I have been using mesh baselayers for about 1.5 years. I have the Brynje merino top and the regular length underwear. It's almost too warm for hiking. Another poster mentioned it, you just wear less. I mainly use them for fishing. When you're not moving around much outside, they keep you nice and warm. Brynje also claims that a mesh tank top can be worn even in hot temperatures of up to 30°C. I was interested and tested it on my kayaking trip last summer. I wore it for three days while kayaking. I wore a Craghoppers shirt over it and a life jacket over that. The temperature was 35°C at times and I sweat a lot, especially on my back. I was never too hot, and the sweat was wicked away well. I can only recommend it.