Synthetic top layer for a winter mummy bag?

good posts

98F – 0.066 atm, 32F – 0.006 atm – so air at 10% relative humidity at 98 F will be 100% at 32 F – the dew point, water vapor will then condense – that’s the problem

” the insensible perspiration displaces other molecules and tends to rise” – I was just reading about that causing thunderstorms

“Last, consider that water vapor can supercool to well below freezing while awaiting a surface or something else to form crystals on.” – yeah, it doesn’t seem like water would condense on the down fibers.  The inside surface of the outer fabric would seem more likely.

“I am looking forward to reports from the field, specifically for reports of apparent success with relatively thin alpha direct at fairly low temperatures at 6000 feet and above”

Yeah, on a few trips of 20 F for 6 days, when I weighed my sleeping bag at the end of the trip, there was no excess weight, so there was never condensation inside my sleeping bag.

Is this just a problem for extreme trips that few people do?

I wonder how higher elevations affect this – air pressure is less which could change things

I don’t see it as a common problem. More for expeditions and a nomad lifestyle. I purchased my alpha and liner to use independently in warmer climates.  Like a kid with tinker toys, legos for the youngsters, I’ve tried different combinations looking for the benefits. While I live at 6,600′, atmospheric pressure is still different at 8,000′. I could not say definitely what happens at higher altitude.

Sweating in a vapor barrier is dependent on how much insulation is used. Personally, I prefer more insulation. YMMV

Jerry, I’ll do most of my camping in January at 8,400 ft.  Hoping to get my cover finished in another week, 2 at the latest. It may be “crude but effective” as the saying goes. Refinements can come later.