Do thin warm undergarments exist?

[Storker]

remember that wool is slightly exothermic when it gets wet

Cite, please? I have a serious problem believing that, and I suspect a misunderstanding somewhere.

 

[grf88]

Not sure if this is the answer to that question:
How does wool keep you warm even when it's wet?

 

[Lorenzoid]

Marketing hype aside, is Smartwool just a brand name for merino wool, or is it something more?

 

[tbone1004]

what @grf88 said, but you won't feel it because it is a small amount of heat that is generated and if you bring in frigid ambient water the temp change from say 5c water that came in and has now shocked your skin being heated to even an extreme of 10c isn't something you will be able to tell. This in contrast to a synthetic fiber that is strictly passive with heat.

Smartwool is a brand name, no different than Nike, Adidas, Under Armour etc. Many of their garments are 100% merino wool which is what they started with, but for some stretch applications they have to put some synthetic fibers in to augment the physical performance of the yarn. Their PhD fabrics are roughly 50/50 Merino/Polyester and the Polyester helps with wicking and evaporative cooling, but their NTS line is 100% merino.