moisture to achieve the desired tropical skin level condition.
If we are exercising and producing large quantities of metabolic heat, we are also likely to need to cool our bodies via sweating. Sweat is released as water vapor, the creation of water vapour results in heat loss (aka. as cooling), etc. If you are using a vapour barrier during intensive exercise, then you will be trapping large quantities of condensed vapor (sweat) in your clothing and against your skin, and will likely suffer from a few olfactory challenges and potentially debilitating "pruning".
On the other hand, when we are sleeping in cold conditions the retention of heat energy is tremendously important and we are unlikely to be producing any excess metabolic heat. Our bodies will only be vaporizing adequate moisture to maintain the ideal skin level of humidity.
Back to the Recommendations:
The Canadian military in researching sleep and clothing systems for usage in arctic conditions did a couple of interesting studies. The studies were looking at deep cold, between 20 and 40 below zero celsius. The following items are useful for us to address at this point
Incontinent water loss is 500 -1000ml/night (think evaporative loss not incontinence)
Sleeping bags gain an average 15g/hour during usage due to ice buildup in the insulation, or approximately 500g during 24 hours of usage
In cold weather we are unable to produce enough heat to push the vapor through our sleeping bags prior to condensation and crystallization within the insulation. This reduces the insulative quality, increases weight, and overtime can reduce our comfort level. By utilizing a VP, we reduce heat loss, reduce moisture/weight buildup in our sleeping bag and increase our overall comfort. But this still leaves us with the final recommendation: to wear it over a thin insulative layer. Simple, plastic bags against our skin feels really nasty.
Another fun-fact, at rest (at room temperature), we lose about 400ml (14oz) of water by insensible perspiration and another 400ml through respiration.
So that's about 130ml (4.5oz) in a eight hour night. But there are many variables, both environmentally and within the body that can skew that average both ways. For example, medical conditions, such as obesity and diabetes, can certainly increase insensible perspiration. So that's an issue for over a 1/3 of the US population ( with the UK catching up fast).
For overnighters, you are unlikely to build up enough moisture in your insulation to worry about. But multi-day trips, it is definitely a consideration.
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Gadget
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Originally Posted by Flash Grundelore
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