>>2042954Reflective insulation sucks dick at sea level pressure, and this applies to everything, Al lined bivvies, survival foils, the reflective coating on sleeping mats as well as any incidental reflection from materials not intended to be reflective insulation.
When you're naked inside your survival foil and it's cold outside, steady state your body is substantially warmer than the foil, the heat you radiate being directed back at you makes a difference while most of the heat you conduct into the foil is gone (though being a thin film and the material also happens to be highly emissive helps with some radiating back). But that situation is not desirable, that shit is supposed to save your life not keep you comfortable.
When you're sleeping inside thick, opaque insulation like a sleeping bag the outside layer of it is almost in equilibrium with the environment already, putting reflective material on top of it does fuck all, the delta in temperature recovered increases conduction and returns the plastic to ambient temp. The inner layer which is in contact with your skin is also almost in equilibrium, putting reflective material inside the bag does fuck all because it's already the same temperature as you. The difference in temperature you gain is lost because it increases conduction until it returns to equilibrium.
Reflective insulation is worth (only with sharp temperature gradients) because you need very little material to achieve it but stops working on thick insulation with shallow temperature gradients
