>>1617229Grid fleece is the best for heat retention, and also breathes well so you don't freeze immediately after working up a sweat. However, all synthetics get manky fast.
Merino is a good choice as well. Almost as good at retaining heat as grid fleece, naturally stank resistant. However, it's the most expensive by a fairly large margin, and needs to be hand-washed or it doesn't last
Avoid cotton. It's less insulating than everything else, loses almost all of its insulation value as soon as it gets even slightly damp, and isn't really any cheaper than the good synthetics any more (which was the only reason it lasted as long as it did, but tech caught up).
If you want good-but-cheap longjohns, google up US military ECWCS gen 3. Layer 1 is the silkweight polypro (ninja suit, the top is a crewneck), they're a decent light baselayer, and a full set of new-in-package should run you $20-25 on ebay or another suprlus outlet. Layer 2 is the grid-fleece (the top is a high-collar halfzip), which are somewhere between a heavyweight and expedition weight, and will run you $20ish a piece gently used on ebay, or about $35 for a new-unissued set (the tops have a much larger market than the bottoms so you get a discount buying the set). Avoid clones like Rothco, they're thinner and the sizing is all fuckered up (I made that mistake, my issued waffle top was a US Large-Regular and is kinda loose on me, I couldn't even get a Rothco Large-Regular on and it's visibly about half the thickness).
