Merino wool base layers in the summer are awesome. I work landscape and I'm sweating constantly, so a wool base layer is a must. It'll feel a little "insulating", but it will keep you DRY AS FUCK. No more swimming in a wet rag the whole day, it wicks right off and cools in a light breeze. And no smell either.
Stick with cotton if you're just chillin' in the heat, but if you're planning on doing some sweating like with a hike, go with a wool micro layer (so long as it's your only layer)
Sierra Trading Post has 'em the cheapest. And RetailMeNot always further coupons.
http://www.sierratradingpost.com/redram-by-icebreaker-merino-wool-base-layer-top-short-sleeve-for-men~p~4238x/?colorFamily=02Get a size smaller for a form-fit. Catches the sweat better.
