Originally Posted by Hang_time
Although not a true article of VBL clothing, I do remember wearing my Houdini windshirt through the night on a cold evening - that seemed to keep me quite warm, and also provided some vapor barrier quality to protect my UQ that night. It worked to some extent, but I see your point of essentially wrapping yourself in the VBL, as opposed to lying on top of it. Good point indeed.
No more space blanket for me, though.
Hang Time, It can truly be one of life's great mysteries. At the moment you're communicating with a man who has used a space blanket, on and off, for six years in his HHSS, And has never had a single condensation problem worth mentioning. Worst case scenario is finding a very few small drops of moisture on the space blanket at the lowest point under the hammock the next morning. But with neither me or any of my insulation perceptively damp. Plus there's all the times I've used a space blanket under the Hammock inside my Peapod. There are at least a few other people here who can report essentially the same experience. So what the heck is going on? Because there's also other people the report situations just as you have. Very hard to figure out. Maybe some of us don't put out any body vapor? I sure do sweat like a horse in warm weather.
Of course there are a bunch of variables to get right with the vapor barrier. Like keeping it warm and having no real insulation between you and the VB. This is one reason I lean towards vapor barrier clothing. So far it just seems nearly impossible for the dampness that occurs to also cause me to be cold. That moisture – whether dampness or sweat – cannot enter into any other insulation other than what I have sacrificed. That is, the very thin layer that is right next to my skin for comfort. In fact my favorite vapor barrier right now has a built-in fuzzy lining, so that I don't even use a separate liner. But then again, even though I like the VB clothing, I do have that long experience with the space blankets and so far without problems. Instead of problems only a significant increase in warmth and dry insulation every time. It's a mystery! To me anyway.
I'm guessing wild here, but what was the temperature rating of your under quilt – and for that matter top quilt? You said the Temps were in the mid-20s. And what else were you wearing other than a top quilt on top and an under quilt on the bottom? IOW, Is it even remotely possible that, before you got cold, that you were sweating? If so, that moisture would normally be absorbed into the down of your under quilt, and you would never notice it. At least until it happened enough to become a problem in the down insulation. Recently, Dirtwheels Used his 45° under quilt at 23° by adding a space blanket. ( and I think he was dry? ) But you can see that, if it's possible to get that much boost from a vapor barrier/radiant barrier that there might be some risk of sweating if used with a – say 20° – under quilt with temps barely into the 20s. I don't know, just guessing wild here!
Still, my experience is that even if I get noticeably wet inside the VB clothing, I don't get cold, in fact I am usually warmer than without the VB. But that is VB clothing, I can see where an SB might be different. With VB clothing, any moisture is pretty much trapped inside the VB with your body. It can't Evaporate (evaporative cooling). And it can't get any other insulation wet, wicking outward and carrying body heat with it and decreasing the loft. Unless you vent on purpose. Not so much with a space blanket. Over heat and sweat, and move around during the night, and I can see where any moisture that got on the Space blanket would be free to evaporate, and maybe float upwards into your top quilt, or into any jacket or long Johns that you might be wearing.
I guess there are all kinds of possible explanations, even though I don't really know what they are. But the mystery I'm always left with is why I ( And a few other folks) have not run into these problems, at least not so far, when other people do. Or to put it another way: why did a SB give Dirtwheels a 20+F boost and cause you to be cold just above the rated temp of your UQ? I don't know!