Almost every source I can find online maintains that two 0.5 cm blankets are significantly warmer than a single 1cm blanket due to air trapped between the thin blankets.
However, the thermal conductivities of air and wool are roughly comparable according to this website. That would seem to mean that air only aids the insulation significantly if the thickness of the air layer is significant compared to the thickness of the blankets, which I would guess it is not.
Further, in order for heat transfer through the air to be dominated by conduction, the air needs to be extremely still, which it seems it would not be around a living, breathing, human. Otherwise the air would mix and not provide significant insulation.
Is there any empirical evidence that two thin blankets are superior? If so, why is that, given the above considerations?
Best Answer
I don't have any accurate measurements, but from my attempts at measuring my blankets with a ruler it looks to me as if two layers of blanket are more than twice as thick as one layer of blanket. I'd guess this is because some fibres of wool stand proud of the surface and they keep the two layers apart.
Anyhow, this implies that two layers will be thicker than a single layer with twice the area density of wool. I can't find figures for wool, but hair is probably similar and the thermal conductivity of hair is three times that of air. Therefore the thicker layer with a larger air-wool ratio formed by the two blankets will have a lower thermal conductivity than a single layer with twice the amount of wool.
So two blankets are warmer than a single thicker blanket.