Originally Posted by nothermark
In use I'd vent over the face area as most of the lost moisture is in the breath. OTOH it's also a lot of heat. If you can capture that under the tarp it sets up another microclimate that might help with the fog on top. Of course if you stay below freezing then fog and dew are not a problem. Lot's to theorize about. ;-)
Yes, lots to theorize about. Some have poo-poo'd my wool undercover/overcover idea, but I like your thinking that breathable on top is more important than breathable on bottom (though the undercover should breathe too). There are all sorts of approaches to this, which makes it fun!
Some folks hate vapor barriers as insulation, but I'm probably going to try it. And some folks have written off wool and Tyvek, but I'm probably going to try them too in various applications. Hang your own hang!