More on the theory of vapor barrier liners, as this thread makes it obvious that many people are not aware of that theory:
https://www.warmlite.com/vapor-barrier/
From the above:
During World War II, US cold weather troops used Vapor Barrier socks to cure frostbite and trench foot. We started promoting use of Vapor Barrier socks (baggies, bread bags, etc.) in 1957, then gloves and shirts, and in sleeping bags in 1967. Vapor barrier in a sleeping bag gives no added warmth when vented, but always protects the insulation from condensation and sweat soaking. Thus, it’s advisable to have Vapor Barrier in your bag for ALL seasons. The surface wickability of Warmlite’s Fuzzy Stuff makes it especially desirable for summer use when you’re sure to overheat.
A common argument against Vapor Barrier is actually a misunderstanding of the function and usefulness of vapor barrier clothing and is, in fact, Vapor Barrier simply doing its job. Using vapor barrier clothing is different than other clothing, such as wickable clothing, and habits and attitudes towards layering will have to be adjusted. Once learned how to be used, however, it is not only helpful but can be an actual life saver. Vapor barrier clothing can increase temperature beside the skin by up to 20°F. Vapor barrier clothing immediately warns the wearer of overheat because it traps over 90% of moisture (sweat) within the barrier, this is what alerts the wearer to overheating. This can be misunderstood as causing the overheating, but it simply all sweat being trapped, protecting upper layers from becoming damp or soaked. It also stops evaporation, chilling, and insensible sweat.
Thank you. There seem to be many people who immediately associate moisture with getting cold without understanding the context. I remember a bunch of times while cold-weather backpacking and my feet started to get cold that I'd slide a ziploc bag over each foot under the sock. It made a HUGE difference.