You don't want to live without these...
After some initial hesitation - Icebreaker T shirts do not come cheap - I have slowly collected a drawer full of Icebreaker Tech Ts, other Ts, and Ibex equivalents. I wear one EVERYDAY, all year round. I change them when I remember to.
I used to spend a lot of time in Phoenix, where the temperature can top 110F. The Tech T is PERFECT for the heat - much more comfortable than a cotton T. Get one - then two or three.. you won't go back!