I used a 'vapour barrier' when my Vango synthetic sleeping bag got damp. (Previous night tent collapse saga)
Full waterproofs +seal ski socks. Over dry thermals. I felt cold at first but after less an hour the bag warmed up and was dry by the morning. (As was I)
If I didn't put the waterproofs on I would've had a long cold & damp night. Possibly getting hypothermic. The temp went down to frost patches on ground in the morning.
It is a 3 season bag. Very interesting wet october first bivvy adventure...
I suppose it wasn't a true vapour barrier but breathable waterproof layer.. and I was using in the opposite way it normally is ..perspiring you+dry insulation Vs dry you damp insulation.

I’ve had an Omin-Heat down sleeping bag for a few years now. I can’t say it keeps me any warmer but the slightly shiny inside feels horrible against the skin! I also have some Omin-Heat leggings and boots. Also don’t seem any warmer.

Interesting thread. Many moons ago I had a week winter mountaineering in Scotland, but we booked a cabin so we never had to consider damp sleeping bags.

https://courses.lumenlearning.com/suny- ... t-balance/

According to this 60% of body heat is lost through radiation, far higher than I expected. So certainly worth looking at ways to reduce it.

Rob S wrote: ↑Sat Oct 29, 2022 12:00 pm https://courses.lumenlearning.com/suny- ... t-balance/

According to this 60% of body heat is lost through radiation, far higher than I expected. So certainly worth looking at ways to reduce it.

When stood still naked with dry skin in unmoved air with 20 °C?

Now I've an image of people riding bikes across a winter moorland wearing tinfoil suits, in a kind of DEVO style....

Alpinum wrote: ↑Sat Oct 29, 2022 1:19 pm

Rob S wrote: ↑Sat Oct 29, 2022 12:00 pm https://courses.lumenlearning.com/suny- ... t-balance/

According to this 60% of body heat is lost through radiation, far higher than I expected. So certainly worth looking at ways to reduce it.

When stood still naked with dry skin in unmoved air with 20 °C?

Ah, I see. As ever the devil is in the detail.

Rob S wrote: ↑Sun Oct 30, 2022 9:46 am Ah, I see. As ever the devil is in the detail.

Pedant mode is often on

I imagine we have far less heat loss through radiation once we wear outdoor clothing, or eg a fleece hat and buff for sleeping etc.
I've spent many hours in space blankets (emergency bags) and whilst they do add a little warmth, I think most comes from blocking the wind and providing a micro climate.
Similar to a bothy bag, which as far as I know are all simply (waterproofed) nylon. And yet, it surely doesn't harm to have a IR reflective fabric.

An issue with reflective properties is durability.
The Therm-a-rest Xlites have some reflective fabrics inside, which after about 50 - 60 nights are down to probably less than about 70 % (only assessed by holding against light and guessing). Space blankets can be used for about 10 - 20 nights before the reflective coating goes missing. How it's done (adding reflective properties) to a down jacket/bag is beyond me, I hope manufacturers found a way to make it durable. With (mountaineering) boots it works well on the other hand. Those high altitude exped boots with (multiple layers of) reflective lining will last for at least a couple of trips (from my limited experience), not the boots, the lining...
Eventually it'll come off at some particular rubbing spots.
...and when you dig your (too long, since on multi week trips such things get forgotten) fingernails into them when forcing your feet inside with numb hands or stiff gloves.