Aye Up Gcckoka,
In the first place,
well done for getting out there and over-nighting in low temps, not many people do that.
Secondly - I am not surprised that you feel the need to change your ground mat - it appears to be placed almost directly onto snow covered, maybe even frozen ground with what appears to be
no natural material beneath it! (The 'reflective' sheet -good for growing something beneath perhaps - as an insulator forget it!
When ground bivvy-ing beneath a well set up, correctly angled tarp/basher the No1 direction for heat loss whilst sleeping is
straight into the ground (as parachutists and seasoned outdoor over-nighters know - gravity is a myth, the Earth sucks!
)
Most (solid) ground 'insulation' mats and even the thin self inflating types are hardly that, most are just good/reasonable moisture barriers.
Relying only on such mats especially in the brown season is folly - I learned (Brit military training) and have subsequently continued to teach that getting a decent amount of natural material
beneath a ground insulation mat is a top priority when setting up your site for comfort and insulation - i.e. grass; fern; bracken; moss; evergreen material etc - if possible stuffed inside one of the large emergency plastic survival bags for example (which all winter campers have with them?) or even in a bivvy bag itself, in order to keep it all in place under you. (If you can't use natural material you can always use the spare mid layer clothing which you carry with you).
Thirdly, regarding the condensation issue - again a well known fact amongst those who overnight beneath tarps/bashers in low temperatures especially with PVC coated (and breathable) materials.
The cause is the 'dew point' - the point where your body heat/perspiration/breath meets a cold surface above you and condenses into water droplets.
To begin with I notice that you have pegged three sides + of your shelter
right to the ground - that means no/limited movement of air within the shelter and so the heat/moisture generated by you is more or less going straight up to the underside of the sheet - where in those conditions it will most likely (did!) almost immediately condense into water droplets! Whenever you can achieve it, it is good practice to allow at least a little through draft to move any moisture laden air towards the open mouth or sides of a tarp/basher. The military style of pitching in the attached image isnt just about rapid exit on all sides its also about condensation limitation!
You can also aim to move the dew point further away from you by -
Sleeping with your head near to the open end of the tarp/basher so that your breath moisture is drawn away from the inside of the (correctly pitched/angled) shelter;
If practical, you can raise the height of the tarp/basher above you so that by the time your warm moisture meets the cold surface it has cooled and dissipated;
You can angle the pitch of the shelter (higher at the head end) so that internal condensation runs towards the lower, foot end of the shelter;
You can use a bivvy bag and a correctly pitched/angled tarp/basher so that body generated heat is dissipated before it reaches the tarp/basher;
I hope that you (and anyone else intending to over-night during low temps) find this info helpful.
Again, well done for having the gonads to get out there in those conditions and thanks for posting your vid.
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