Cotton base layers trap sweat and stink. Merino regulates temperature and doesn't smell like a gym bag.
Why Not Just Buy the Cheap One?
Cotton base layers absorb sweat and keep moisture against your skin, making you cold in winter and creating an environment where bacteria thrive—you'll smell sour within hours. Merino wool actively regulates temperature (warm when dry, cooling when damp), naturally resists odor, and doesn't itch like cheap itchy wool blends do.
The Buy-Once Math
You'll buy cotton base layers twice a year ($15-25 each time) because they stink, pill, and lose elasticity fast—that's $180-300 over a decade. A Patagonia merino set costs $150-200 and actually works for 5+ years, staying fresh longer between washes (saving water and detergent). Merino is the only base layer you'll ever actually want to wear.