hey john ...
Looks like you did most things right ... what i suspect happened was that the dew point was in yr bivy ... i think that the reason most of the condensation ended up at yr feet was because yr feet dont produce as much heat as yr core ... so with less heat pushing out the dew point was somewhere in yr system in that area, see the pics below ... same with the head, exhalled breath often gets trapped ... IMO a synthetic jacket inside a down bag isnt optimum for moisture management, especially if there's moisture in the jacket ... lack of ventilation in the bivy exaberates the problem
chances are that the temp dropped below the dew point somewhere in the morning
you can try
- use yr bivy unzipped, you only need splash protection at the head and feet
- don't use a bivy, ventilation will be improved, though i guess is certain situations the dew point could end up in yr bag rather than the bivy ... putting yr rain jacket (better ventilation) over yr feet and yr windshirt on the head will likely give you enough protection in an A frame pitch
- increase the heat inside that bag as that will push out moisture, it wastes fuel but a hot nalgene might help at night , heat warmers may also work
- put yr synthetic jacket over yr sleeping bag, rather than wearing it inside ... this wil act like a partial synth overbag ... you jacket may get damp instead, but since its synth you can dry it on the move ... remember that moisture travels outward, where do you want it to end up?
- use a VBL
just keep on testing in yr back yard as much as possible
sometimes condensation, especially in freezing and humid conditions, are just a fact of life, you just need to deal with it
i personally use synth bags so i dont need to worry about it as much
for more ...
No fabric, however, can completely eliminate condensation inside a bivy sack. Any time cold temperatures are mixed with a warm body that emits perspiration, and that body is confined in a small space such as a bivy sack, condensation will occur. Most often, condensation occurs in the head end of the bivy sack (where a significant quantity of moisture from respiration is difficult to ventilate) and the foot end of the bivy sack (where a lack of body heat creates very cold surfaces on the bivy sack shell, causing even minute quantities of moisture vapor to condense into liquid moisture.