So OK, the points you are making above seem to be in opposition to those that MacEntyre was postulating: That cold air coming in from the outside might be the problem. And what you are saying is closer to my previous "understanding" of how this stuff works.
But as for the SB letting vapor through: that would surprise me. But I have never really tested a WM space blanket or Heatsheet for water proofing, I have just assumed it was there. I have found some few water drops on top of the space blanket, but not underneath it.
Have you ever tested your space blanket ( is it a Wallyworld special?) for being WP, by any chance? If mine were not put up in a box, I would go test them right now. I would be particularly interested to see if any vapor could get through with me trying to blow through it onto a cold surface.
If, as you suspect, body moisture can get past the space blanket, then I would expect it to condense down in the UC with it's cold material.
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