Mike7
WKR
Elkhunter, that all makes sense to me what you are saying. I understand how the inner waterproof barrier should prevent the loss of 540 cal per gram of water vaporized since the inner barrier is not allowing ongoing escape and vaporization of perspiration.
I was just under the impression that Tyvek had some breathability, so wondered weather this is enough to prevent moisture build up inside of an outer Tyvek bivy sack as long as a person had the sack cinched down around the neck tightly...in order to prevent the addition of extra moisture getting into the bivy sack from the environment and from breathing. I wonder this, because I have not yet had any noted condensation inside of my Montbell down bag/outer bivy sack combination in any cold humid weather as long as the bag combo is cinched down around my neck. I do however run a 30 degree bag and sleep down into the teens in this combo, so I never seem to be overheating/perspiring much in these temps (I just add a little clothing as needed depending upon the temp).
I was just under the impression that Tyvek had some breathability, so wondered weather this is enough to prevent moisture build up inside of an outer Tyvek bivy sack as long as a person had the sack cinched down around the neck tightly...in order to prevent the addition of extra moisture getting into the bivy sack from the environment and from breathing. I wonder this, because I have not yet had any noted condensation inside of my Montbell down bag/outer bivy sack combination in any cold humid weather as long as the bag combo is cinched down around my neck. I do however run a 30 degree bag and sleep down into the teens in this combo, so I never seem to be overheating/perspiring much in these temps (I just add a little clothing as needed depending upon the temp).
