Do not wear under the membrane: woolen, cotton socks, synthetic content is required (+/- 25%). It could be a training sock. The standard sock of the APU also has a synthetic content. No need to wear thermal socks! Also, pulling on the foot more and, as it seems to you, warmer, for example, grandmother's wool knitting, you only make it worse, as you eliminate the minimum air cushion inside the boot, where heat should accumulate and remain. When choosing or selecting winter shoes, your leg, foot should feel free inside the boot, as they say, without pressure.