I bicycle 20 miles per day, and saving weight over sleep comfort is straining out the gnats whilst letting the elephants thru. How much weight will the heavier/bulkier bag add to your load, the bicycle carries the weight and if the climbs are a problem then gearing is your solution. Come nightfall my money rides on comfort, for a good nights sleep makes for an enjoyable ride.
Your vapor barrier will certainly trap vapor, but probably not much heat. I would opt for a silk bag liner in lieu of taking a vapor barrier if you decide to take the lighter bag.
Anyhow here is how Vapor gets around, in any system that is separated by a semi-permeable barrier (in our case skin) there will always be an attempt for the pressurized liquid (inside the skin) to seek to pass thru the skin in order to better equalize the excessive water. There have been studies that suggest that during sleep the body and mind can actually loose 1-2 lbs of water. In most cases this is water vapor borne on the exhaled air as we breathe. A VBL may trap most of moisture (escaping from the skin, the bodies largest organ), and keep the interior of the liner at a higher humidity than the air outside the VBL, unless it is a good rain or fog outdoors. Your breath will have a more active impact on the sleeping bag, as the outside air temperature on the outer surface of the sleeping bag will be far colder than your exhaled breath. So, a wool wrap over you mouth and nose will trap that moisture and help to prevent it from encountering the cold surface of the sleeping bag and condensing on it.
So, condensation forms when warm humid air (water is in its gaseous state) encounters a surface that is colder. If the temperature differential between the humid air and the VBL or Tent fabric is sufficiently far apart, the warm humid air condenses on the inside surface.
Take a teapot and heat some boiling water, when the water is rapidly boiling, open the Freezer Door and observe closely how the steam is deposited on the cold surfaces. The colder air will not support the steam cloud because the air is too cold to support the heated and excited motions of the water vapor molecules of steam. Because the water vapor is temperature dependent, the colder air slows the movement of water vapor molecules, and the steam begins to revert back to water droplets from its vapor stage. Therefore the water vapor condenses on the colder surface. Much like a glass of Iced Water will form droplets on the outside of the glass. Same principal.
Your situation is dependent upon how much water intake you have, versus how much your internal engine, and mind burn up during the night. The water has to go somewhere, and no it is not in the bladder. We are talking water loss here. So, the answer is that it depends, everyone's internal engine and mind and sleep patterns will invoke differing results under very similar conditions.
Think of it this way, some are soggy sleepers, others are high and dry sleepers...not using much water at night and so on and so forth. Everyone's experience will differ as will the same individual's experience. Under similar conditions even the same individual may very well set up circumstances that are inversely proportional to the exact same conditions on a given basis. For instance salt intake, liquid intake, and intake of foods that tend to absorb liquids. Diabetics will have differing results dependent upon blood sugar levels. Lots of variables factor in this equation.
My best advice is just quadruple your salt intake
My experience comes from being an Electronic Technician and being assigned 2 Hemodialysis Clinics where the Physicians would filter the blood by diffusion, ultrafiltration and osmosis. The addition of saline to increase water intake for the patient was incorporated to at the end of treatments in order to satisfy the Prescription that was made.
You will get many stories, all of them are true and perhaps some will mimic your experiences.
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Originally Posted by joe_guilbeau
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